Fair Value Measurements - Summary of Cash and Cash Equivalents and Investments' Costs, Gross Unrealized Gains (Losses), and Fair Value by Major Security Type Recorded as Cash and Cash Equivalents or Short-Term or Long-Term Investments (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	$ 228,020	$ 409,949
Short-term Investments	534,887	376,995
Long-term Investments	229,027	187,921
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	228,020	409,949
Gross unrealized gains (losses)	2,118	637
Cash and cash equivalents and investment, cost	989,816	974,228
Cash and cash equivalents and investments, fair value	991,934	974,865
Short-term Investments	534,887	376,995
Long-term Investments	229,027	187,921
Fair Value, Measurements, Recurring | Cash
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	64,782	53,039
Cash and cash equivalents, fair value	64,782	53,039
Fair Value, Measurements, Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	77,563	223,580
Cash and cash equivalents, fair value	77,563	223,580
Fair Value, Measurements, Recurring | Level 1 | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	77,563	223,580
Cash and cash equivalents, fair value	77,563	223,580
Fair Value, Measurements, Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	85,675	133,330
Investments, cost	847,471	697,609
Gross unrealized gains (losses)	2,118	637
Investments, fair value	849,589	698,246
Short-term Investments	534,887	376,995
Long-term Investments	229,027	187,921
Fair Value, Measurements, Recurring | Level 2 | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	56,966	98,325
Investments, cost	256,539	217,140
Gross unrealized gains (losses)	(38)	(6)
Investments, fair value	256,501	217,134
Short-term Investments	199,535	118,809
Long-term Investments	0	0
Fair Value, Measurements, Recurring | Level 2 | Certificates of deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, cost	1,000	1,000
Gross unrealized gains (losses)	0	0
Investments, fair value	1,000	1,000
Short-term Investments	1,000	0
Long-term Investments	0	1,000
Fair Value, Measurements, Recurring | Level 2 | U.S. Treasury and agency bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	25,559	35,005
Investments, cost	378,558	294,953
Gross unrealized gains (losses)	2,691	199
Investments, fair value	381,249	295,152
Short-term Investments	161,444	161,767
Long-term Investments	194,246	98,380
Fair Value, Measurements, Recurring | Level 2 | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	3,150
Investments, cost	211,374	184,516
Gross unrealized gains (losses)	(535)	444
Investments, fair value	210,839	184,960
Short-term Investments	172,908	96,419
Long-term Investments	34,781	88,541
Fair Value, Measurements, Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, cost	0	0
Gross unrealized gains (losses)	0	0
Investments, fair value	0	0
Short-term Investments	0	0
Long-term Investments	$ 0	$ 0