Fair Value Measurements - Schedule of Fair Value of Financial Assets (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash Equivalents, Amortized Cost		$ 566,581
Cash Equivalents, Fair Value		566,581
Marketable Securities, Amortized Cost	$ 1,868,987	1,609,833
Marketable Securities, Unrealized Gains	9,806	3,383
Marketable Securities, Unrealized Losses	(3,133)	(134)
Marketable Securities, Fair Value	1,875,660	1,613,082
Total Financial Assets, Amortized Cost	2,224,763	2,180,643
Total Financial Assets, Fair Value	2,231,436	2,183,892
Level I
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash Equivalents, Fair Value		562,580
Marketable Securities, Fair Value	583,602	519,278
Total Financial Assets, Fair Value	939,378	1,086,087
Level II
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash Equivalents, Fair Value		4,001
Marketable Securities, Fair Value	1,292,058	1,093,804
Total Financial Assets, Fair Value	1,292,058	1,097,805
Level III
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash Equivalents, Fair Value		0
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	351,469	562,580
Cash Equivalents, Fair Value	351,469	562,580
Money market funds | Level I
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash Equivalents, Fair Value	351,469	562,580
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	83,261	66,717
Marketable Securities, Unrealized Gains	0	0
Marketable Securities, Unrealized Losses	0	0
Marketable Securities, Fair Value	83,261	66,717
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	83,261	66,717
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]
Cash Equivalents, Amortized Cost		4,001
Cash Equivalents, Fair Value		4,001
Marketable Securities, Amortized Cost	3,000	3,000
Marketable Securities, Unrealized Gains		0
Marketable Securities, Unrealized Losses		0
Marketable Securities, Fair Value	3,000	3,000
Certificate of deposits | Level I
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable Securities, Fair Value		0
Certificate of deposits | Level II
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash Equivalents, Fair Value		4,001
Marketable Securities, Fair Value	3,000	3,000
Certificate of deposits | Level III
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable Securities, Fair Value		0
Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable Securities, Amortized Cost	923,194	787,741
Marketable Securities, Unrealized Gains	1,753	2,392
Marketable Securities, Unrealized Losses	(3,116)	(73)
Marketable Securities, Fair Value	921,831	790,060
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	921,831	790,060
Corporate bonds | 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	576,488	518,884
Marketable Securities, Unrealized Gains	7,114	414
Marketable Securities, Unrealized Losses	0	(20)
Marketable Securities, Fair Value	583,602	519,278
U.S. government notes | Level I
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable Securities, Fair Value	583,602	519,278
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
Agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable Securities, Amortized Cost	283,044	233,491
Marketable Securities, Unrealized Gains	939	577
Marketable Securities, Unrealized Losses	(17)	(41)
Marketable Securities, Fair Value	283,966	234,027
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	283,966	234,027
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	4,307	4,229
Other Assets, Fair Value	4,307	4,229
Money market funds - restricted | Level I
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other Assets, Fair Value	4,307	4,229
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