Consolidated Statements of Changes in Stockholders' Equity - USD ($) shares in Thousands, $ in Thousands	Common Stock [Member]	Capital Surplus [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss), Net [Member]	Total
Balance at Dec. 31, 2016	$ 291,358	$ 1,698,253	$ 850,689	$ (120,532)	$ 2,719,768
Balance, Shares Issued at Dec. 31, 2016	87,495
Net income			160,183		160,183
Other comprehensive income				22,381	22,381
Comprehensive income			160,183	22,381	182,564
Cash dividends declared			(62,400)		(62,400)
Common stock activity, long-term incentive plan		20,910	119		21,029
Issuance of stock from dividend reinvestment		2,314			2,314
Balance at Sep. 30, 2017	$ 291,358	1,721,477	948,591	(98,151)	2,863,275
Balance, Shares Issued at Sep. 30, 2017	87,495
Balance at Dec. 31, 2017	$ 292,716	1,718,117	1,008,518	(134,402)	$ 2,884,949
Balance, Shares Issued at Dec. 31, 2017	87,903				87,903
Net income			227,530		$ 227,530
Other comprehensive income				(85,777)	(85,777)
Comprehensive income			227,530	(85,777)	141,753
Cash dividends declared			(65,287)		(65,287)
Common stock activity, long-term incentive plan		14,832	136		14,968
Issuance of stock from dividend reinvestment		2,495			2,495
Balance at Sep. 30, 2018	$ 292,716	$ 1,735,444	$ 1,170,897	$ (220,179)	$ 2,978,878
Balance, Shares Issued at Sep. 30, 2018	87,903				87,903