Short-term Investments - Available-For-Sale Securities Reported At Fair Value (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Level 2 securities, Gross Unrealized Gains	$ 1,049	$ 154
Level 2 securities, Gross Unrealized Losses	(160)	(1,464)
Total, Amortized Cost	2,077,127	2,318,114
Total, Estimated Fair Value	2,078,016	2,316,804
Level 1 Securities [Member] | Money market funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash and money market funds, Amortized Cost	543,544	107,199
Cash and money market funds, Estimated Fair Value	543,544	107,199
Level 2 Securities [Member] | Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Level 2 securities, Amortized Cost	225,081	240,867
Level 2 securities, Gross Unrealized Gains	711	154
Level 2 securities, Gross Unrealized Losses	(70)	(409)
Level 2 securities, Estimated Fair Value	225,722	240,612
Level 2 Securities [Member] | Government securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Level 2 securities, Amortized Cost	231,048	235,252
Level 2 securities, Gross Unrealized Gains	333	0
Level 2 securities, Gross Unrealized Losses	(90)	(1,046)
Level 2 securities, Estimated Fair Value	231,291	234,206
Level 2 Securities [Member] | Agency Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Level 2 securities, Amortized Cost	10,209	26,576
Level 2 securities, Gross Unrealized Gains	5	0
Level 2 securities, Gross Unrealized Losses	0	(9)
Level 2 securities, Estimated Fair Value	10,214	26,567
Cash [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash and money market funds, Amortized Cost	1,067,245	1,708,220
Cash and money market funds, Estimated Fair Value	1,067,245	1,708,220
Fair Value, Measurements, Recurring [Member] | Level 1 Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash, Cash Equivalents And Restricted Cash, Noncurrent Assets	2,963	4,461
Fair Value, Measurements, Recurring [Member] | Level 1 Securities [Member] | Money market funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash and money market funds, Estimated Fair Value	540,581	102,738
Fair Value, Measurements, Recurring [Member] | Level 2 Securities [Member] | Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Level 2 securities, Estimated Fair Value	225,722	240,612
Fair Value, Measurements, Recurring [Member] | Level 2 Securities [Member] | Government securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Level 2 securities, Estimated Fair Value	231,291	234,206
Fair Value, Measurements, Recurring [Member] | Level 2 Securities [Member] | Agency Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Level 2 securities, Estimated Fair Value	10,214	26,567
Fair Value, Measurements, Recurring [Member] | Cash [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash and money market funds, Estimated Fair Value	1,064,663	1,706,592
Cash, Cash Equivalents And Restricted Cash, Noncurrent Assets	2,582	1,628
Cash and Cash Equivalents [Member] | Fair Value, Measurements, Recurring [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, Estimated Fair Value	1,605,244	1,809,330
Non-current Assets [Member] | Fair Value, Measurements, Recurring [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, Estimated Fair Value	5,545	6,089
Short-term Investments [Member] | Fair Value, Measurements, Recurring [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, Estimated Fair Value	$ 467,227	$ 501,385