CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY (Unaudited) - USD ($) $ in Thousands	Common Stock Shares Outstanding [Member]	Common Stock Amount Issued [Member]	Additional Paid-In Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive (Loss) [Member]	Treasury Stock [Member]	Deferred Compensation Arrangements [Member]	Total
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of change in accounting principle - equity securities				$ 208	$ (208)			$ 0
Balance at Dec. 31, 2017		$ 51,264	$ 894,879	700,557	(3,699)	$ (21,014)	$ 13,328	1,635,315
Balance (in shares) at Dec. 31, 2017	50,696,077
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income				84,712				84,712
Other comprehensive income (loss), net of tax					(42,584)			(42,584)
Cash dividends declared:
Common				(34,698)				(34,698)
Common stock activity under employee stock ownership plan		248	4,396					4,644
Common stock activity under employee stock ownership plan (in shares)	248,151
Stock-based compensation			3,396					3,396
Distribution of stock under deferred compensation arrangements						1,898	(1,898)	0
Distribution of stock under deferred compensation arrangements (in shares)	35,233
Treasury stock issued to benefit plans, net			2,242			3,782	150	6,174
Treasury stock issued to benefit plans, net (in shares)	106,413
Balance at Jun. 30, 2018		51,512	904,913	750,779	(46,491)	(15,334)	11,580	1,656,959
Balance (in shares) at Jun. 30, 2018	51,085,874
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of change in accounting principle - equity securities				208	(208)			0
Balance at Mar. 31, 2018		51,374	898,036	723,404	(35,226)	(17,633)	11,511	1,631,466
Balance (in shares) at Mar. 31, 2018	50,883,603
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income				44,606				44,606
Other comprehensive income (loss), net of tax					(11,057)			(11,057)
Cash dividends declared:
Common				(17,439)				(17,439)
Common stock activity under employee stock ownership plan		138	3,936					4,074
Common stock activity under employee stock ownership plan (in shares)	137,738
Stock-based compensation			1,681					1,681
Treasury stock issued to benefit plans, net			1,260			2,299	69	3,628
Treasury stock issued to benefit plans, net (in shares)	64,533
Balance at Jun. 30, 2018		51,512	904,913	750,779	(46,491)	(15,334)	11,580	1,656,959
Balance (in shares) at Jun. 30, 2018	51,085,874
Balance at Dec. 31, 2018		51,577	911,748	795,563	(45,305)	(11,528)	11,728	1,713,783
Balance (in shares) at Dec. 31, 2018	51,257,824
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income				86,961				86,961
Other comprehensive income (loss), net of tax					40,684			40,684
Cash dividends declared:
Common				(39,236)				(39,236)
Common stock activity under employee stock ownership plan		229	992					1,221
Common stock activity under employee stock ownership plan (in shares)	228,585
Stock-based compensation			2,757					2,757
Distribution of stock under deferred compensation arrangements			1,064			830	(1,894)	0
Distribution of stock under deferred compensation arrangements (in shares)	32,431
Treasury stock issued to benefit plans, net			1,294			1,901	145	3,340
Treasury stock issued to benefit plans, net (in shares)	51,768
Balance at Jun. 30, 2019		51,806	917,855	843,288	(4,621)	(8,797)	9,979	1,809,510
Balance (in shares) at Jun. 30, 2019	51,570,608
Balance at Mar. 31, 2019		51,728	913,917	817,933	(26,762)	(9,601)	9,913	1,757,128
Balance (in shares) at Mar. 31, 2019	51,471,371
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income				45,015				45,015
Other comprehensive income (loss), net of tax					22,141			22,141
Cash dividends declared:
Common				(19,660)				(19,660)
Common stock activity under employee stock ownership plan		78	1,987					2,065
Common stock activity under employee stock ownership plan (in shares)	77,666
Stock-based compensation			1,366					1,366
Treasury stock issued to benefit plans, net			585			804	66	1,455
Treasury stock issued to benefit plans, net (in shares)	21,571
Balance at Jun. 30, 2019		$ 51,806	$ 917,855	$ 843,288	$ (4,621)	$ (8,797)	$ 9,979	$ 1,809,510
Balance (in shares) at Jun. 30, 2019	51,570,608