Cash and Investments - Cash, Cash Equivalents and Available-for-Sale Investments (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015	Jun. 30, 2015	Dec. 31, 2014
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents, amortized cost	$ 719,807	$ 546,237	$ 450,991	$ 460,887
Total cash, cash equivalents, and short-term investments, amortized cost	3,304,852	3,123,576
Total cash, cash equivalents, and short-term investments, gross unrealized gains	6,804	188
Total cash, cash equivalents, and short-term investments, gross unrealized losses	(140)	(4,382)
Total cash, cash equivalents, and short-term investments, estimated fair market value	3,311,516	3,119,382
Certificates of deposit [Member]
Cash and Cash Equivalents [Line Items]
Short-term investments, amortized cost	10,000
Short-term investments, gross unrealized gains	5
Short-term investments, estimated fair market value	10,005
Commercial paper [Member]
Cash and Cash Equivalents [Line Items]
Short-term investments, amortized cost	26,633	55,350
Short-term investments, gross unrealized gains	29	47
Short-term investments, gross unrealized losses	0	(23)
Short-term investments, estimated fair market value	26,662	55,374
U.S. treasury securities [Member]
Cash and Cash Equivalents [Line Items]
Short-term investments, amortized cost	942,885	874,968
Short-term investments, gross unrealized gains	1,048	20
Short-term investments, gross unrealized losses	(11)	(959)
Short-term investments, estimated fair market value	943,922	874,029
U.S. agency securities [Member]
Cash and Cash Equivalents [Line Items]
Short-term investments, amortized cost	446,114	606,924
Short-term investments, gross unrealized gains	400	17
Short-term investments, gross unrealized losses	(4)	(739)
Short-term investments, estimated fair market value	446,510	606,202
Corporate debt securities [Member]
Cash and Cash Equivalents [Line Items]
Short-term investments, amortized cost	1,151,409	1,029,463
Short-term investments, gross unrealized gains	5,309	96
Short-term investments, gross unrealized losses	(120)	(2,661)
Short-term investments, estimated fair market value	1,156,598	1,026,898
Municipal securities [Member]
Cash and Cash Equivalents [Line Items]
Short-term investments, amortized cost	8,000	10,636
Short-term investments, gross unrealized gains	12	6
Short-term investments, gross unrealized losses		0
Short-term investments, estimated fair market value	8,012	10,642
Cash [Member]
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents, amortized cost	132,287	180,790
Cash and cash equivalents, estimated fair market value	132,287	180,790
Money market funds [Member]
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents, amortized cost	419,858	276,898
Cash and cash equivalents, estimated fair market value	419,858	276,898
Commercial paper [Member]
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents, amortized cost		12,048
Cash and cash equivalents, gross unrealized gain		1
Cash and cash equivalents, estimated fair market value		12,049
U.S. treasury securities [Member]
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents, amortized cost	130,312	24,999
Cash and cash equivalents, gross unrealized gain	1	1
Cash and cash equivalents, estimated fair market value	130,313	25,000
U.S. agency securities [Member]
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents, amortized cost	37,354	51,500
Cash and cash equivalents, gross unrealized gain		0
Cash and cash equivalents, gross unrealized loss	(5)
Cash and cash equivalents, estimated fair market value	$ 37,349	$ 51,500