Consolidated Statements of Changes in Stockholders' Equity - 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, 2016	$ 2,719,768		$ 291,358	$ 1,698,253	$ 850,689	$ (120,532)
Balance, Shares Issued at Dec. 31, 2016			87,495
Net income	215,632				215,632
Other comprehensive income (loss)	11,460					11,460
Comprehensive income	227,092				215,632	11,460
Reclassification of certain tax effects from accumulated other comprehensive loss	25,330	[1]			25,330	(25,330)
Cash dividends declared	(83,266)				(83,266)
Common stock activity, long-term incentive plan	18,135		$ 1,358	16,644	133
Common stock activity, long-term incentive plan, Shares Issued			408
Issuance of stock from dividend reinvestment and stock purchase plans	3,220			3,220
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	323,770				323,770
Other comprehensive income (loss)	(46,307)					(46,307)
Comprehensive income	277,463				323,770	(46,307)
Cash dividends declared	(88,838)				(88,838)
Common stock activity, long-term incentive plan	12,624			12,482	142
Issuance of stock from dividend reinvestment and stock purchase plans	3,409			3,409
Purchase of common stock under stock buyback program	(8,267)			(8,267)
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	$ 327,380				327,380
Other comprehensive income (loss)	125,985					125,985
Comprehensive income	453,365				327,380	125,985
Cash dividends declared	(94,871)				(94,871)
Common stock issued as consideration in business combination	193,849		$ 16,797	177,052
Common stock issued as consideration in business combination, Shares Issued			5,044
Common stock activity, long-term incentive plan	15,388			15,257	131
Issuance of stock from dividend reinvestment and stock purchase plans	3,614			3,614
Initial delivery of shares under accelerated share repurchase agreement	(138,768)			(138,768)
Forward contract for accelerated share repurchase agreement	(46,232)			(46,232)
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

[1] Represents the reclassification of stranded income tax effects to Retained Earnings upon adoption of ASU 2018-02.