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)	(228,105)				(228,105)
Other comprehensive income (loss)	157,450						157,450
Comprehensive income (loss)	(70,655)				(228,105)		157,450
Cash dividends declared	(47,817)				(47,817)
Common stock activity, long-term incentive plans	9,619			9,564	55
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	2,018			2,018
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
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
Net income (loss)	(117,072)				(117,072)
Other comprehensive income (loss)	29,460						29,460
Comprehensive income (loss)	(87,612)				(117,072)		29,460
Cash dividends declared	(23,789)				(23,789)
Common stock activity, long-term incentive plans	5,460			5,450	10
Issuance of stock from dividend reinvestment and stock purchase plans	1,034			1,034
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
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)	$ 195,890				195,890
Other comprehensive income (loss)	(37,207)						(37,207)
Comprehensive income (loss)	158,683				195,890		(37,207)
Cash dividends declared	(48,044)				(48,044)
Common stock activity, long-term incentive plans	11,275			11,074	201
Issuance of stock from dividend reinvestment and stock purchase plans	1,962			1,962
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		92,947
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
Net income (loss)	88,718				88,718
Other comprehensive income (loss)	74,305						74,305
Comprehensive income (loss)	163,023				88,718		74,305
Cash dividends declared	(24,023)				(24,023)
Common stock activity, long-term incentive plans	6,040			5,870	170
Issuance of stock from dividend reinvestment and stock purchase plans	958			958
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		92,947