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	227,530			227,530
Other comprehensive income (loss)	(85,777)				(85,777)
Comprehensive income	141,753			227,530	(85,777)
Cash dividends declared	(65,287)			(65,287)
Common stock activity, long-term incentive plan	14,968		14,832	136
Issuance of stock from dividend reinvestment and stock purchase plans	2,495		2,495
Balance at Sep. 30, 2018	2,978,878	$ 292,716	1,735,444	1,170,897	(220,179)
Balance, Shares Issued at Sep. 30, 2018		87,903
Balance at Jun. 30, 2018	2,929,555	$ 292,716	1,729,542	1,110,506	(203,209)
Balance, Shares Issued at Jun. 30, 2018		87,903
Net income	83,878			83,878
Other comprehensive income (loss)	(16,970)				(16,970)
Comprehensive income	66,908			83,878	(16,970)
Cash dividends declared	(23,512)			(23,512)
Common stock activity, long-term incentive plan	5,078		5,053	25
Issuance of stock from dividend reinvestment and stock purchase plans	849		849
Balance at Sep. 30, 2018	2,978,878	$ 292,716	1,735,444	1,170,897	(220,179)
Balance, Shares Issued at Sep. 30, 2018		87,903
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	$ 235,248			235,248
Other comprehensive income (loss)	129,553				129,553
Comprehensive income	364,801			235,248	129,553
Cash dividends declared	(70,771)			(70,771)
Common stock issued in business combination	193,849	$ 16,797	177,052
Common stock issued in business combination, Shares Issued		5,044
Common stock activity, long-term incentive plan	14,469		14,355	114
Issuance of stock from dividend reinvestment and stock purchase plans	2,692		2,692
Balance at Sep. 30, 2019	$ 3,586,380	$ 309,513	1,919,840	1,408,183	(51,156)
Balance, Shares Issued at Sep. 30, 2019	92,947	92,947
Balance at Jun. 30, 2019	$ 3,318,915	$ 292,716	1,737,492	1,363,910	(75,203)
Balance, Shares Issued at Jun. 30, 2019		87,903
Net income	67,807			67,807
Other comprehensive income (loss)	24,047				24,047
Comprehensive income	91,854			67,807	24,047
Cash dividends declared	(23,597)			(23,597)
Common stock issued in business combination	193,849	$ 16,797	177,052
Common stock issued in business combination, Shares Issued		5,044
Common stock activity, long-term incentive plan	4,470		4,407	63
Issuance of stock from dividend reinvestment and stock purchase plans	889		889
Balance at Sep. 30, 2019	$ 3,586,380	$ 309,513	$ 1,919,840	$ 1,408,183	$ (51,156)
Balance, Shares Issued at Sep. 30, 2019	92,947	92,947