Fair Value Measurements - Assets Measured at Fair Value on a Recurring Basis (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Aug. 31, 2019	Dec. 31, 2018
Fair Value Measurements, Financial Assets
Cash and cash equivalents	$ 189,052,000		$ 465,217,000
Total amortized cost	1,594,334,000		261,468,000
Marketable securities, accumulated gross unrealized gain, before tax	4,757,000		23,000
Marketable securities, gross unrealized losses less than 12 months	(58,000)		(130,000)
Marketable securities, gross unrealized losses more than 12 months	0		(233,000)
Marketable securities, aggregate fair value	1,599,033,000		261,128,000
Strategic investments	5,500,000
Total financial assets, amortized cost or carrying value	1,788,886,000		726,685,000
Total financial assets	1,793,585,000		726,345,000
Note receivable		$ 5,500,000
Impairment of debt security	0
U.S. Treasury securities
Fair Value Measurements, Financial Assets
Total amortized cost	215,847,000		59,785,000
Marketable securities, accumulated gross unrealized gain, before tax	241,000		0
Marketable securities, gross unrealized losses less than 12 months	(3,000)		(7,000)
Marketable securities, gross unrealized losses more than 12 months			(9,000)
Marketable securities, aggregate fair value	216,085,000		59,769,000
Corporate debt securities and commercial paper
Fair Value Measurements, Financial Assets
Total amortized cost	1,378,487,000		201,683,000
Marketable securities, accumulated gross unrealized gain, before tax	4,516,000		23,000
Marketable securities, gross unrealized losses less than 12 months	(55,000)		(123,000)
Marketable securities, gross unrealized losses more than 12 months	0		(224,000)
Marketable securities, aggregate fair value	1,382,948,000		201,359,000
Level 1
Fair Value Measurements, Financial Assets
Cash and cash equivalents	153,252,000		420,234,000
Marketable securities, aggregate fair value	221,085,000		59,769,000
Total financial assets	374,337,000		480,003,000
Level 1 | U.S. Treasury securities
Fair Value Measurements, Financial Assets
Marketable securities, aggregate fair value	216,085,000		59,769,000
Level 1 | Corporate debt securities and commercial paper
Fair Value Measurements, Financial Assets
Marketable securities, aggregate fair value	5,000,000
Level 2
Fair Value Measurements, Financial Assets
Cash and cash equivalents	35,800,000		44,983,000
Marketable securities, aggregate fair value	1,377,948,000		201,359,000
Total financial assets	1,413,748,000		246,342,000
Level 2 | U.S. Treasury securities
Fair Value Measurements, Financial Assets
Marketable securities, aggregate fair value	0		0
Level 2 | Corporate debt securities and commercial paper
Fair Value Measurements, Financial Assets
Marketable securities, aggregate fair value	1,377,948,000		201,359,000
Level 3
Fair Value Measurements, Financial Assets
Strategic investments	5,500,000
Total financial assets	5,500,000		0
Carrying Value | Money market funds
Fair Value Measurements, Financial Assets
Cash and cash equivalents	153,252,000		420,234,000
Carrying Value | Reverse repurchase agreements
Fair Value Measurements, Financial Assets
Cash and cash equivalents	35,800,000		35,000,000
Carrying Value | Commercial Paper
Fair Value Measurements, Financial Assets
Cash and cash equivalents			9,983,000
Aggregate Fair Value | Money market funds
Fair Value Measurements, Financial Assets
Cash and cash equivalents	153,252,000		420,234,000
Aggregate Fair Value | Reverse repurchase agreements
Fair Value Measurements, Financial Assets
Cash and cash equivalents	35,800,000		35,000,000.0
Aggregate Fair Value | Commercial Paper
Fair Value Measurements, Financial Assets
Cash and cash equivalents			9,983,000
Aggregate Fair Value | Level 1 | Money market funds
Fair Value Measurements, Financial Assets
Cash and cash equivalents	153,252,000		420,234,000
Aggregate Fair Value | Level 2 | Reverse repurchase agreements
Fair Value Measurements, Financial Assets
Cash and cash equivalents	$ 35,800,000		35,000,000
Aggregate Fair Value | Level 2 | Commercial Paper
Fair Value Measurements, Financial Assets
Cash and cash equivalents			$ 9,983,000