Stockholders' Equity - Sales Agreement (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Equity [Abstract]
Stock issued (shares)	10,890,915
Proceeds from common stock sold under an at-market-issuance sales agreement (Note 7)	$ 73,869	$ 0	$ 0