CONSOLIDATED STATEMENT OF CHANGES IN SHAREHOLDERS' EQUITY (Unaudited) - 6 months ended Jun. 30, 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			84,712				84,712
Other comprehensive loss, net of tax				(42,584)			(42,584)
Cash dividends declared:
Common, $0.68 per share			(34,698)				(34,698)
Common stock issued under employee stock ownership plan	$ 248	4,396					4,644
Common stock issued 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