Marketable Securities and Equity Investments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Available-for-sale Debt Securities
Fair Value	$ 1,648,745	$ 1,033,419
Equity Securities, FV-NI and without Readily Determinable Fair Value [Abstract]
Amortized Cost	484,473
Gross Unrealized Gains	40,646
Gross Unrealized Losses	7,011
Fair Value	518,108
Amortized Cost		392,225
Gross Unrealized Gains		31,608
Gross Unrealized Losses		(579)
Fair Value		423,254
Money market funds
Summary of cash, cash equivalents and marketable securities
Amortized Cost	1,226,603	614,951
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	1,226,603	614,951
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
Government-sponsored enterprise securities
Summary of cash, cash equivalents and marketable securities
Amortized Cost	7,124	12,679
Gross Unrealized Gains	0	0
Gross Unrealized Losses	1	1
Fair Value	7,123	12,678
Commercial paper
Summary of cash, cash equivalents and marketable securities
Amortized Cost	58,271	57,371
Gross Unrealized Gains	0	0
Gross Unrealized Losses	3	14
Fair Value	58,268	57,357
Cash Equivalents
Summary of cash, cash equivalents and marketable securities
Amortized Cost	1,297,965	685,001
Gross Unrealized Gains	0	0
Gross Unrealized Losses	5	15
Fair Value	1,297,960	684,986
Total marketable debt securities
Available-for-sale Debt Securities
Amortized Cost	351,316	349,012
Gross Unrealized Gains	27	0
Gross Unrealized Losses	(558)	(579)
Fair Value	350,785	348,433
US Treasury securities (matures within 1 year)
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 (matures within 1 year)
Available-for-sale Debt Securities
Amortized Cost	10,704	2,304
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	(1)
Fair Value	10,704	2,303
Corporate debt securities (matures within 1 year)
Available-for-sale Debt Securities
Amortized Cost	232,845	215,639
Gross Unrealized Gains	25	0
Gross Unrealized Losses	(450)	(363)
Fair Value	232,420	215,276
Corporate debt securities (matures after 1 year through 5 years)
Available-for-sale Debt Securities
Amortized Cost	1,243	50,697
Gross Unrealized Gains	2	0
Gross Unrealized Losses	0	(106)
Fair Value	1,245	50,591
Commercial Paper (matures within 1 year)
Available-for-sale Debt Securities
Amortized Cost	100,498	80,372
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(108)	(109)
Fair Value	100,390	80,263
Corporate equity securities
Equity Securities, FV-NI and without Readily Determinable Fair Value [Abstract]
Amortized Cost	133,157
Gross Unrealized Gains	40,619
Gross Unrealized Losses	6,453
Fair Value	$ 167,323
Amortized Cost		43,213
Gross Unrealized Gains		31,608
Gross Unrealized Losses		0
Fair Value		$ 74,821