Annual report pursuant to Section 13 and 15(d)

Balance Sheet Account Details - Investment Categories (Details)

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Balance Sheet Account Details - Investment Categories (Details) - USD ($)
$ in Thousands
Dec. 31, 2018
Dec. 31, 2017
Short-term investments    
Debt Securities, Available-for-sale [Line Items]    
Cost $ 600,154 $ 179,528
Gross unrealized gains 1,226 1,695
Gross unrealized losses (163) (182)
Estimated fair value 601,217 181,041
Bank deposits    
Debt Securities, Available-for-sale [Line Items]    
Cost 311,066 80,095
Gross unrealized gains 26 6
Gross unrealized losses (29) (42)
Estimated fair value 311,063 80,059
Corporate bonds    
Debt Securities, Available-for-sale [Line Items]    
Cost 53,223 55,335
Gross unrealized gains 1 0
Gross unrealized losses (45) (96)
Estimated fair value 53,179 55,239
Corporate equity securities    
Debt Securities, Available-for-sale [Line Items]    
Cost 135 207
Gross unrealized gains 1,191 1,689
Gross unrealized losses 0 0
Estimated fair value 1,326 1,896
Commercial paper    
Debt Securities, Available-for-sale [Line Items]    
Cost 225,731 27,933
Gross unrealized gains 8 0
Gross unrealized losses (76) (20)
Estimated fair value 225,663 27,913
Agency bonds    
Debt Securities, Available-for-sale [Line Items]    
Cost   4,991
Gross unrealized gains   0
Gross unrealized losses   (1)
Estimated fair value   4,990
U.S. Government bonds    
Debt Securities, Available-for-sale [Line Items]    
Cost 7,982 8,939
Gross unrealized gains 0 0
Gross unrealized losses (9) (10)
Estimated fair value 7,973 8,929
Municipal bonds    
Debt Securities, Available-for-sale [Line Items]    
Cost 2,017 2,028
Gross unrealized gains 0 0
Gross unrealized losses (4) (13)
Estimated fair value $ 2,013 $ 2,015