Fair Value Measurements - Summary of Fair Value Assets and Liabilities Measured on Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Assets
Assets fair value	$ 427,939	$ 275,401
Money market funds
Assets
Assets fair value	348,525	223,568
Fair Value Measurements on Recurring Basis | Level 1
Assets
Assets fair value	348,525	223,568
Fair Value Measurements on Recurring Basis | Level 1 | Money market funds
Assets
Assets fair value	348,525	223,568
Fair Value Measurements on Recurring Basis | Level 2
Assets
Assets fair value	79,414	51,833
U.S. government agency obligations
Assets
Assets fair value	23,819	27,878
U.S. government agency obligations | Fair Value Measurements on Recurring Basis | Level 2
Assets
Assets fair value	23,819	27,878
Corporate securities
Assets
Assets fair value	55,595	23,955
Corporate securities | Fair Value Measurements on Recurring Basis | Level 2
Assets
Assets fair value	$ 55,595	$ 23,955