CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY (Unaudited) - USD ($) $ in Thousands	Common Stock 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
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				40,106				40,106
Other comprehensive income (loss), net of tax					(31,527)			(31,527)
Dividends declared:
Common				(17,259)				(17,259)
Common stock activity under employee stock ownership plan		110	460					570
Common stock activity under employee stock ownership plan (in shares)	110,413
Stock-based compensation			1,715					1,715
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			982			1,483	81	2,546
Treasury stock issued to benefit plans, net (in shares)	41,880
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
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				41,946				41,946
Other comprehensive income (loss), net of tax					18,543			18,543
Dividends declared:
Common				(19,576)				(19,576)
Common stock activity under employee stock ownership plan		151	(995)					(844)
Common stock activity under employee stock ownership plan (in shares)	150,919
Stock-based compensation			1,391					1,391
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			709			1,097	79	1,885
Treasury stock issued to benefit plans, net (in shares)	30,197
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