Earnings Per Common Share - Computation of basic and diluted earnings per common share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Basic Earnings Per Share:
Net income available to common shareholders	$ 30,848	$ 25,655	$ 29,057	$ (22,483)	$ 12,518	$ 13,171	$ 12,199	$ 11,482	$ 63,077	$ 49,370	$ 46,249
Less: Income allocated to participating securities									89	36	16
Net income allocated to common shareholders									$ 62,988	$ 49,334	$ 46,233
Weighted average common shares outstanding Including participating securities									35,952	19,844	20,358
Less: Participating securities									50	20	9
Average common shares									35,902	19,824	20,349
Basic earnings per common share	$ 0.83	$ 0.69	$ 0.78	$ (0.71)	$ 0.63	$ 0.67	$ 0.62	$ 0.57	$ 1.75	$ 2.49	$ 2.27
Diluted Earnings Per Share:
Net income allocated to common shareholders									$ 62,988	$ 49,334	$ 46,233
Weighted average common shares outstanding for basic earnings per common share									35,902	19,824	20,349
Add: Dilutive effects of stock options and restricted stock units									47	107	119
Average shares and dilutive potential common shares									35,949	19,931	20,468
Diluted earnings per common share	$ 0.82	$ 0.69	$ 0.78	$ (0.71)	$ 0.63	$ 0.66	$ 0.61	$ 0.57	$ 1.75	$ 2.48	$ 2.26