CONSOLIDATED STATEMENT OF CHANGES IN SHAREHOLDERS' EQUITY (Unaudited) - 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
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 loss, net of tax				(58,235)			(58,235)
Cash dividends declared:
Common, $1.06 per share			(54,180)				(54,180)
Common stock issued under employee stock ownership plan	$ 283	5,511					5,794
Common stock issued 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