Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Assets measured at fair value on a recurring basis:
Transfer of assets from level 1 to level 2	$ 0	$ 0
Transfer of assets from level 2 to level 1	0	0
Transfer of liabilities from level 1 to level 2	0	0
Transfer of liabilities from level 2 to level 1	0	0
Recurring basis | Actively Quoted Markets for Identical Assets Level 1
Assets measured at fair value on a recurring basis:
Total assets at fair value	10,625		12,397
Recurring basis | Actively Quoted Markets for Identical Assets Level 1 | Money market deposits
Assets measured at fair value on a recurring basis:
Total assets at fair value	10,625		12,397
Recurring basis | Significant Other Observable Inputs Level 2
Assets measured at fair value on a recurring basis:
Total assets at fair value	105,042		76,184
Recurring basis | Significant Other Observable Inputs Level 2 | Commercial paper
Assets measured at fair value on a recurring basis:
Total assets at fair value	19,748		29,749
Recurring basis | Significant Other Observable Inputs Level 2 | Corporate debt securities
Assets measured at fair value on a recurring basis:
Total assets at fair value	48,292		46,435
Recurring basis | Significant Other Observable Inputs Level 2 | Corporate equity securities
Assets measured at fair value on a recurring basis:
Total assets at fair value	37,002
Recurring basis | Fair value
Assets measured at fair value on a recurring basis:
Total assets at fair value	115,667		88,581
Recurring basis | Fair value | Money market deposits
Assets measured at fair value on a recurring basis:
Total assets at fair value	10,625		12,397
Recurring basis | Fair value | Commercial paper
Assets measured at fair value on a recurring basis:
Total assets at fair value	19,748		29,749
Recurring basis | Fair value | Corporate debt securities
Assets measured at fair value on a recurring basis:
Total assets at fair value	48,292		46,435
Recurring basis | Fair value | Corporate equity securities
Assets measured at fair value on a recurring basis:
Total assets at fair value	$ 37,002