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			101,281		101,281
Other comprehensive income				16,093	16,093
Comprehensive income			101,281	16,093	117,374
Cash dividends declared ($0.48 per common share)			(41,594)		(41,594)
Common stock activity, long-term incentive plan		16,801	83		16,884
Issuance of stock from dividend reinvestment and stock repurchase plan		1,530			1,530
Balance at Jun. 30, 2017	$ 291,358	1,716,584	910,459	(104,439)	2,813,962
Balance, Shares Issued at Jun. 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			143,652		$ 143,652
Other comprehensive income				(68,807)	(68,807)
Comprehensive income			143,652	(68,807)	74,845
Cash dividends declared ($0.48 per common share)			(41,775)		(41,775)
Common stock activity, long-term incentive plan		9,779	111		9,890
Issuance of stock from dividend reinvestment and stock repurchase plan		1,646			1,646
Balance at Jun. 30, 2018	$ 292,716	$ 1,729,542	$ 1,110,506	$ (203,209)	$ 2,929,555
Balance, Shares Issued at Jun. 30, 2018	87,903				87,903