Stockholders' Equity - 2017 Public Offerings of Common Stock (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended		3 Months Ended		9 Months Ended
	Jan. 31, 2018	Aug. 31, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Sale of stock
Proceeds from the issuance of common stock, net						$ 18,536
Stock-based compensation expense			$ 721	$ 967	$ 2,827	$ 2,727
2017 Public Offering | Common Stock
Sale of stock
Shares issued (in shares)		5,333,334
Share price (in dollars per share)		$ 3.75
Proceeds from the issuance of common stock, net		$ 18,500
2014 Equity Incentive Plan | Performance-based vesting | Executive officers
Sale of stock
Granted (in shares)	365,000
Stock-based compensation expense			$ 0		$ 0