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)	167,441				167,441
Other comprehensive income	105,506						105,506
Comprehensive income (loss)	272,947				167,441		105,506
Cash dividends declared	(47,174)				(47,174)
Common stock activity, long-term incentive plan	9,999			9,948	51
Issuance of stock from dividend reinvestment and stock purchase plans	1,803			1,803
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
Balance at Mar. 31, 2019	3,190,575		$ 292,716	1,731,148	1,299,220		(132,509)
Balance, Shares Issued at Mar. 31, 2019			87,903
Net income (loss)	88,277				88,277
Other comprehensive income	57,306						57,306
Comprehensive income (loss)	145,583				88,277		57,306
Cash dividends declared	(23,593)				(23,593)
Common stock activity, long-term incentive plan	5,426			5,420	6
Issuance of stock from dividend reinvestment and stock purchase plans	924			924
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
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)	$ (228,105)				(228,105)
Other comprehensive income	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 plan	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		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	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 plan	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		92,947