EPS (Computation of Basic and Diluted Earnings Per Share) (Details) - USD ($) $ / shares in Units, $ 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
Earnings Per Share, Basic and Diluted, Other Disclosures [Abstract]
Net income	$ 18,278	$ 16,775	$ 10,940	$ 13,493	$ 15,238	$ 14,488	$ 13,204	$ 14,273	$ 59,486	$ 57,203	$ 53,071
Dividends and undistributed earnings allocated to participating securities(1)									(149)	(120)	(148)
Net income available to common shareholders									$ 59,337	$ 57,083	$ 52,923
Weighted-average common shares outstanding for basic EPS									14,985,235	15,407,289	15,571,387
Dilutive effect of stock-based awards									38,532	45,733	54,916
Weighted-average common and potential common shares for diluted EPS									15,023,767	15,453,022	15,626,303
Basic EPS	$ 1.22	$ 1.12	$ 0.73	$ 0.89	$ 1.00	$ 0.94	$ 0.85	$ 0.91	$ 3.96	$ 3.70	$ 3.40
Diluted EPS	$ 1.22	$ 1.11	$ 0.73	$ 0.89	$ 0.99	$ 0.94	$ 0.85	$ 0.91	$ 3.95	$ 3.69	$ 3.39