Investments - Schedule of Investments (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Available-for-sale securities:
Amortized Cost	$ 395,928
Estimated Fair Value	396,310
Held-to-maturity securities:
Estimated Fair Value	3,589
Long-term Investments
Amortized Cost	43,041	$ 80,923
Gross Unrealized Gains	102	136
Gross Unrealized Losses, Less than One Year	(88)	(210)
Gross Unrealized Losses, Greater than One Year	(50)	(105)
Total long-term investments	43,005	80,744
Total investments
Amortized Cost	399,415	353,758
Gross Unrealized Gains	901	393
Gross Unrealized Losses, Less than One Year	(103)	(291)
Gross Unrealized Losses, Greater than One Year	(314)	(583)
Estimated Fair Value	399,899	353,277
Short-term investments
Available-for-sale securities:
Amortized Cost	356,374	272,343
Gross Unrealized Gains	799	257
Gross Unrealized Losses, Less than One Year	(15)	(81)
Gross Unrealized Losses, Greater than One Year	(264)	(478)
Estimated Fair Value	356,894	272,041
Held-to-maturity securities:
Amortized Cost		272,835
Gross Unrealized Gains		257
Gross Unrealized Losses, Less than One Year		(81)
Gross Unrealized Losses, Greater than One Year		(478)
Estimated Fair Value		272,533
Short-term investments | Corporate debt securities
Available-for-sale securities:
Amortized Cost	156,443	120,197
Gross Unrealized Gains	349	57
Gross Unrealized Losses, Less than One Year	(11)	(62)
Gross Unrealized Losses, Greater than One Year	(149)	(274)
Estimated Fair Value	156,632	119,918
Held-to-maturity securities:
Amortized Cost		492
Estimated Fair Value		492
Short-term investments | U.S. government and agency debt securities
Available-for-sale securities:
Amortized Cost	108,113	80,055
Gross Unrealized Gains	257	115
Gross Unrealized Losses, Less than One Year	(4)	(11)
Gross Unrealized Losses, Greater than One Year	(45)	(87)
Estimated Fair Value	108,321	80,072
Short-term investments | International government agency debt securities
Available-for-sale securities:
Amortized Cost	91,818	72,091
Gross Unrealized Gains	193	85
Gross Unrealized Losses, Less than One Year		(8)
Gross Unrealized Losses, Greater than One Year	(70)	(117)
Estimated Fair Value	91,941	72,051
Long-term investments
Available-for-sale securities:
Amortized Cost	39,554	77,436
Gross Unrealized Losses, Less than One Year	(88)	(210)
Gross Unrealized Losses, Greater than One Year	(50)	(105)
Estimated Fair Value	39,416	77,121
Held-to-maturity securities:
Amortized Cost	3,487	3,487
Gross Unrealized Gains	102	136
Estimated Fair Value	3,589	3,623
Long-term investments | Certificates of deposit
Held-to-maturity securities:
Amortized Cost	1,820	1,820
Estimated Fair Value	1,820	1,820
Long-term investments | Corporate debt securities
Available-for-sale securities:
Amortized Cost	28,570	53,505
Gross Unrealized Losses, Less than One Year	(84)	(185)
Gross Unrealized Losses, Greater than One Year	(45)	(93)
Estimated Fair Value	28,441	53,227
Long-term investments | U.S. government and agency debt securities
Available-for-sale securities:
Amortized Cost	10,984	18,474
Gross Unrealized Losses, Less than One Year	(4)	(21)
Gross Unrealized Losses, Greater than One Year	(5)	(12)
Estimated Fair Value	10,975	18,441
Long-term investments | International government agency debt securities
Available-for-sale securities:
Amortized Cost		5,457
Gross Unrealized Losses, Less than One Year		(4)
Estimated Fair Value		5,453
Long-term investments | Fixed Term Deposit Account
Held-to-maturity securities:
Amortized Cost	1,667	1,667
Gross Unrealized Gains	102	136
Estimated Fair Value	$ 1,769	$ 1,803