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, 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
Net income (loss)	(148,749)				(148,749)
Other comprehensive 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
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 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
Balance at Dec. 31, 2020	$ 3,439,025		$ 309,513	1,757,937	1,291,506		80,069
Balance, Shares Issued at Dec. 31, 2020	92,947		92,947
Net income (loss)	$ 325,472				325,472
Other comprehensive loss	(79,871)						(79,871)
Comprehensive income	245,601				325,472		(79,871)
Cash dividends declared	(72,046)				(72,046)
Common stock activity, long-term incentive plans	16,755			16,506	249
Repurchase of common stock	(2,509)			(2,509)
Issuance of stock from dividend reinvestment and stock purchase plans	2,940			2,940
Balance at Sep. 30, 2021	$ 3,629,766		$ 309,513	1,774,874	1,545,181		198
Balance, Shares Issued at Sep. 30, 2021	92,947		92,947
Balance at Jun. 30, 2021	$ 3,562,901		$ 309,513	1,770,973	1,439,553		42,862
Balance, Shares Issued at Jun. 30, 2021			92,947
Net income (loss)	129,582				129,582
Other comprehensive loss	(42,664)						(42,664)
Comprehensive income	86,918				129,582		(42,664)
Cash dividends declared	(24,002)				(24,002)
Common stock activity, long-term incentive plans	5,480			5,432	48
Repurchase of common stock	(2,509)			(2,509)
Issuance of stock from dividend reinvestment and stock purchase plans	978			978
Balance at Sep. 30, 2021	$ 3,629,766		$ 309,513	$ 1,774,874	$ 1,545,181		$ 198
Balance, Shares Issued at Sep. 30, 2021	92,947		92,947