Marketable Securities and Equity Investments - Summary of Cash Equivalents and Marketable Securities (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Available-for-sale Debt Securities
Fair Value	$ 1,786,188	$ 1,648,745
Equity Securities, FV-NI and without Readily Determinable Fair Value [Abstract]
Amortized Cost	506,368	484,473
Gross Unrealized Gains	79,217	40,646
Gross Unrealized Losses	(1,435)	(7,011)
Fair Value	584,150	518,108
Money market funds
Summary of cash, cash equivalents and marketable securities
Amortized Cost	1,354,656	1,226,603
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	1,354,656	1,226,603
U.S. Treasury securities
Summary of cash, cash equivalents and marketable securities
Amortized Cost	5,996	5,967
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	(1)
Fair Value	5,996	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	11,787	7,124
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(1)	(1)
Fair Value	11,786	7,123
Available-for-sale Debt Securities
Amortized Cost	15,180	10,704
Gross Unrealized Gains	2	0
Gross Unrealized Losses	(1)	0
Fair Value	15,181	10,704
Corporate debt securities
Summary of cash, cash equivalents and marketable securities
Amortized Cost	4,519
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Fair Value	4,519
Available-for-sale Debt Securities
Amortized Cost	231,516	234,088
Gross Unrealized Gains	88	27
Gross Unrealized Losses	(32)	(450)
Fair Value	231,572	233,665
Commercial paper
Summary of cash, cash equivalents and marketable securities
Amortized Cost	35,959	58,271
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(4)	(3)
Fair Value	35,955	58,268
Available-for-sale Debt Securities
Amortized Cost	126,515	100,498
Gross Unrealized Gains	34	0
Gross Unrealized Losses	(26)	(108)
Fair Value	126,523	100,390
Cash Equivalents
Summary of cash, cash equivalents and marketable securities
Amortized Cost	1,412,917	1,297,965
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(5)	(5)
Fair Value	1,412,912	1,297,960
Total marketable debt securities
Available-for-sale Debt Securities
Amortized Cost	373,211	351,316
Gross Unrealized Gains	124	27
Gross Unrealized Losses	(59)	(558)
Fair Value	373,276	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	79,093	40,619
Gross Unrealized Losses	(1,376)	(6,453)
Fair Value	$ 210,874	$ 167,323