Short-term Investments - Available-For-Sale Securities Reported At Fair Value (Details) - USD ($) $ in Thousands		Sep. 30, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Level 2 securities, Gross Unrealized Gains		$ 933	$ 154
Level 2 securities, Gross Unrealized Losses		(108)	(1,464)
Total, amortized cost		1,346,523	2,318,114
Total, estimated fair value		1,347,348	2,316,804
Level 1 Securities | Money market funds
Schedule of Available-for-sale Securities [Line Items]
Cash and money market funds, Amortized Cost		52,737	107,199
Cash and money market funds, Estimated Fair Value		52,737	107,199
Level 2 Securities | Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Level 2 securities, Amortized Cost		220,826	240,867
Level 2 securities, Gross Unrealized Gains		770	154
Level 2 securities, Gross Unrealized Losses		(68)	(409)
Level 2 securities, Estimated Fair Value		221,528	240,612
Level 2 Securities | Government securities
Schedule of Available-for-sale Securities [Line Items]
Level 2 securities, Amortized Cost		110,860	235,252
Level 2 securities, Gross Unrealized Gains		159	0
Level 2 securities, Gross Unrealized Losses		0	(1,046)
Level 2 securities, Estimated Fair Value		111,019	234,206
Level 2 Securities | Certificates of deposit/commercial
Schedule of Available-for-sale Securities [Line Items]
Level 2 securities, Amortized Cost		9,817
Level 2 securities, Estimated Fair Value		9,817
Level 2 Securities | Agency securities
Schedule of Available-for-sale Securities [Line Items]
Level 2 securities, Amortized Cost		31,770	26,576
Level 2 securities, Gross Unrealized Gains		4	0
Level 2 securities, Gross Unrealized Losses		(40)	(9)
Level 2 securities, Estimated Fair Value		31,734	26,567
Cash
Schedule of Available-for-sale Securities [Line Items]
Cash and money market funds, Amortized Cost		920,513	1,708,220
Cash and money market funds, Estimated Fair Value		920,513	1,708,220
Fair Value, Measurements, Recurring | Level 1 Securities
Schedule of Available-for-sale Securities [Line Items]
Cash, Cash Equivalents and Restricted Cash, Noncurrent Assets	[1]	1,515	4,461
Fair Value, Measurements, Recurring | Level 1 Securities | Money market funds
Schedule of Available-for-sale Securities [Line Items]
Cash and money market funds, Estimated Fair Value		51,222	102,738
Fair Value, Measurements, Recurring | Level 2 Securities | Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Level 2 securities, Estimated Fair Value		221,528	240,612
Fair Value, Measurements, Recurring | Level 2 Securities | Government securities
Schedule of Available-for-sale Securities [Line Items]
Level 2 securities, Estimated Fair Value		111,019	234,206
Fair Value, Measurements, Recurring | Level 2 Securities | Agency securities
Schedule of Available-for-sale Securities [Line Items]
Level 2 securities, Estimated Fair Value		31,734	26,567
Fair Value, Measurements, Recurring | Cash
Schedule of Available-for-sale Securities [Line Items]
Cash and money market funds, Estimated Fair Value		917,936	1,706,592
Cash, Cash Equivalents and Restricted Cash, Noncurrent Assets	[1]	2,577	1,628
Cash and Cash Equivalents | Fair Value, Measurements, Recurring
Schedule of Available-for-sale Securities [Line Items]
Total, estimated fair value		969,158	1,809,330
Short-term Investments | Fair Value, Measurements, Recurring
Schedule of Available-for-sale Securities [Line Items]
Total, estimated fair value		374,098	501,385
Non-current Assets | Fair Value, Measurements, Recurring
Schedule of Available-for-sale Securities [Line Items]
Total, estimated fair value	[1]	$ 4,092	$ 6,089

[1] Primarily restricted cash that is related to workers compensation deposits and letter of credit agreements.