Earnings Per Share - Computation of Basic and Diluted Earnings Per 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 common share computation
Net income available to common stockholders									$ 31,596	$ 40,081	$ 33,719
Net earnings allocated to participating securities									(100)	(147)	(150)
Net income available to common stockholders									$ 31,496	$ 39,934	$ 33,569
Weighted average common shares outstanding, including shares considered participating securities (in shares)									16,048	15,219	15,274
Less: Average participating securities (in shares)									(48)	(55)	(64)
Weighted average shares (in shares)									16,000	15,164	15,210
Basic earnings per common share (in dollars per share)	$ 0.40	$ 0.47	$ 0.54	$ 0.57	$ 0.69	$ 0.68	$ 0.64	$ 0.62	$ 1.97	$ 2.63	$ 2.21
Diluted earnings per common share computation
Net earnings allocated to common stock									$ 31,496	$ 39,934	$ 33,569
Weighted average common shares outstanding for basic earnings per common share (in shares)									16,000	15,164	15,210
Add: Dilutive effects of assumed exercises of stock options (in shares)									0	0	0
Weighted average shares and dilutive potential common shares (in shares)									16,000	15,164	15,210
Diluted earnings per common share (in dollars per share)	$ 0.40	$ 0.47	$ 0.54	$ 0.57	$ 0.69	$ 0.68	$ 0.64	$ 0.62	$ 1.97	$ 2.63	$ 2.21
Distributed earnings allocated to common stock
Basic earnings per common share computation
Net income available to common stockholders									$ 10,942	$ 10,317	$ 10,186
Undistributed earnings allocated to common stock
Basic earnings per common share computation
Net income available to common stockholders									$ 20,554	$ 29,617	$ 23,383