Fair Value Measurements (Details) (Recurring basis, USD $) In Thousands, unless otherwise specified	Dec. 31, 2013
Total
Financial assets carried at fair value:
Total	$ 1,151,466
Total | Money market funds
Financial assets carried at fair value:
Cash equivalents:	255,689
Total | Government-sponsored enterprise securities
Financial assets carried at fair value:
Marketable securities:	600,450
Total | Commercial paper
Financial assets carried at fair value:
Marketable securities:	83,493
Total | Corporate debt securities
Financial assets carried at fair value:
Marketable securities:	211,834
Level 1
Financial assets carried at fair value:
Total	856,139
Level 1 | Money market funds
Financial assets carried at fair value:
Cash equivalents:	255,689
Level 1 | Government-sponsored enterprise securities
Financial assets carried at fair value:
Marketable securities:	600,450
Level 1 | Commercial paper
Financial assets carried at fair value:
Marketable securities:	0
Level 1 | Corporate debt securities
Financial assets carried at fair value:
Marketable securities:	0
Level 2
Financial assets carried at fair value:
Total	295,327
Level 2 | Money market funds
Financial assets carried at fair value:
Cash equivalents:	0
Level 2 | Government-sponsored enterprise securities
Financial assets carried at fair value:
Marketable securities:	0
Level 2 | Commercial paper
Financial assets carried at fair value:
Marketable securities:	83,493
Level 2 | Corporate debt securities
Financial assets carried at fair value:
Marketable securities:	211,834
Level 3
Financial assets carried at fair value:
Total	0
Level 3 | Money market funds
Financial assets carried at fair value:
Cash equivalents:	0
Level 3 | Government-sponsored enterprise securities
Financial assets carried at fair value:
Marketable securities:	0
Level 3 | Commercial paper
Financial assets carried at fair value:
Marketable securities:	0
Level 3 | Corporate debt securities
Financial assets carried at fair value:
Marketable securities:	$ 0