CONSOLIDATED STATEMENT OF CHANGES IN SHAREHOLDERS' EQUITY (Unaudited) - 3 months ended Mar. 31, 2018 - USD ($) $ in Thousands	Common Stock [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 loss, net of tax				(31,527)			(31,527)
Cash dividends declared:
Common, $0.34 per share			(17,259)				(17,259)
Common stock issued under employee stock ownership plan	$ 110	460					570
Common stock issued 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