Earnings per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Basic earnings per common share
Net income									$ 49,166	$ 43,905	$ 55,324
Less income allocated to participating securities									639	584	715
Net income allocated to common shareholders	$ 11,560	$ 12,429	$ 25,474	$ (297)	$ 14,696	$ 9,921	$ 21,164	$ (1,876)	$ 48,527	$ 43,321	$ 54,609
Weighted average common shares outstanding									22,779,057	22,681,888	22,576,381
Basic gain per common share (in dollars per share)	$ 0.50	$ 0.54	$ 1.10	$ (0.01)	$ 0.64	$ 0.43	$ 0.92	$ (0.08)	$ 2.13	$ 1.91	$ 2.42
Earnings per common share assuming dilution
Net income									$ 49,166	$ 43,905	$ 55,324
Less income allocated to participating securities									639	584	715
Net income allocated to common shareholders	$ 11,560	$ 12,429	$ 25,474	$ (297)	$ 14,696	$ 9,921	$ 21,164	$ (1,876)	$ 48,527	$ 43,321	$ 54,609
Weighted average common shares outstanding									22,779,057	22,681,888	22,576,381
Incremental shares applicable to non-participating RSUs									34,654	22,968	11,267
Weighted average common shares assuming dilution									22,813,711	22,704,856	22,587,648
Gain per common share assuming dilution (in dollars per share)	$ 0.50	$ 0.53	$ 1.10	$ (0.01)	$ 0.63	$ 0.43	$ 0.91	$ (0.08)	$ 2.11	$ 1.89	$ 2.40