Fair Value Measurements - Summary of Fair Value Assets Measured on Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Assets
Assets fair value	$ 627,321	$ 393,906
Money market funds
Assets
Assets fair value	465,257	245,194
Fair Value Measurements on Recurring Basis | Level 1
Assets
Assets fair value	465,257	245,194
Fair Value Measurements on Recurring Basis | Level 2
Assets
Assets fair value	162,064	148,712
Fair Value Measurements on Recurring Basis | Money market funds | Level 1
Assets
Assets fair value	465,257	245,194
U.S. government agency obligations
Assets
Assets fair value	98,165	85,033
U.S. government agency obligations | Fair Value Measurements on Recurring Basis | Level 2
Assets
Assets fair value	98,165	85,033
Corporate securities
Assets
Assets fair value	63,899	63,679
Corporate securities | Fair Value Measurements on Recurring Basis | Level 2
Assets
Assets fair value	$ 63,899	$ 63,679