Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Major categories of assets and liabilities measured on a recurring basis, at fair value
Short-term investments	$ 68,877	$ 120,572
Transfer of assets between levels	0
Level 3
Major categories of assets and liabilities measured on a recurring basis, at fair value
Assets measured at fair value	0	0
Liabilities measured at fair value	0	0
Assets and liabilities measured on a recurring basis | U.S. treasuries
Major categories of assets and liabilities measured on a recurring basis, at fair value
Short-term investments	54,731	81,527
Assets and liabilities measured on a recurring basis | Government agency securities
Major categories of assets and liabilities measured on a recurring basis, at fair value
Short-term investments	2,999
Assets and liabilities measured on a recurring basis | Corporate debt
Major categories of assets and liabilities measured on a recurring basis, at fair value
Short-term investments	11,147	39,045
Assets and liabilities measured on a recurring basis | Level 1 | U.S. treasuries
Major categories of assets and liabilities measured on a recurring basis, at fair value
Short-term investments	54,731	81,527
Assets and liabilities measured on a recurring basis | Level 2 | Government agency securities
Major categories of assets and liabilities measured on a recurring basis, at fair value
Short-term investments	2,999
Assets and liabilities measured on a recurring basis | Level 2 | Corporate debt
Major categories of assets and liabilities measured on a recurring basis, at fair value
Short-term investments	$ 11,147	$ 39,045