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				127,818				127,818
Other comprehensive income (loss), net of tax					(58,235)			(58,235)
Cash dividends declared:
Common				(54,180)				(54,180)
Common stock activity under employee stock ownership plan		283	5,511					5,794
Common stock activity under employee stock ownership plan (in shares)	283,412
Stock-based compensation			4,607					4,607
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			2,693			4,313	220	7,226
Treasury stock issued to benefit plans, net (in shares)	121,954
Balance at Sep. 30, 2018		51,547	907,690	774,403	(62,142)	(14,803)	11,650	1,668,345
Balance (in shares) at Sep. 30, 2018	51,136,676
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]
Net income				43,106				43,106
Other comprehensive income (loss), net of tax					(15,651)			(15,651)
Cash dividends declared:
Common				(19,482)				(19,482)
Common stock activity under employee stock ownership plan		35	1,115					1,150
Common stock activity under employee stock ownership plan (in shares)	35,261
Stock-based compensation			1,211					1,211
Treasury stock issued to benefit plans, net			451			531	70	1,052
Treasury stock issued to benefit plans, net (in shares)	15,541
Balance at Sep. 30, 2018		51,547	907,690	774,403	(62,142)	(14,803)	11,650	1,668,345
Balance (in shares) at Sep. 30, 2018	51,136,676
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				126,179				126,179
Other comprehensive income (loss), net of tax					47,775			47,775
Cash dividends declared:
Common				(60,488)				(60,488)
Common stock activity under employee stock ownership plan		266	2,246					2,512
Common stock activity under employee stock ownership plan (in shares)	266,127
Stock-based compensation			3,998					3,998
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			2,626			3,823	213	6,662
Treasury stock issued to benefit plans, net (in shares)	103,672
Balance at Sep. 30, 2019		51,843	921,682	861,254	2,470	(6,875)	10,047	1,840,421
Balance (in shares) at Sep. 30, 2019	51,660,054
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
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income				39,218				39,218
Other comprehensive income (loss), net of tax					7,091			7,091
Cash dividends declared:
Common				(21,252)				(21,252)
Common stock activity under employee stock ownership plan		37	1,254					1,291
Common stock activity under employee stock ownership plan (in shares)	37,542
Stock-based compensation			1,241					1,241
Treasury stock issued to benefit plans, net			1,332			1,922	68	3,322
Treasury stock issued to benefit plans, net (in shares)	51,904
Balance at Sep. 30, 2019		$ 51,843	$ 921,682	$ 861,254	$ 2,470	$ (6,875)	$ 10,047	$ 1,840,421
Balance (in shares) at Sep. 30, 2019	51,660,054