Fair Value Measurements and Short-Term Investments - Fair Value of Financial Assets and Liabilities Measured on Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Redeemable Convertible Preferred Stock Warrant Liability
Liabilities:
Total		$ 1,708	$ 1,193
Fair Value Measurements Recurring
Assets:
Total	$ 195,396	18,960
Liabilities:
Total		1,708
Fair Value Measurements Recurring | Redeemable Convertible Preferred Stock Warrant Liability
Liabilities:
Total		1,708
Fair Value Measurements Recurring | Level 1
Assets:
Total	116,202	6,578
Liabilities:
Total		0
Fair Value Measurements Recurring | Level 1 | Redeemable Convertible Preferred Stock Warrant Liability
Liabilities:
Total		0
Fair Value Measurements Recurring | Level 2
Assets:
Total	79,194	12,382
Liabilities:
Total		0
Fair Value Measurements Recurring | Level 2 | Redeemable Convertible Preferred Stock Warrant Liability
Liabilities:
Total		0
Fair Value Measurements Recurring | Level 3
Assets:
Total	0	0
Liabilities:
Total		1,708
Fair Value Measurements Recurring | Level 3 | Redeemable Convertible Preferred Stock Warrant Liability
Liabilities:
Total		1,708
Money Market Funds | Fair Value Measurements Recurring
Assets:
Total	116,202	6,578
Money Market Funds | Fair Value Measurements Recurring | Level 1
Assets:
Total	116,202	6,578
Money Market Funds | Fair Value Measurements Recurring | Level 2
Assets:
Total	0	0
Money Market Funds | Fair Value Measurements Recurring | Level 3
Assets:
Total	0	0
Commercial Paper | Fair Value Measurements Recurring
Assets:
Total	26,625	7,689
Commercial Paper | Fair Value Measurements Recurring | Level 1
Assets:
Total	0	0
Commercial Paper | Fair Value Measurements Recurring | Level 2
Assets:
Total	26,625	7,689
Commercial Paper | Fair Value Measurements Recurring | Level 3
Assets:
Total	0	0
Corporate Debt Securities | Fair Value Measurements Recurring
Assets:
Total	11,774	800
Corporate Debt Securities | Fair Value Measurements Recurring | Level 1
Assets:
Total	0	0
Corporate Debt Securities | Fair Value Measurements Recurring | Level 2
Assets:
Total	11,774	800
Corporate Debt Securities | Fair Value Measurements Recurring | Level 3
Assets:
Total	0	0
Asset-backed Securities | Fair Value Measurements Recurring
Assets:
Total	16,899
Asset-backed Securities | Fair Value Measurements Recurring | Level 1
Assets:
Total	0
Asset-backed Securities | Fair Value Measurements Recurring | Level 2
Assets:
Total	16,899
Asset-backed Securities | Fair Value Measurements Recurring | Level 3
Assets:
Total	0
U.S. Government Agency Securities | Fair Value Measurements Recurring
Assets:
Total	23,896	3,893
U.S. Government Agency Securities | Fair Value Measurements Recurring | Level 1
Assets:
Total	0	0
U.S. Government Agency Securities | Fair Value Measurements Recurring | Level 2
Assets:
Total	23,896	3,893
U.S. Government Agency Securities | Fair Value Measurements Recurring | Level 3
Assets:
Total	$ 0	$ 0