Marketable Securities and Equity Investments - Summary of Cash Equivalents and Marketable Securities (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Available-for-sale Debt Securities
Fair Value	$ 1,907,842	$ 1,648,745
Equity Securities, FV-NI and without Readily Determinable Fair Value [Abstract]
Amortized Cost	494,783	484,473
Gross Unrealized Gains	108,057	40,646
Gross Unrealized Losses	(4,450)	(7,011)
Fair Value	598,390	518,108
Money market funds
Summary of cash, cash equivalents and marketable securities
Amortized Cost	1,476,254	1,226,603
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	1,476,254	1,226,603
U.S. Treasury securities
Summary of cash, cash equivalents and marketable securities
Amortized Cost	5,892	5,967
Gross Unrealized Gains	1	0
Gross Unrealized Losses	0	(1)
Fair Value	5,893	5,966
Available-for-sale Debt Securities
Amortized Cost		6,026
Gross Unrealized Gains		0
Gross Unrealized Losses		0
Fair Value		6,026
Government-sponsored enterprise securities
Summary of cash, cash equivalents and marketable securities
Amortized Cost	13,370	7,124
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	(1)
Fair Value	13,370	7,123
Available-for-sale Debt Securities
Amortized Cost	14,188	10,704
Gross Unrealized Gains	29	0
Gross Unrealized Losses	0	0
Fair Value	14,217	10,704
Commercial paper
Summary of cash, cash equivalents and marketable securities
Amortized Cost	50,125	58,271
Gross Unrealized Gains	1	0
Gross Unrealized Losses	(7)	(3)
Fair Value	50,119	58,268
Available-for-sale Debt Securities
Amortized Cost	82,825	100,498
Gross Unrealized Gains	113	0
Gross Unrealized Losses	0	(108)
Fair Value	82,938	100,390
Cash equivalents
Summary of cash, cash equivalents and marketable securities
Amortized Cost	1,545,641	1,297,965
Gross Unrealized Gains	2	0
Gross Unrealized Losses	(7)	(5)
Fair Value	1,545,636	1,297,960
Corporate debt securities
Available-for-sale Debt Securities
Amortized Cost	264,613	234,088
Gross Unrealized Gains	465	27
Gross Unrealized Losses	(27)	(450)
Fair Value	265,051	233,665
Marketable securities
Available-for-sale Debt Securities
Amortized Cost	361,626	351,316
Gross Unrealized Gains	607	27
Gross Unrealized Losses	(27)	(558)
Fair Value	362,206	350,785
Corporate equity securities
Equity Securities, FV-NI and without Readily Determinable Fair Value [Abstract]
Amortized Cost	133,157	133,157
Gross Unrealized Gains	107,450	40,619
Gross Unrealized Losses	(4,423)	(6,453)
Fair Value	$ 236,184	$ 167,323