Investments - Schedule of Available-for-sale Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Short-Term Marketable Securities
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	$ 479,791	$ 110,095
Unrealized Gain	503
Unrealized Loss	(4)	(107)
Estimated Fair Value	480,290	109,988
Long-Term Marketable Securities
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	105,263
Unrealized Gain	181
Unrealized Loss	(9)
Estimated Fair Value	105,435
Commercial Paper | Short-Term Marketable Securities
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	121,866	16,887
Estimated Fair Value	121,866	16,887
U.S. Government Debt and Agency Securities | Short-Term Marketable Securities
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	285,963	85,722
Unrealized Gain	394
Unrealized Loss	(1)	(99)
Estimated Fair Value	286,356	85,623
U.S. Government Debt and Agency Securities | Long-Term Marketable Securities
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	90,750
Unrealized Gain	146
Unrealized Loss	(9)
Estimated Fair Value	90,887
Corporate Bonds | Short-Term Marketable Securities
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	71,962	7,486
Unrealized Gain	109
Unrealized Loss	(3)	(8)
Estimated Fair Value	72,068	$ 7,478
Corporate Bonds | Long-Term Marketable Securities
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	14,513
Unrealized Gain	35
Estimated Fair Value	$ 14,548