Fair Value Measurements - Fair Value of Financial Assets and Liabilities Measured on Recurring Basis (Details) - Fair Value Measurements Recurring - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Assets:
Total	$ 166,045	$ 195,396
Level 1
Assets:
Total	17,209	116,202
Level 2
Assets:
Total	148,836	79,194
Level 3
Assets:
Total	0	0
Money Market Funds
Assets:
Total	17,209	116,202
Money Market Funds | Level 1
Assets:
Total	17,209	116,202
Money Market Funds | Level 2
Assets:
Total	0	0
Money Market Funds | Level 3
Assets:
Total	0	0
Commercial Paper
Assets:
Total	38,810	26,625
Commercial Paper | Level 1
Assets:
Total	0	0
Commercial Paper | Level 2
Assets:
Total	38,810	26,625
Commercial Paper | Level 3
Assets:
Total	0	0
Corporate Debt Securities
Assets:
Total	50,609	11,774
Corporate Debt Securities | Level 1
Assets:
Total	0	0
Corporate Debt Securities | Level 2
Assets:
Total	50,609	11,774
Corporate Debt Securities | Level 3
Assets:
Total	0	0
Asset-backed Securities
Assets:
Total	32,515	16,899
Asset-backed Securities | Level 1
Assets:
Total	0	0
Asset-backed Securities | Level 2
Assets:
Total	32,515	16,899
Asset-backed Securities | Level 3
Assets:
Total	0	0
U.S. Government Agency Securities
Assets:
Total	26,902	23,896
U.S. Government Agency Securities | Level 1
Assets:
Total	0	0
U.S. Government Agency Securities | Level 2
Assets:
Total	26,902	23,896
U.S. Government Agency Securities | Level 3
Assets:
Total	$ 0	$ 0