Consolidated Statements of Changes in Stockholders' Equity (Unaudited) - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock [Member]	Capital Surplus [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss), Net [Member]
Balance at Dec. 31, 2017	$ 2,884,949	$ 292,716	$ 1,718,117	$ 1,008,518	$ (134,402)
Balance, Shares Issued at Dec. 31, 2017		87,903
Net income	72,475			72,475
Other comprehensive income (loss)	(46,147)				(46,147)
Comprehensive income	26,328			72,475	(46,147)
Cash dividends declared	(20,881)			(20,881)
Common stock activity, long-term incentive plan	4,805		4,735	70
Issuance of stock from dividend reinvestment and stock purchase plans	837		837
Balance, Shares Issued at Mar. 31, 2018		87,903
Balance at Mar. 31, 2018	2,896,038	$ 292,716	1,723,689	1,060,182	(180,549)
Balance at Dec. 31, 2018	$ 3,081,340	$ 292,716	1,725,741	1,243,592	(180,709)
Balance, Shares Issued at Dec. 31, 2018	87,903	87,903
Net income	$ 79,164			79,164
Other comprehensive income (loss)	48,200				48,200
Comprehensive income	127,364			79,164	48,200
Cash dividends declared	(23,581)			(23,581)
Common stock activity, long-term incentive plan	4,573		4,528	45
Issuance of stock from dividend reinvestment and stock purchase plans	$ 879		879
Balance, Shares Issued at Mar. 31, 2019	87,903	87,903
Balance at Mar. 31, 2019	$ 3,190,575	$ 292,716	$ 1,731,148	$ 1,299,220	$ (132,509)