Assets (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Major categories of assets and liabilities measured on a recurring basis, at fair value
Short-term investments	$ 88,997	$ 120,572
Assets held for sale	6,000	6,000
Transfer of assets between levels	0
Transfer of liabilities 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 | Level 1 | U.S. treasuries
Major categories of assets and liabilities measured on a recurring basis, at fair value
Short-term investments	59,732	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	4,999
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	24,266	39,045
Assets and liabilities measured on a recurring basis | Total | U.S. treasuries
Major categories of assets and liabilities measured on a recurring basis, at fair value
Short-term investments	59,732	81,527
Assets and liabilities measured on a recurring basis | Total | Government agency securities
Major categories of assets and liabilities measured on a recurring basis, at fair value
Short-term investments	4,999
Assets and liabilities measured on a recurring basis | Total | Corporate debt
Major categories of assets and liabilities measured on a recurring basis, at fair value
Short-term investments	$ 24,266	$ 39,045