Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ 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			49,014		49,014
Other comprehensive income				2,020	2,020
Comprehensive income			49,014	2,020	51,034
Cash dividends declared ($0.24 per common share)			(20,793)		(20,793)
Common stock activity, long-term incentive plan		12,815	43		12,858
Issuance of stock from dividend reinvestment and stock repurchase plans		755			755
Balance at Mar. 31, 2017	$ 291,358	1,711,823	878,953	(118,512)	2,763,622
Balance, Shares Issued at Mar. 31, 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				87,903,000
Net income			72,475		$ 72,475
Other comprehensive income				(46,147)	(46,147)
Comprehensive income			72,475	(46,147)	26,328
Cash dividends declared ($0.24 per common share)			(20,881)		(20,881)
Common stock activity, long-term incentive plan		4,735	70		4,805
Issuance of stock from dividend reinvestment and stock repurchase plans		837			837
Balance at Mar. 31, 2018	$ 292,716	$ 1,723,689	$ 1,060,182	$ (180,549)	$ 2,896,038
Balance, Shares Issued at Mar. 31, 2018	87,903				87,903,000