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)	(111,033)				(111,033)
Other comprehensive income (loss)	127,990						127,990
Comprehensive income (loss)	16,957				(111,033)		127,990
Cash dividends declared	(24,028)				(24,028)
Common stock activity, long-term incentive plan	4,159			4,114	45
Repurchase of common stock	(12,716)			(12,716)
Net settlement of accelerated share repurchase agreement (1,001,472 shares)	12,110			12,110
Issuance of stock from dividend reinvestment and stock purchase plans	984			984
Balance at Mar. 31, 2020	3,421,064		$ 309,513	1,741,156	1,297,129		73,266
Balance, Shares Issued at Mar. 31, 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)	$ 107,172				107,172
Other comprehensive income (loss)	(111,512)						(111,512)
Comprehensive income (loss)	(4,340)				107,172		(111,512)
Cash dividends declared	(24,021)				(24,021)
Common stock activity, long-term incentive plan	5,235			5,204	31
Issuance of stock from dividend reinvestment and stock purchase plans	1,004			1,004
Balance at Mar. 31, 2021	$ 3,416,903		$ 309,513	$ 1,764,145	$ 1,374,688		$ (31,443)
Balance, Shares Issued at Mar. 31, 2021	92,947		92,947