Consolidated Statements of Changes in Stockholders' Equity (Unaudited) - USD ($) shares in Thousands, $ in Thousands	Total	Cumulative Effect, Period of Adoption, Adjustment [Member]	Common Stock [Member]	Capital Surplus [Member]	Retained Earnings [Member]	Retained Earnings [Member] Cumulative Effect, Period of Adoption, Adjustment [Member]	Accumulated Other Comprehensive Income (Loss), Net [Member]
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
Net income (loss)	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 plans	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
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 (loss)	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 plans	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
Balance at Dec. 31, 2019	$ 3,467,685	$ (44,087)	$ 309,513	1,736,664	1,476,232	$ (44,087)	(54,724)
Balance, Shares Issued at Dec. 31, 2019	92,947		92,947
Net income (loss)	$ (148,749)				(148,749)
Other comprehensive income (loss)	153,662						153,662
Comprehensive income	4,913				(148,749)		153,662
Cash dividends declared	(71,620)				(71,620)
Common stock activity, long-term incentive plans	16,261			16,159	102
Net settlement of accelerated share repurchase agreement (1,001,472 shares)	12,110			12,110
Repurchase of common stock	(12,716)			(12,716)
Issuance of stock from dividend reinvestment and stock purchase plans	3,098			3,098
Balance at Sep. 30, 2020	$ 3,375,644		$ 309,513	1,755,315	1,211,878		98,938
Balance, Shares Issued at Sep. 30, 2020	92,947		92,947
Balance at Jun. 30, 2020	$ 3,316,157		$ 309,513	1,747,640	1,156,278		102,726
Balance, Shares Issued at Jun. 30, 2020			92,947
Net income (loss)	79,356				79,356
Other comprehensive income (loss)	(3,788)						(3,788)
Comprehensive income	75,568				79,356		(3,788)
Cash dividends declared	(23,803)				(23,803)
Common stock activity, long-term incentive plans	6,642			6,595	47
Issuance of stock from dividend reinvestment and stock purchase plans	1,080			1,080
Balance at Sep. 30, 2020	$ 3,375,644		$ 309,513	$ 1,755,315	$ 1,211,878		$ 98,938
Balance, Shares Issued at Sep. 30, 2020	92,947		92,947