Quarterly report pursuant to Section 13 or 15(d)

Cover
- Document and Entity Information
Financial Statements
Notes to Financial Statements
Accounting Policies
- Basis of Presentation and Summary of Significant Accounting Policies (Policies)
Notes Tables
Notes Details

Basis of Presentation and Summary of Significant Accounting Policies - Investment Categories (Details)

v3.10.0.1

Basis of Presentation and Summary of Significant Accounting Policies - Investment Categories (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Amortized cost	$ 889,653	$ 179,528
Gross unrealized gains	1,654	1,695
Gross unrealized losses	179	182
Estimated fair value	891,128	181,041
Bank deposits
Debt Securities, Available-for-sale [Line Items]
Amortized cost	411,166	80,095
Gross unrealized gains	98	6
Gross unrealized losses	17	42
Estimated fair value	411,247	80,059
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Amortized cost	76,961	55,335
Gross unrealized gains	4	0
Gross unrealized losses	52	96
Estimated fair value	76,913	55,239
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Amortized cost	288,549	27,933
Gross unrealized gains	3	0
Gross unrealized losses	72	20
Estimated fair value	288,480	27,913
U.S. Government bonds
Debt Securities, Available-for-sale [Line Items]
Amortized cost	110,834	8,939
Gross unrealized gains	0	0
Gross unrealized losses	25	10
Estimated fair value	110,809	8,929
Agency bonds
Debt Securities, Available-for-sale [Line Items]
Amortized cost	0	4,991
Gross unrealized gains	0	0
Gross unrealized losses	0	1
Estimated fair value	0	4,990
Municipal bonds
Debt Securities, Available-for-sale [Line Items]
Amortized cost	2,008	2,028
Gross unrealized gains	0	0
Gross unrealized losses	13	13
Estimated fair value	1,995	2,015
Corporate equity securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost	135	207
Gross unrealized gains	1,549	1,689
Gross unrealized losses	0	0
Estimated fair value	$ 1,684	$ 1,896

- Definition

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

+ Details

Name:	us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

+ Details

Name:	us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

+ Details

Name:	us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26610-111562

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesDebtSecurities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_DemandDepositsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CorporateDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CommercialPaperMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USGovernmentAgenciesDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_MunicipalBondsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_EquitySecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: