Cash and Investments - Cash, Cash Equivalents and Available-for-Sale Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Cash, Cash Equivalents and Short-term Investments [Line Items]
Cash and cash equivalents, amortized cost	$ 546,237	$ 460,887	$ 803,089	$ 270,408
Total cash, cash equivalents and marketable securities, amortized cost	3,123,576	3,444,170
Total cash, cash equivalent and marketable securities, gross unrealized gains	188	726
Total cash, cash equivalent and marketable securities, gross unrealized losses	(4,382)	(1,587)
Total cash, cash equivalent and marketable securities, estimated fair market value	3,119,382	3,443,309
Commercial paper [Member]
Cash, Cash Equivalents and Short-term Investments [Line Items]
Marketable securities, amortized cost	55,350	122,345
Marketable securities, gross unrealized gains	47	33
Marketable securities, gross unrealized losses	(23)	(7)
Marketable securities, estimated fair market value	55,374	122,371
Certificates of deposit [Member]
Cash, Cash Equivalents and Short-term Investments [Line Items]
Marketable securities, amortized cost		5,925
Marketable securities, gross unrealized gains		2
Marketable securities, estimated fair market value		5,927
U.S. treasury securities [Member]
Cash, Cash Equivalents and Short-term Investments [Line Items]
Marketable securities, amortized cost	874,968	1,234,870
Marketable securities, gross unrealized gains	20	64
Marketable securities, gross unrealized losses	(959)	(366)
Marketable securities, estimated fair market value	874,029	1,234,568
U.S. agency securities [Member]
Cash, Cash Equivalents and Short-term Investments [Line Items]
Marketable securities, amortized cost	606,924	881,843
Marketable securities, gross unrealized gains	17	393
Marketable securities, gross unrealized losses	(739)	(274)
Marketable securities, estimated fair market value	606,202	881,962
Corporate debt securities [Member]
Cash, Cash Equivalents and Short-term Investments [Line Items]
Marketable securities, amortized cost	1,029,463	723,412
Marketable securities, gross unrealized gains	96	225
Marketable securities, gross unrealized losses	(2,661)	(932)
Marketable securities, estimated fair market value	1,026,898	722,705
Municipal securities [Member]
Cash, Cash Equivalents and Short-term Investments [Line Items]
Marketable securities, amortized cost	10,636	14,893
Marketable securities, gross unrealized gains	6	4
Marketable securities, gross unrealized losses		(8)
Marketable securities, estimated fair market value	10,642	14,889
Cash [Member]
Cash, Cash Equivalents and Short-term Investments [Line Items]
Cash and cash equivalents, amortized cost	180,790	213,724
Cash and cash equivalents, estimated fair market value	180,790	213,724
Money market funds [Member]
Cash, Cash Equivalents and Short-term Investments [Line Items]
Cash and cash equivalents, amortized cost	276,898	95,470
Cash and cash equivalents, estimated fair market value	276,898	95,470
Commercial paper [Member]
Cash, Cash Equivalents and Short-term Investments [Line Items]
Cash and cash equivalents, amortized cost	12,048	54,340
Cash and cash equivalents, gross unrealized gain	1	4
Cash and cash equivalents, estimated fair market value	12,049	54,344
U.S. treasury securities [Member]
Cash, Cash Equivalents and Short-term Investments [Line Items]
Cash and cash equivalents, amortized cost	24,999	54,349
Cash and cash equivalents, gross unrealized gain	1
Cash and cash equivalents, estimated fair market value	25,000	54,349
U.S. agency securities [Member]
Cash, Cash Equivalents and Short-term Investments [Line Items]
Cash and cash equivalents, amortized cost	51,500	42,999
Cash and cash equivalents, gross unrealized gain		1
Cash and cash equivalents, estimated fair market value	$ 51,500	$ 43,000