Fair Value Measurements (Details) - Recurring basis - Quoted prices in active markets for identical assets (Level 1) - USD ($) $ in Thousands	Dec. 31, 2014	Dec. 31, 2013
Money Market Funds
Fair Value Measurements
Investments in U.S. Treasury money market funds and U.S. Treasury securities with maturities less than 90 days		$ 203,828
U.S. Treasury money market funds
Fair Value Measurements
Investments in U.S. Treasury money market funds and U.S. Treasury securities with maturities less than 90 days	$ 35,111
U.S. Treasury securities with maturities less than one year
Fair Value Measurements
Investments in U.S. Treasury securities	$ 423,746