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	Jun. 30, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	$ 128,175	$ 409,949
Short-term Investments	607,620	376,995
Long-term Investments	238,643	187,921
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	128,175	409,949
Gross unrealized gains (losses)	3,680	637
Cash and cash equivalents and investment, cost	970,758	974,228
Cash and cash equivalents and investments, fair value	974,438	974,865
Short-term Investments	607,620	376,995
Long-term Investments	238,643	187,921
Fair Value, Measurements, Recurring | Cash
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	42,475	53,039
Cash and cash equivalents, fair value	42,475	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	57,500	223,580
Cash and cash equivalents, fair value	57,500	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	57,500	223,580
Cash and cash equivalents, fair value	57,500	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	28,200	133,330
Investments, cost	870,783	697,609
Gross unrealized gains (losses)	3,680	637
Investments, fair value	874,463	698,246
Short-term Investments	607,620	376,995
Long-term Investments	238,643	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	2,000	98,325
Investments, cost	127,310	217,140
Gross unrealized gains (losses)	120	(6)
Investments, fair value	127,430	217,134
Short-term Investments	125,430	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	26,200	35,005
Investments, cost	537,097	294,953
Gross unrealized gains (losses)	2,210	199
Investments, fair value	539,307	295,152
Short-term Investments	312,160	161,767
Long-term Investments	200,947	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	0
Investments, cost	205,376	184,516
Gross unrealized gains (losses)	1,350	444
Investments, fair value	206,726	184,960
Short-term Investments	169,030	96,419
Long-term Investments	37,696	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