Investments - Schedule of Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Available-for-sale securities:
Amortized Cost	$ 405,738
Estimated Fair Value	407,010
Held-to-maturity securities:
Estimated Fair Value	3,589
Long-term Investments
Total long-term investments	79,391	$ 80,744
Total investments
Amortized Cost	409,225	353,758
Gross Unrealized Gains	1,460	393
Gross Unrealized Losses, Less than One Year	(78)	(291)
Gross Unrealized Losses, Greater than One Year	(8)	(583)
Estimated Fair Value	410,599	353,277
Short-term Investments
Amortized Cost	329,862	272,835
Gross Unrealized Gains	1,358	257
Gross Unrealized Losses, Less than One Year	(11)	(81)
Gross Unrealized Losses, Greater than One Year	(1)	(478)
Total short-term investments	331,208	272,533
Short-term investments
Available-for-sale securities:
Amortized Cost		272,343
Gross Unrealized Gains		257
Gross Unrealized Losses, Less than One Year		(81)
Gross Unrealized Losses, Greater than One Year		(478)
Estimated Fair Value		272,041
Short-term investments | U.S. government and agency debt securities
Available-for-sale securities:
Amortized Cost	112,948	80,055
Gross Unrealized Gains	434	115
Gross Unrealized Losses, Less than One Year	(1)	(11)
Gross Unrealized Losses, Greater than One Year	(1)	(87)
Estimated Fair Value	113,380	80,072
Short-term investments | Corporate debt securities
Available-for-sale securities:
Amortized Cost	144,161	120,197
Gross Unrealized Gains	676	57
Gross Unrealized Losses, Less than One Year		(62)
Gross Unrealized Losses, Greater than One Year		(274)
Estimated Fair Value	144,837	119,918
Held-to-maturity securities:
Amortized Cost		492
Estimated Fair Value		492
Short-term investments | International government agency debt securities
Available-for-sale securities:
Amortized Cost	72,753	72,091
Gross Unrealized Gains	248	85
Gross Unrealized Losses, Less than One Year	(10)	(8)
Gross Unrealized Losses, Greater than One Year		(117)
Estimated Fair Value	72,991	72,051
Long-term investments
Available-for-sale securities:
Amortized Cost	75,876	77,436
Gross Unrealized Losses, Less than One Year	(67)	(210)
Gross Unrealized Losses, Greater than One Year	(7)	(105)
Estimated Fair Value	75,802	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
Amortized Cost	79,363	80,923
Gross Unrealized Gains	102	136
Gross Unrealized Losses, Less than One Year	(67)	(210)
Gross Unrealized Losses, Greater than One Year	(7)	(105)
Total long-term investments	79,391	80,744
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 | U.S. government and agency debt securities
Available-for-sale securities:
Amortized Cost	4,000	18,474
Gross Unrealized Losses, Less than One Year	(4)	(21)
Gross Unrealized Losses, Greater than One Year		(12)
Estimated Fair Value	3,996	18,441
Long-term investments | Corporate debt securities
Available-for-sale securities:
Amortized Cost	51,070	53,505
Gross Unrealized Losses, Less than One Year	(45)	(185)
Gross Unrealized Losses, Greater than One Year	(7)	(93)
Estimated Fair Value	51,018	53,227
Long-term investments | International government agency debt securities
Available-for-sale securities:
Amortized Cost	20,806	5,457
Gross Unrealized Losses, Less than One Year	(18)	(4)
Estimated Fair Value	20,788	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