CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY (Unaudited) - USD ($) $ in Thousands	Total	Common Stock [Member]	Paid-in Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Treasury Stock [Member]
Common Stock, Shares Outstanding, Beginning Balance at Dec. 31, 2018		17,302,000
Shareholders' Equity, Beginning Balance at Dec. 31, 2018	$ 448,184	$ 1,081	$ 119,888	$ 355,524	$ (28,309)	$ 0
Net income	34,768			34,768
Total other comprehensive income (loss), net of tax	16,443				16,443
Cash dividends declared	(17,141)			(17,141)
Share-based compensation	1,503		1,503
Exercise of stock options and issuance of other compensation-related equity awards, shares		34,000
Exercise of stock options and issuance of other compensation-related equity awards, value	(274)	$ 2	(276)			0
Common Stock, Shares Outstanding, Ending Balance at Jun. 30, 2019		17,336,000
Shareholders' Equity, Ending Balance at Jun. 30, 2019	484,205	$ 1,083	121,115	373,873	(11,866)	0
Common Stock, Shares Outstanding, Beginning Balance at Mar. 31, 2019		17,305,000
Shareholders' Equity, Beginning Balance at Mar. 31, 2019	469,843	$ 1,082	120,743	365,521	(17,503)	0
Net income	17,273			17,273
Total other comprehensive income (loss), net of tax	5,637				5,637
Cash dividends declared	(8,921)			(8,921)
Share-based compensation	764		764
Exercise of stock options and issuance of other compensation-related equity awards, shares		31,000
Exercise of stock options and issuance of other compensation-related equity awards, value	(391)	$ 1	(392)			0
Common Stock, Shares Outstanding, Ending Balance at Jun. 30, 2019		17,336,000
Shareholders' Equity, Ending Balance at Jun. 30, 2019	484,205	$ 1,083	121,115	373,873	(11,866)	0
Cumulative effect of change in accounting principle	$ 722			722
Common Stock, Shares Outstanding, Beginning Balance at Dec. 31, 2019	17,363,455	17,363,000
Shareholders' Equity, Beginning Balance at Dec. 31, 2019	$ 503,492	$ 1,085	123,281	390,363	(11,237)	0
Net income	32,941			32,941
Total other comprehensive income (loss), net of tax	10,775				10,775
Cash dividends declared	(17,810)			(17,810)
Share-based compensation	1,613		1,613
Exercise of stock options and issuance of other compensation-related equity awards, shares		22,000
Exercise of stock options and issuance of other compensation-related equity awards, value	$ (418)	$ 0	(1,210)			792
Treasury stock purchased under 2019 Stock Repurchase Program, shares acquired	(124,863)
Treasury stock purchased under 2019 Stock Repurchase Program, amount acquired	$ (4,322)			0	0	(4,322)
Common Stock, Shares Outstanding, Ending Balance at Jun. 30, 2020	17,260,217	17,260,000
Shareholders' Equity, Ending Balance at Jun. 30, 2020	$ 520,163	$ 1,085	123,684	399,386	(462)	(3,530)
Common Stock, Shares Outstanding, Beginning Balance at Mar. 31, 2020		17,252,000
Shareholders' Equity, Beginning Balance at Mar. 31, 2020	508,597	$ 1,085	123,167	387,243	929	(3,827)
Net income	21,040			21,040
Total other comprehensive income (loss), net of tax	(1,391)				(1,391)
Cash dividends declared	(8,897)			(8,897)
Share-based compensation	855		855
Exercise of stock options and issuance of other compensation-related equity awards, shares		8,000
Exercise of stock options and issuance of other compensation-related equity awards, value	$ (41)	$ 0	(338)			297
Common Stock, Shares Outstanding, Ending Balance at Jun. 30, 2020	17,260,217	17,260,000
Shareholders' Equity, Ending Balance at Jun. 30, 2020	$ 520,163	$ 1,085	$ 123,684	399,386	$ (462)	$ (3,530)
Cumulative effect of change in accounting principle	$ (6,108)			$ (6,108)