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, 2017
Dec. 31, 2016
Short-term investments    
Schedule of Available-for-sale Securities [Line Items]    
Cost $ 179,528 $ 120,758
Gross unrealized gains 1,695 1,564
Gross unrealized losses (182) (26)
Estimated fair value 181,041 122,296
Bank deposits    
Schedule of Available-for-sale Securities [Line Items]    
Cost 80,095 40,715
Gross unrealized gains 6 19
Gross unrealized losses (42) 0
Estimated fair value 80,059 40,734
Corporate bonds    
Schedule of Available-for-sale Securities [Line Items]    
Cost 55,335 11,031
Gross unrealized gains 0 0
Gross unrealized losses (96) (5)
Estimated fair value 55,239 11,026
Corporate equity securities    
Schedule of Available-for-sale Securities [Line Items]    
Cost 207 1,512
Gross unrealized gains 1,689 1,542
Gross unrealized losses 0 0
Estimated fair value 1,896 3,054
Commercial paper    
Schedule of Available-for-sale Securities [Line Items]    
Cost 27,933 33,074
Gross unrealized gains 0 2
Gross unrealized losses (20) (9)
Estimated fair value 27,913 33,067
Agency bonds    
Schedule of Available-for-sale Securities [Line Items]    
Cost 4,991 7,294
Gross unrealized gains 0 1
Gross unrealized losses (1) 0
Estimated fair value 4,990 7,295
U.S. Government bonds    
Schedule of Available-for-sale Securities [Line Items]    
Cost 8,939 7,508
Gross unrealized gains 0 0
Gross unrealized losses (10) (1)
Estimated fair value 8,929 7,507
Municipal bonds    
Schedule of Available-for-sale Securities [Line Items]    
Cost 2,028 19,624
Gross unrealized gains 0 0
Gross unrealized losses (13) (11)
Estimated fair value $ 2,015 $ 19,613