Earnings Per Common Share - Reconciliation of Weighted-Average Common Shares Outstanding Used in Calculation of Basic EPS to Diluted EPS (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Numerator:
Net income	$ 14,510	$ 11,311	$ 42,384	$ 23,622	$ 25,874
Less: Net income allocated to participating securities	(84)	(83)	(282)	(277)
Numerator for basic and diluted earnings available to common stockholders	$ 14,426	$ 11,228	$ 42,102	$ 23,345
Denominator:
Weighted-average basic common shares outstanding (in shares)	81,490	75,471	77,811	71,693
Weighted-average diluted common shares outstanding (in shares)	81,490	75,471	77,811	71,693
Earnings per common share, basic (in usd per share)	$ 0.18	$ 0.15	$ 0.54	$ 0.33
Earnings per common share, diluted (in usd per share)	$ 0.18	$ 0.15	$ 0.54	$ 0.33