Marketable Securities and Equity Investments - Summary of Cash Equivalents and Marketable Securities (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Available-for-sale Debt Securities
Fair Value	$ 1,997,660	$ 1,648,745
Equity Securities, FV-NI and without Readily Determinable Fair Value [Abstract]
Amortized Cost	521,785	484,473
Gross Unrealized Gains	140,883	40,646
Gross Unrealized Losses	(6,130)	(7,011)
Fair Value	656,538	518,108
Money market funds
Summary of cash, cash equivalents and marketable securities
Amortized Cost	1,561,770	1,226,603
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	1,561,770	1,226,603
U.S. Treasury securities
Summary of cash, cash equivalents and marketable securities
Amortized Cost		5,967
Gross Unrealized Gains		0
Gross Unrealized Losses		(1)
Fair Value		5,966
Available-for-sale Debt Securities
Amortized Cost	5,891	6,026
Gross Unrealized Gains	1	0
Gross Unrealized Losses	0	0
Fair Value	5,892	6,026
Government-sponsored enterprise securities
Summary of cash, cash equivalents and marketable securities
Amortized Cost	24,518	7,124
Gross Unrealized Gains	2	0
Gross Unrealized Losses	(1)	(1)
Fair Value	24,519	7,123
Available-for-sale Debt Securities
Amortized Cost	8,240	10,704
Gross Unrealized Gains	7	0
Gross Unrealized Losses	0	0
Fair Value	8,247	10,704
Corporate debt securities
Available-for-sale Debt Securities
Amortized Cost	249,076	234,088
Gross Unrealized Gains	373	27
Gross Unrealized Losses	(23)	(450)
Fair Value	249,426	233,665
Commercial paper
Summary of cash, cash equivalents and marketable securities
Amortized Cost	22,233	58,271
Gross Unrealized Gains	2	0
Gross Unrealized Losses	(1)	(3)
Fair Value	22,234	58,268
Available-for-sale Debt Securities
Amortized Cost	125,421	100,498
Gross Unrealized Gains	174	0
Gross Unrealized Losses	(23)	(108)
Fair Value	125,572	100,390
Cash Equivalents
Summary of cash, cash equivalents and marketable securities
Amortized Cost	1,608,521	1,297,965
Gross Unrealized Gains	4	0
Gross Unrealized Losses	(2)	(5)
Fair Value	1,608,523	1,297,960
Total marketable debt securities
Available-for-sale Debt Securities
Amortized Cost	388,628	351,316
Gross Unrealized Gains	555	27
Gross Unrealized Losses	(46)	(558)
Fair Value	389,137	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	140,328	40,619
Gross Unrealized Losses	(6,084)	(6,453)
Fair Value	$ 267,401	$ 167,323