Fair Value Measurements - Assets measured on a recurring basis - (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Assets, Fair Value Disclosure [Abstract]
Total assets measured at fair value	$ 202,424	$ 222,637
Level 1
Assets, Fair Value Disclosure [Abstract]
Total assets measured at fair value	175,482	187,582
Level 2
Assets, Fair Value Disclosure [Abstract]
Total assets measured at fair value	26,942	35,055
U.S. Treasury notes
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	110,019	157,981
U.S. Treasury notes | Level 1
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	110,019	157,981
U. S. Government agency bonds
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	26,942	35,055
U. S. Government agency bonds | Level 2
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	26,942	35,055
Money market funds
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	65,463	29,601
Money market funds | Level 1
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	$ 65,463	$ 29,601