Investments - Schedule of Investments (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Available-for-sale securities:
Amortized Cost	$ 343,526
Estimated Fair Value	347,010
Held-to-maturity securities:
Estimated Fair Value	3,584
Long-term Investments
Amortized Cost	6,940	$ 79,363
Gross Unrealized Gains	96	102
Gross Unrealized Losses, Less than One Year	(3)	(67)
Gross Unrealized Losses, Greater than One Year		(7)
Total long-term investments	7,033	79,391
Total investments
Amortized Cost	347,014	409,225
Gross Unrealized Gains	3,616	1,460
Gross Unrealized Losses, Less than One Year	(33)	(78)
Gross Unrealized Losses, Greater than One Year	(3)	(8)
Estimated Fair Value	350,594	410,599
Short-term Investments
Amortized Cost	340,074	329,862
Gross Unrealized Gains	3,520	1,358
Gross Unrealized Losses, Less than One Year	(30)	(11)
Gross Unrealized Losses, Greater than One Year	(3)	(1)
Total short-term investments	343,561	331,208
Short-term investments | Corporate debt securities
Available-for-sale securities:
Amortized Cost	163,761	144,161
Gross Unrealized Gains	1,774	676
Gross Unrealized Losses, Less than One Year	(13)
Gross Unrealized Losses, Greater than One Year	(3)
Estimated Fair Value	165,519	144,837
Short-term investments | U.S. government and agency debt securities
Available-for-sale securities:
Amortized Cost	98,689	112,948
Gross Unrealized Gains	827	434
Gross Unrealized Losses, Less than One Year	(2)	(1)
Gross Unrealized Losses, Greater than One Year		(1)
Estimated Fair Value	99,514	113,380
Short-term investments | International government agency debt securities
Available-for-sale securities:
Amortized Cost	77,624	72,753
Gross Unrealized Gains	919	248
Gross Unrealized Losses, Less than One Year	(15)	(10)
Estimated Fair Value	78,528	72,991
Long-term investments
Available-for-sale securities:
Amortized Cost	3,452	75,876
Gross Unrealized Losses, Less than One Year	(3)	(67)
Gross Unrealized Losses, Greater than One Year		(7)
Estimated Fair Value	3,449	75,802
Held-to-maturity securities:
Amortized Cost	3,488	3,487
Gross Unrealized Gains	96	102
Estimated Fair Value	3,584	3,589
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	3,452	51,070
Gross Unrealized Losses, Less than One Year	(3)	(45)
Gross Unrealized Losses, Greater than One Year		(7)
Estimated Fair Value	3,449	51,018
Long-term investments | U.S. government and agency debt securities
Available-for-sale securities:
Amortized Cost		4,000
Gross Unrealized Losses, Less than One Year		(4)
Estimated Fair Value		3,996
Long-term investments | International government agency debt securities
Available-for-sale securities:
Amortized Cost		20,806
Gross Unrealized Losses, Less than One Year		(18)
Estimated Fair Value		20,788
Long-term investments | Fixed Term Deposit Account
Held-to-maturity securities:
Amortized Cost	1,668	1,667
Gross Unrealized Gains	96	102
Estimated Fair Value	$ 1,764	$ 1,769