Fair Value Measurements (Details) (USD $)	Mar. 31, 2014
Recurring basis | Total
Financial assets carried at fair value:
Total	$ 1,041,738,000
Recurring basis | Total | Money market funds
Financial assets carried at fair value:
Cash equivalents:	141,583,000
Recurring basis | Total | Government-sponsored enterprise securities
Financial assets carried at fair value:
Marketable securities:	607,923,000
Recurring basis | Total | Commercial paper
Financial assets carried at fair value:
Marketable securities:	81,010,000
Recurring basis | Total | Corporate debt securities
Financial assets carried at fair value:
Marketable securities:	211,222,000
Recurring basis | Level 1
Financial assets carried at fair value:
Total	749,506,000
Recurring basis | Level 1 | Money market funds
Financial assets carried at fair value:
Cash equivalents:	141,583,000
Recurring basis | Level 1 | Government-sponsored enterprise securities
Financial assets carried at fair value:
Marketable securities:	607,923,000
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	292,232,000
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:	81,010,000
Recurring basis | Level 2 | Corporate debt securities
Financial assets carried at fair value:
Marketable securities:	211,222,000
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
Foreign Currency Forward Contract
Financial assets carried at fair value:
Notional amount of foreign currency forward contract	$ 24,600,000