Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Basic
Net income available to common shareholders	$ 77,588	$ 81,718	$ 82,308
Less: income attributable to unvested stock-based compensation awards	(857)	(1,306)	(1,315)
Net earnings allocated to common shareholders	$ 76,731	$ 80,412	$ 80,993
Weighted average shares outstanding, including unvested stock-based compensation awards (in shares)	14,933,990	15,149,535	15,283,914
Less: average unvested stock-based compensation awards (in shares)	(230,600)	(242,478)	(244,685)
Weighted average shares outstanding - Basic (in shares)	14,703,390	14,907,057	15,039,229
Diluted
Net earnings allocated to common shareholders	$ 76,731	$ 80,412	$ 80,993
Weighted average shares outstanding - Basic (in shares)	14,703,390	14,907,057	15,039,229
Plus: incremental shares from assumed conversion of stock-based compensation awards (in shares)	38,650	66,894	93,028
Weighted average shares outstanding - Diluted (in shares)	14,742,040	14,973,951	15,132,257
Basic EPS (in dollars per share)	$ 5.22	$ 5.39	$ 5.39
Diluted EPS (in dollars per share)	$ 5.20	$ 5.37	$ 5.35
Antidilutive securities excluded from computation of earning per share (in shares)	7,591	14,982	10,013