Consolidated Statements of Changes in Stockholders' Equity - USD ($) shares in Thousands, $ in Thousands	Common Stock [Member]	Capital Surplus [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Loss, Net [Member]		Total
Balance at Dec. 31, 2015	$ 291,346	$ 1,424,448	$ 777,944	$ (80,595)		$ 2,413,143
Balance, Shares Issued at Dec. 31, 2015	87,491
Net income			149,296			149,296
Other comprehensive loss				(39,937)		(39,937)
Comprehensive income			149,296	(39,937)		109,359
Cash dividends declared			(76,551)			(76,551)
Common stock activity, long-term incentive plan	$ 12	12,991				13,003
Common stock activity, long-term incentive plan, Shares Issued	4
Issuance of stock from dividend reinvestment and stock purchase plans		1,515				1,515
Common stock issued in public stock offering (6,325 shares)		259,299				259,299
Balance at Dec. 31, 2016	$ 291,358	1,698,253	850,689	(120,532)		2,719,768
Balance, Shares Issued at Dec. 31, 2016	87,495
Net income			215,632			215,632
Other comprehensive loss				11,460		11,460
Comprehensive income			215,632	11,460		227,092
Reclassification of certain tax effects from accumulated other comprehensive loss			25,330	(25,330)	[1]
Cash dividends declared			(83,266)			(83,266)
Common stock activity, long-term incentive plan	$ 1,358	16,644	133			18,135
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	$ 292,716	1,718,117	1,008,518	(134,402)		$ 2,884,949
Balance, Shares Issued at Dec. 31, 2017	87,903					87,903
Net income			323,770			$ 323,770
Other comprehensive loss				(46,307)		(46,307)
Comprehensive income			323,770	(46,307)		277,463
Cash dividends declared			(88,838)			(88,838)
Common stock activity, long-term incentive plan		12,482	142			12,624
Common stock activity, long-term incentive plan, Shares Issued
Issuance of stock from dividend reinvestment and stock purchase plans		3,409				3,409
Purchase of common stock under stock buyback program (200,000 shares)		(8,267)				(8,267)
Balance at Dec. 31, 2018	$ 292,716	$ 1,725,741	$ 1,243,592	$ (180,709)		$ 3,081,340
Balance, Shares Issued at Dec. 31, 2018	87,903					87,903

[1] Represents the reclassification of stranded income tax effects to Retained Earnings upon adoption of ASU 2018-02. The adjustment is discussed in more detail later in this footnote.