Fair Value Measurements Fair Value, Assets and Liabilities Measured on a Recurring Basis (details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Marketable securities:
Total	$ 80,987	$ 80,947
Quoted Prices in Active Markets for Identical Assets (Level 1)
Marketable securities:
Total	80,938	80,899
Significant Other Observable Inputs (Level 2)
Marketable securities:
Total	49	48
Marketable Securities
Marketable securities:
U.S. treasury securities	30,026	30,020
Certificates of deposit	49	48
Mutual funds	543	520
Total marketable securities	30,618	30,588
Marketable Securities | Quoted Prices in Active Markets for Identical Assets (Level 1)
Marketable securities:
U.S. treasury securities	30,026	30,020
Certificates of deposit	0	0
Mutual funds	543	520
Total marketable securities	30,569	30,540
Marketable Securities | Significant Other Observable Inputs (Level 2)
Marketable securities:
U.S. treasury securities	0	0
Certificates of deposit	49	48
Mutual funds	0	0
Total marketable securities	49	48
Cash Equivalents
Cash equivalents:
Money market funds	50,369	50,359
Total cash equivalents	50,369	50,359
Cash Equivalents | Quoted Prices in Active Markets for Identical Assets (Level 1)
Cash equivalents:
Money market funds	50,369	50,359
Total cash equivalents	50,369	50,359
Cash Equivalents | Significant Other Observable Inputs (Level 2)
Cash equivalents:
Money market funds	0	0
Total cash equivalents	$ 0	$ 0