Earnings Per Common Share - Calculation of basic and diluted EPS (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Numerator:
Net income	$ 2,252	$ 11,311	$ 2,030	$ 10,281	$ 8,388	$ 7,832	$ 7,631	$ 5,502	$ 25,874	$ 29,353	$ 10,034
Less: Net income allocated to participating securities									(354)	(260)	(286)
Numerator for basic and diluted earnings available to common stockholders									$ 25,520	$ 29,093	$ 9,748
Denominator:
Weighted-average basic common shares outstanding (shares)	75,476	75,471	72,564	66,951	60,875	57,595	57,478	48,101	72,647	56,030	37,380
Weighted-average diluted common shares outstanding (shares)	75,476	75,471	72,564	66,951	60,875	57,595	57,478	48,101	72,647	56,030	37,380
Earnings per common share, basic (usd per share)	$ 0.03	$ 0.15	$ 0.03	$ 0.15	$ 0.14	$ 0.13	$ 0.13	$ 0.11	$ 0.35	$ 0.52	$ 0.26
Earnings per common share, diluted (usd per share)	$ 0.03	$ 0.15	$ 0.03	$ 0.15	$ 0.14	$ 0.13	$ 0.13	$ 0.11	$ 0.35	$ 0.52	$ 0.26