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, 2017	$ 2,884,949	$ 292,716	$ 1,718,117	$ 1,008,518	$ (134,402)
Balance, Shares Issued at Dec. 31, 2017		87,903
Net income	143,652			143,652
Other comprehensive income (loss)	(68,807)				(68,807)
Comprehensive income	74,845			143,652	(68,807)
Cash dividends declared	(41,775)			(41,775)
Common stock activity, long-term incentive plan	9,890		9,779	111
Issuance of stock from dividend reinvestment and stock purchase plans	1,646		1,646
Balance at Jun. 30, 2018	2,929,555	$ 292,716	1,729,542	1,110,506	(203,209)
Balance, Shares Issued at Jun. 30, 2018		87,903
Balance at Mar. 31, 2018	2,896,038	$ 292,716	1,723,689	1,060,182	(180,549)
Balance, Shares Issued at Mar. 31, 2018		87,903
Net income	71,177			71,177
Other comprehensive income (loss)	(22,660)				(22,660)
Comprehensive income	48,517			71,177	(22,660)
Cash dividends declared	(20,894)			(20,894)
Common stock activity, long-term incentive plan	5,085		5,044	41
Issuance of stock from dividend reinvestment and stock purchase plans	809		809
Balance at Jun. 30, 2018	2,929,555	$ 292,716	1,729,542	1,110,506	(203,209)
Balance, Shares Issued at Jun. 30, 2018		87,903
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	87,903
Net income	$ 167,441			167,441
Other comprehensive income (loss)	105,506				105,506
Comprehensive income	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	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	88,277			88,277
Other comprehensive income (loss)	57,306				57,306
Comprehensive income	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	87,903