Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Basic earnings (loss) per common share
Net income									$ 43,905	$ 55,324	$ 39,009
Less income (loss) allocated to participating securities									584	715	540
Net income (loss) attributable to common shareholders	$ 14,696	$ 9,921	$ 21,164	$ (1,876)	$ 34,528	$ 9,327	$ 14,746	$ (3,277)	$ 43,321	$ 54,609	$ 38,469
Weighted average common shares outstanding									22,681,888	22,576,381	22,480,679
Basic earnings (loss) per common share (in dollars per share)	$ 0.64	$ 0.43	$ 0.92	$ (0.08)	$ 1.51	$ 0.41	$ 0.64	$ (0.14)	$ 1.91	$ 2.42	$ 1.71
Earnings per common share assuming dilution
Net income									$ 43,905	$ 55,324	$ 39,009
Less income (loss) allocated to participating securities									584	715	540
Net income (loss) attributable to common shareholders	$ 14,696	$ 9,921	$ 21,164	$ (1,876)	$ 34,528	$ 9,327	$ 14,746	$ (3,277)	$ 43,321	$ 54,609	$ 38,469
Weighted average common shares outstanding									22,681,888	22,576,381	22,480,679
Incremental shares applicable to non-participating RSUs									22,968	11,267
Weighted average common shares assuming dilution									22,704,856	22,587,648	22,480,679
Earnings (loss) per common share assuming dilution (in dollars per share)	$ 0.63	$ 0.43	$ 0.91	$ (0.08)	$ 1.50	$ 0.40	$ 0.64	$ (0.14)	$ 1.89	$ 2.40	$ 1.70