Summary of Significant Accounting Policies - Liquidity (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Accumulated deficit	$ 157,177	$ 139,724
Net cash used in operations	14,519	17,530
Cash and cash equivalents	25,120	22,408	$ 27,900
Short-term investments	97,103	$ 0
Debt covenant minimum cash balance	$ 5,000
Minimum
Debt Covenant [Line Items]
Quarterly increase in revenue target (percent)	2.00%
Maximum
Debt Covenant [Line Items]
Quarterly increase in revenue target (percent)	4.00%