Consolidated Statements of Changes in Stockholders' Equity (Unaudited) - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock [Member]	Capital Surplus [Member]	Retained Earnings [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)	79,164			79,164
Other comprehensive income (loss)	48,200				48,200
Comprehensive income	127,364			79,164	48,200
Cash dividends declared	(23,581)			(23,581)
Common stock activity, long-term incentive plan	4,573		4,528	45
Issuance of stock from dividend reinvestment and stock purchase plans	879		879
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
Balance at Dec. 31, 2019	$ 3,467,685	$ 309,513	1,736,664	1,476,232	(54,724)
Balance, Shares Issued at Dec. 31, 2019	92,947	92,947
Net income (loss)	$ (111,033)			(111,033)
Other comprehensive income (loss)	127,990				127,990
Comprehensive income	16,957			(111,033)	127,990
Cumulative effect of change in accounting principle	(44,087)			(44,087)
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 forward contract portion 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	92,947