CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY (Unaudited) - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]
Common Stock, Shares Outstanding, Beginning Balance at Dec. 31, 2017		17,227,000
Shareholders' Equity, Beginning Balance at Dec. 31, 2017	$ 413,284	$ 1,077	$ 117,961	$ 317,756	$ (23,510)
Net income	51,396			51,396
Total other comprehensive income (loss), net of tax	(15,567)				(15,567)
Cash dividends declared	(22,467)			(22,467)
Share-based compensation	1,977		1,977
Exercise of stock options and issuance of other compensation-related equity awards, shares		63,000
Exercise of stock options and issuance of other compensation-related equity awards, value	(714)	$ 4	(718)
Common Stock, Shares Outstanding, Ending Balance at Sep. 30, 2018		17,290,000
Shareholders' Equity, Ending Balance at Sep. 30, 2018	427,909	$ 1,081	119,220	346,685	(39,077)
Common Stock, Shares Outstanding, Beginning Balance at Jun. 30, 2018		17,278,000
Shareholders' Equity, Beginning Balance at Jun. 30, 2018	421,571	$ 1,080	118,883	336,670	(35,062)
Net income	17,511			17,511
Total other comprehensive income (loss), net of tax	(4,015)				(4,015)
Cash dividends declared	(7,496)			(7,496)
Share-based compensation	658		658
Exercise of stock options and issuance of other compensation-related equity awards, shares		12,000
Exercise of stock options and issuance of other compensation-related equity awards, value	(320)	$ 1	(321)
Common Stock, Shares Outstanding, Ending Balance at Sep. 30, 2018		17,290,000
Shareholders' Equity, Ending Balance at Sep. 30, 2018	$ 427,909	$ 1,081	119,220	346,685	(39,077)
Common Stock, Shares Outstanding, Beginning Balance at Dec. 31, 2018	17,302,037	17,302,000
Shareholders' Equity, Beginning Balance at Dec. 31, 2018	$ 448,184	$ 1,081	119,888	355,524	(28,309)
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)
Common Stock, Shares Outstanding, Ending Balance at Sep. 30, 2019	17,338,348	17,338,000
Shareholders' Equity, Ending Balance at Sep. 30, 2019	$ 497,825	$ 1,084	121,900	383,765	(8,924)
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)
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)
Common Stock, Shares Outstanding, Ending Balance at Sep. 30, 2019	17,338,348	17,338,000
Shareholders' Equity, Ending Balance at Sep. 30, 2019	$ 497,825	$ 1,084	$ 121,900	$ 383,765	$ (8,924)