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		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Numerator:
Net income	$ 13,267	$ 2,030	$ 27,874	$ 12,311	$ 25,874
Less: Net income allocated to participating securities	(96)	(91)	(198)	(193)
Numerator for basic and diluted earnings available to common stockholders	$ 13,171	$ 1,939	$ 27,676	$ 12,118
Denominator:
Weighted-average basic common shares outstanding (in shares)	76,374	72,564	75,941	69,773
Weighted-average diluted common shares outstanding (in shares)	76,374	72,564	75,941	69,773
Earnings per common share, basic (in usd per share)	$ 0.17	$ 0.03	$ 0.36	$ 0.17
Earnings per common share, diluted (in usd per share)	$ 0.17	$ 0.03	$ 0.36	$ 0.17