Consolidated Statement of Changes in Shareholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	Common Stock [Member]	Surplus [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Treasury Stock [Member]
Beginning Balance at Dec. 31, 2017	$ 3,240,530	$ 262,675	$ 2,129,077	$ 891,816	$ (42,025)	$ (1,013)
Beginning Balance, shares at Dec. 31, 2017		105,069,821
Comprehensive income:
Net income	127,980	$ 0	0	127,980	0	0
Other comprehensive income, net of tax	(21,487)	0	0	0	(21,487)	0
Total comprehensive income, net of tax	106,493
Stock based compensation expense	1,992	0	1,992	0	0	0
Purchase of treasury stock	(35,984)	0	0	0	0	(35,984)
Cash dividends	(71,332)			(71,332)	0	0
Grant of restricted stock	0	$ 243	(243)	0	0	0
Grant of restricted stock, shares		97,004
Forfeiture of restricted stock	0	$ 0	109	0	0	(109)
Common stock options exercised	$ 866	$ 104	762	0	0	0
Common stock options exercised, shares	42,089	42,089
Ending Balance at Jun. 30, 2018	$ 3,242,565	$ 263,022	2,131,697	954,953	(70,001)	(37,106)
Ending Balance, shares at Jun. 30, 2018		105,208,914
Comprehensive income:
Net income	66,274
Other comprehensive income, net of tax	(5,448)
Total comprehensive income, net of tax	60,826
Ending Balance at Jun. 30, 2018	3,242,565	$ 263,022	$ 2,131,697	954,953	(70,001)	$ (37,106)
Ending Balance, shares at Jun. 30, 2018		105,208,914
Cumulative effect of adopting Accounting Standard Update 2016-01	0			136	(136)
Reclass due to adopting Accounting Standard Update 2018-02	$ 0			$ 6,353	$ (6,353)