Fair Value Measurements - Summary of Fair Value Assets and Liabilities Measured on Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Assets
Assets fair value	$ 393,906	$ 275,401
Money market funds
Assets
Assets fair value	245,194	223,568
U.S. government agency obligations
Assets
Assets fair value	85,033	27,878
Corporate securities
Assets
Assets fair value	63,679	23,955
Cash equivalents (due within 90 days)
Assets
Assets fair value	245,194	223,568
Short-term investments (due within one year)
Assets
Assets fair value	68,564	31,933
Long-term investments (due between one and two years)
Assets
Assets fair value	80,148	19,900
Fair Value Measurements on Recurring Basis | Level 1
Assets
Assets fair value	245,194	223,568
Fair Value Measurements on Recurring Basis | Level 1 | Money market funds
Assets
Assets fair value	245,194	223,568
Fair Value Measurements on Recurring Basis | Level 2
Assets
Assets fair value	148,712	51,833
Fair Value Measurements on Recurring Basis | Level 2 | U.S. government agency obligations
Assets
Assets fair value	85,033	27,878
Fair Value Measurements on Recurring Basis | Level 2 | Corporate securities
Assets
Assets fair value	$ 63,679	$ 23,955