Fair Value Measurements (Details) (Recurring basis, USD $) In Thousands, unless otherwise specified	Jun. 30, 2014
Total
Financial assets carried at fair value:
Total	$ 922,072
Total | Money market funds
Financial assets carried at fair value:
Cash equivalents:	123,469
Total | Government-sponsored enterprise securities
Financial assets carried at fair value:
Marketable securities:	497,587
Total | Commercial paper
Financial assets carried at fair value:
Marketable securities:	69,243
Total | Corporate debt securities
Financial assets carried at fair value:
Marketable securities:	231,773
Level 1
Financial assets carried at fair value:
Total	621,056
Level 1 | Money market funds
Financial assets carried at fair value:
Cash equivalents:	123,469
Level 1 | Government-sponsored enterprise securities
Financial assets carried at fair value:
Marketable securities:	497,587
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	301,016
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:	69,243
Level 2 | Corporate debt securities
Financial assets carried at fair value:
Marketable securities:	231,773
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