Marketable Securities and Equity Investments - Summary of cash equivalents and marketable securities (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Available-for-sale debt securities
Fair Value	$ 1,382,910	$ 1,033,419
Equity securities
Amortized Cost	445,009
Gross Unrealized Gains	178,067
Gross Unrealized Losses	(680)
Fair Value	622,396
Amortized Cost		392,225
Gross Unrealized Gains		31,608
Gross Unrealized Losses		(579)
Fair Value		423,254
US Treasury Securities
Available-for-sale debt securities
Amortized Cost	11,022
Gross Unrealized Gains	0
Gross Unrealized Losses	(2)
Fair Value	11,020
Money market funds
Available-for-sale debt securities
Amortized Cost	971,044	614,951
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	971,044	614,951
Government-sponsored enterprise securities
Available-for-sale debt securities
Amortized Cost	10,981	12,679
Gross Unrealized Gains	1	0
Gross Unrealized Losses	0	(1)
Fair Value	10,982	12,678
Commercial paper
Available-for-sale debt securities
Amortized Cost	21,270	57,371
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(7)	(14)
Fair Value	21,263	57,357
Cash Equivalents
Available-for-sale debt securities
Amortized Cost	1,003,295	685,001
Gross Unrealized Gains	1	0
Gross Unrealized Losses	(7)	(15)
Fair Value	1,003,289	684,986
Total marketable debt securities
Available-for-sale debt securities
Amortized Cost	380,296	349,012
Gross Unrealized Gains	5	0
Gross Unrealized Losses	(680)	(579)
Fair Value	379,621	348,433
Government-sponsored enterprise securities (matures within 1 year)
Available-for-sale debt securities
Amortized Cost	2,491	2,304
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	(1)
Fair Value	2,491	2,303
Corporate debt securities (matures within 1 year)
Available-for-sale debt securities
Amortized Cost	178,692	215,639
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(564)	(363)
Fair Value	178,128	215,276
Corporate debt securities (matures after 1 year through 5 years)
Available-for-sale debt securities
Amortized Cost	54,613	50,697
Gross Unrealized Gains	1	0
Gross Unrealized Losses	(38)	(106)
Fair Value	54,576	50,591
Commercial paper (matures within 1 year)
Available-for-sale debt securities
Amortized Cost	133,478	80,372
Gross Unrealized Gains	4	0
Gross Unrealized Losses	(76)	(109)
Fair Value	133,406	80,263
Corporate equity securities
Equity Securities, FV-NI and without Readily Determinable Fair Value [Abstract]
Amortized Cost	64,713
Gross Unrealized Gains	178,062
Gross Unrealized Losses	0
Fair Value	$ 242,775
Equity securities
Amortized Cost		43,213
Gross Unrealized Gains		31,608
Gross Unrealized Losses		0
Fair Value		$ 74,821