Fair Value Measurements - Schedule of Fair Value of Financial Assets (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable Securities, Amortized Cost	$ 2,183,707	$ 1,978,283
Marketable Securities, Unrealized Gains	1,186	1,700
Marketable Securities, Unrealized Losses	(563)	(334)
Marketable Securities, Fair Value	2,184,330	1,979,649
Total Financial Assets, Amortized Cost	2,567,288	2,421,372
Total Financial Assets, Fair Value	2,567,911	2,422,738
Level I
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable Securities, Fair Value	661,518	523,506
Total Financial Assets, Fair Value	1,045,099	966,595
Level II
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable Securities, Fair Value	1,522,812	1,456,143
Total Financial Assets, Fair Value	1,522,812	1,456,143
Level III
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable Securities, Fair Value	0	0
Total Financial Assets, Fair Value	0	0
Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash Equivalents, Amortized Cost	379,345	438,854
Cash Equivalents, Fair Value	379,345	438,854
Money market funds | Level I
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash Equivalents, Fair Value	379,345	438,854
Money market funds | Level II
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash Equivalents, Fair Value	0	0
Money market funds | Level III
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash Equivalents, Fair Value	0	0
Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable Securities, Amortized Cost	46,658	51,211
Marketable Securities, Unrealized Gains	0	0
Marketable Securities, Unrealized Losses	0	0
Marketable Securities, Fair Value	46,658	51,211
Commercial paper | Level I
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable Securities, Fair Value	0	0
Commercial paper | Level II
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable Securities, Fair Value	46,658	51,211
Commercial paper | Level III
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable Securities, Fair Value	0	0
Certificate of deposits
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable Securities, Amortized Cost	80,069	50,136
Marketable Securities, Unrealized Gains	0	3
Marketable Securities, Unrealized Losses	0	0
Marketable Securities, Fair Value	80,069	50,139
Certificate of deposits | Level I
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable Securities, Fair Value	0	0
Certificate of deposits | Level II
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable Securities, Fair Value	80,069	50,139
Certificate of deposits | Level III
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable Securities, Fair Value	0	0
U.S. government notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable Securities, Amortized Cost	661,366	523,320
Marketable Securities, Unrealized Gains	190	187
Marketable Securities, Unrealized Losses	(38)	(1)
Marketable Securities, Fair Value	661,518	523,506
U.S. government notes | Level I
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable Securities, Fair Value	661,518	523,506
U.S. government notes | Level II
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable Securities, Fair Value	0	0
U.S. government notes | Level III
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable Securities, Fair Value	0	0
Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable Securities, Amortized Cost	972,063	878,484
Marketable Securities, Unrealized Gains	709	1,167
Marketable Securities, Unrealized Losses	(522)	(330)
Marketable Securities, Fair Value	972,250	879,321
Corporate bonds | Level I
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable Securities, Fair Value	0	0
Corporate bonds | Level II
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable Securities, Fair Value	972,250	879,321
Corporate bonds | Level III
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable Securities, Fair Value	0	0
Agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable Securities, Amortized Cost	423,551	475,132
Marketable Securities, Unrealized Gains	287	343
Marketable Securities, Unrealized Losses	(3)	(3)
Marketable Securities, Fair Value	423,835	475,472
Agency securities | Level I
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable Securities, Fair Value	0	0
Agency securities | Level II
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable Securities, Fair Value	423,835	475,472
Agency securities | Level III
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable Securities, Fair Value	0	0
Money market funds - restricted
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other Assets, Amortized Cost	4,236	4,235
Other Assets, Fair Value	4,236	4,235
Money market funds - restricted | Level I
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other Assets, Fair Value	4,236	4,235
Money market funds - restricted | Level II
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other Assets, Fair Value	0	0
Money market funds - restricted | Level III
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other Assets, Fair Value	$ 0	$ 0