Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Fair value of financial assets and liabilities
Senior secured term loan	$ 294,740	$ 0
Recurring basis | Total
Financial assets carried at fair value:
Total	1,102,303
Recurring basis | Total | Money market funds
Financial assets carried at fair value:
Cash equivalents:	343,129
Recurring basis | Total | Government-sponsored enterprise securities
Financial assets carried at fair value:
Marketable securities:	467,868
Recurring basis | Total | Commercial paper
Financial assets carried at fair value:
Marketable securities:	46,496
Recurring basis | Total | Corporate debt securities
Financial assets carried at fair value:
Marketable securities:	244,810
Recurring basis | Level 1
Financial assets carried at fair value:
Total	810,997
Recurring basis | Level 1 | Money market funds
Financial assets carried at fair value:
Cash equivalents:	343,129
Recurring basis | Level 1 | Government-sponsored enterprise securities
Financial assets carried at fair value:
Marketable securities:	467,868
Recurring basis | Level 1 | Commercial paper
Financial assets carried at fair value:
Marketable securities:	0
Recurring basis | Level 1 | Corporate debt securities
Financial assets carried at fair value:
Marketable securities:	0
Recurring basis | Level 2
Financial assets carried at fair value:
Total	291,306
Recurring basis | Level 2 | Money market funds
Financial assets carried at fair value:
Cash equivalents:	0
Recurring basis | Level 2 | Government-sponsored enterprise securities
Financial assets carried at fair value:
Marketable securities:	0
Recurring basis | Level 2 | Commercial paper
Financial assets carried at fair value:
Marketable securities:	46,496
Recurring basis | Level 2 | Corporate debt securities
Financial assets carried at fair value:
Marketable securities:	244,810
Recurring basis | Level 3
Financial assets carried at fair value:
Total	0
Recurring basis | Level 3 | Money market funds
Financial assets carried at fair value:
Cash equivalents:	0
Recurring basis | Level 3 | Government-sponsored enterprise securities
Financial assets carried at fair value:
Marketable securities:	0
Recurring basis | Level 3 | Commercial paper
Financial assets carried at fair value:
Marketable securities:	0
Recurring basis | Level 3 | Corporate debt securities
Financial assets carried at fair value:
Marketable securities:	$ 0