Fair Value Measurements - Summary of Fair Value Assets and Liabilities Measured on Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Assets
Assets fair value	$ 259,157	$ 275,401
Money market funds
Assets
Assets fair value	183,714	223,568
Fair Value Measurements on Recurring Basis | Level 1
Assets
Assets fair value	183,714	223,568
Fair Value Measurements on Recurring Basis | Level 1 | Money market funds
Assets
Assets fair value	183,714	223,568
Fair Value Measurements on Recurring Basis | Level 2
Assets
Assets fair value	75,443	51,833
U.S. government agency obligations
Assets
Assets fair value	23,875	27,878
U.S. government agency obligations | Fair Value Measurements on Recurring Basis | Level 2
Assets
Assets fair value	23,875	27,878
Corporate securities
Assets
Assets fair value	51,568	23,955
Corporate securities | Fair Value Measurements on Recurring Basis | Level 2
Assets
Assets fair value	$ 51,568	$ 23,955