Short-term Investments - Available-For-Sale Securities Reported At Fair Value (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Other than temporary impairment losses, investments, available-for-sale securities		$ 0	$ 0
Available-for-sale securities, gross realized gain (loss), excluding other than temporary impairments		0	$ 0
Level 1 Securities | Money market funds
Schedule of Available-for-sale Securities [Line Items]
Cash and Money Market Funds, Estimated Fair Value		8,730		$ 204,967
Level 2 Securities | Certificates of deposit/commercial
Schedule of Available-for-sale Securities [Line Items]
Level 2 securities, Estimated Fair Value				9,833
Cash
Schedule of Available-for-sale Securities [Line Items]
Cash and Money Market Funds, Estimated Fair Value		1,073,756		1,267,523
Fair Value, Measurements, Recurring
Schedule of Available-for-sale Securities [Line Items]
Level 2 securities, Gross Unrealized Gains		177		110
Level 2 securities, Gross Unrealized Losses		(950)		(1,087)
Total, amortized cost		1,346,664		1,739,673
Total, estimated fair value		1,345,891		1,738,696
Fair Value, Measurements, Recurring | Level 1 Securities | Money market funds
Schedule of Available-for-sale Securities [Line Items]
Cash and Money Market Funds, Amortized Cost		8,730		204,967
Cash and Money Market Funds, Estimated Fair Value		7,463		203,450
Cash, Cash Equivalents and Restricted Cash, Noncurrent Assets		1,267		1,517
Fair Value, Measurements, Recurring | Level 2 Securities | Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Level 2 securities, Amortized Cost		204,507		199,843
Level 2 securities, Gross Unrealized Gains		177		110
Level 2 securities, Gross Unrealized Losses		(592)		(731)
Level 2 securities, Estimated Fair Value		204,092		199,222
Fair Value, Measurements, Recurring | Level 2 Securities | Government securities
Schedule of Available-for-sale Securities [Line Items]
Level 2 securities, Amortized Cost		35,925		35,944
Level 2 securities, Gross Unrealized Losses		(142)		(128)
Level 2 securities, Estimated Fair Value		35,783		35,816
Fair Value, Measurements, Recurring | Level 2 Securities | Certificates of deposit/commercial
Schedule of Available-for-sale Securities [Line Items]
Level 2 securities, Amortized Cost		2,180		9,833
Level 2 securities, Estimated Fair Value		2,180		9,833
Fair Value, Measurements, Recurring | Level 2 Securities | Agency securities
Schedule of Available-for-sale Securities [Line Items]
Level 2 securities, Amortized Cost		21,566		21,563
Level 2 securities, Gross Unrealized Losses		(216)		(228)
Level 2 securities, Estimated Fair Value		21,350		21,335
Fair Value, Measurements, Recurring | Cash
Schedule of Available-for-sale Securities [Line Items]
Cash and Money Market Funds, Amortized Cost		1,073,756		1,267,523
Cash and Money Market Funds, Estimated Fair Value		1,070,361		1,264,126
Cash, Cash Equivalents and Restricted Cash, Noncurrent Assets		3,395		3,397
Cash and Cash Equivalents | Fair Value, Measurements, Recurring
Schedule of Available-for-sale Securities [Line Items]
Total, estimated fair value		1,077,824		1,467,576
Short-term Investments | Fair Value, Measurements, Recurring
Schedule of Available-for-sale Securities [Line Items]
Total, estimated fair value		263,405		266,206
Non-current Assets | Fair Value, Measurements, Recurring
Schedule of Available-for-sale Securities [Line Items]
Total, estimated fair value	[1]	$ 4,662		$ 4,914

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