Stockholders' Equity - 2016 and 2017 Public Offerings of Common Stock (Details) $ / shares in Units, $ in Thousands	1 Months Ended			2 Months Ended	12 Months Ended
	Aug. 31, 2017 USD ($) $ / shares shares	Feb. 29, 2016 $ / shares shares	Jan. 31, 2016 USD ($) shareholder $ / shares shares	Feb. 29, 2016 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) shares	Dec. 31, 2016 USD ($) shares
Sale of stock
Proceeds from issuance of common stock, net of offering costs | $					$ 18,535	$ 37,528
Common Stock
Sale of stock
Shares issued (in shares)					5,333,334	6,338,583
2016 Public Offering | Stockholders affiliated with members of board of directors
Sale of stock
Number of stockholders affiliated with members of board of directors who purchased shares | shareholder			1
2016 Public Offering | Common Stock
Sale of stock
Shares issued (in shares)			5,511,812	6,338,583
Share price (in dollars per share) | $ / shares			$ 6.35
Proceeds from issuance of common stock, net of offering costs | $				$ 37,500
2016 Public Offering | Common Stock | Stockholders affiliated with members of board of directors
Sale of stock
Shares issued (in shares)			393,700
Proceeds from issuance of common stock, net of offering costs | $			$ 2,500
Underwriter purchase option | Common Stock
Sale of stock
Shares issued (in shares)		826,771
Share price (in dollars per share) | $ / shares		$ 6.35		$ 6.35
2017 Public Offering | Common Stock
Sale of stock
Shares issued (in shares)	5,333,334
Share price (in dollars per share) | $ / shares	$ 3.75
Proceeds from issuance of common stock, net of offering costs | $	$ 18,500