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	53,582			53,582
Total other comprehensive income (loss), net of tax	19,385				19,385
Cash dividends declared	(26,063)			(26,063)
Share-based compensation	2,295		2,295
Exercise of stock options and issuance of other compensation-related equity awards, shares		36,000
Exercise of stock options and issuance of other compensation-related equity awards, value	(280)	$ 3	(283)			0
Common Stock, Shares Outstanding, Ending Balance at Sep. 30, 2019		17,338,000
Shareholders' Equity, Ending Balance at Sep. 30, 2019	497,825	$ 1,084	121,900	383,765	(8,924)	0
Common Stock, Shares Outstanding, Beginning Balance at Jun. 30, 2019		17,336,000
Shareholders' Equity, Beginning Balance at Jun. 30, 2019	484,205	$ 1,083	121,115	373,873	(11,866)	0
Net income	18,814			18,814
Total other comprehensive income (loss), net of tax	2,942				2,942
Cash dividends declared	(8,922)			(8,922)
Share-based compensation	792		792
Exercise of stock options and issuance of other compensation-related equity awards, shares		2,000
Exercise of stock options and issuance of other compensation-related equity awards, value	(6)	$ 1	(7)			0
Common Stock, Shares Outstanding, Ending Balance at Sep. 30, 2019		17,338,000
Shareholders' Equity, Ending Balance at Sep. 30, 2019	497,825	$ 1,084	121,900	383,765	(8,924)	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	51,263			51,263
Total other comprehensive income (loss), net of tax	7,834				7,834
Cash dividends declared	(26,745)			(26,745)
Share-based compensation	2,696		2,696
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	$ (417)	$ 0	(1,209)			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 Sep. 30, 2020	17,260,219	17,260,000
Shareholders' Equity, Ending Balance at Sep. 30, 2020	$ 527,693	$ 1,085	124,768	408,773	(3,403)	(3,530)
Common Stock, Shares Outstanding, Beginning Balance at Jun. 30, 2020		17,260,000
Shareholders' Equity, Beginning Balance at Jun. 30, 2020	520,163	$ 1,085	123,684	399,386	(462)	(3,530)
Net income	18,322			18,322
Total other comprehensive income (loss), net of tax	(2,941)				(2,941)
Cash dividends declared	(8,935)			(8,935)
Share-based compensation	1,083		1,083
Exercise of stock options and issuance of other compensation-related equity awards, shares		0
Exercise of stock options and issuance of other compensation-related equity awards, value	$ 1	$ 0	1			0
Common Stock, Shares Outstanding, Ending Balance at Sep. 30, 2020	17,260,219	17,260,000
Shareholders' Equity, Ending Balance at Sep. 30, 2020	$ 527,693	$ 1,085	$ 124,768	408,773	$ (3,403)	$ (3,530)
Cumulative effect of change in accounting principle	$ (6,108)			$ (6,108)