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	Sep. 30, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	$ 95,952	$ 409,949
Short-term Investments	627,259	376,995
Long-term Investments	259,277	187,921
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	95,952	409,949
Gross unrealized gains (losses)	2,386	637
Cash and cash equivalents and investment, cost	980,102	974,228
Cash and cash equivalents and investments, fair value	982,488	974,865
Short-term Investments	627,259	376,995
Long-term Investments	259,277	187,921
Fair Value, Measurements, Recurring | Cash
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	49,548	53,039
Cash and cash equivalents, fair value	49,548	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	31,433	223,580
Cash and cash equivalents, fair value	31,433	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	31,433	223,580
Cash and cash equivalents, fair value	31,433	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	14,971	133,330
Investments, cost	899,121	697,609
Gross unrealized gains (losses)	2,386	637
Investments, fair value	901,507	698,246
Short-term Investments	627,259	376,995
Long-term Investments	259,277	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	14,971	98,325
Investments, cost	116,849	217,140
Gross unrealized gains (losses)	22	(6)
Investments, fair value	116,871	217,134
Short-term Investments	101,900	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	2,800	1,000
Gross unrealized gains (losses)	0	0
Investments, fair value	2,800	1,000
Short-term Investments	2,800	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	0	35,005
Investments, cost	592,734	294,953
Gross unrealized gains (losses)	1,723	199
Investments, fair value	594,457	295,152
Short-term Investments	393,501	161,767
Long-term Investments	200,956	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	186,738	184,516
Gross unrealized gains (losses)	641	444
Investments, fair value	187,379	184,960
Short-term Investments	129,058	96,419
Long-term Investments	58,321	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