Fair Value Measurements - Summary of Fair Value Assets and Liabilities Measured on Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Assets
Assets fair value	$ 283,462	$ 152,555
Money market funds
Assets
Assets fair value	227,466	136,481
Fair Value Measurements on Recurring Basis | Level 1
Assets
Assets fair value	227,466	136,481
Fair Value Measurements on Recurring Basis | Level 1 | Money market funds
Assets
Assets fair value	227,466	136,481
Fair Value Measurements on Recurring Basis | Level 2
Assets
Assets fair value	55,996	16,074
U.S. government agency obligations
Assets
Assets fair value	31,963	12,075
U.S. government agency obligations | Fair Value Measurements on Recurring Basis | Level 2
Assets
Assets fair value	31,963	12,075
Corporate securities
Assets
Assets fair value	24,033	3,999
Corporate securities | Fair Value Measurements on Recurring Basis | Level 2
Assets
Assets fair value	$ 24,033	$ 3,999