Consolidated Statements of Changes in Stockholders' Equity - USD ($) shares in Thousands, $ in Thousands	Common Stock [Member]	Capital Surplus [Member]	Retained Earnings [Member]	Accumulated Other ComprehensiveLoss, Net [Member]		Total
Balance at Dec. 31, 2014	$ 291,307	$ 1,507,673	$ 723,496	$ (50,074)		$ 2,472,402
Balance, Shares Issued at Dec. 31, 2014	87,480
Net income			131,461			131,461
Other comprehensive loss, net of tax				(30,521)		(30,521)
Comprehensive income			131,461	(30,521)		100,940
Cash dividends declared ($0.96 per common share)			(77,013)			(77,013)
Common stock activity, long-term incentive plan	$ 39	12,388				12,427
Common stock activity, long-term incentive plan, Shares Issued	11
Purchase of common stock under stock buyback program (3,305 shares)		(95,613)				(95,613)
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, net of tax				(39,937)		(39,937)
Comprehensive income			149,296	(39,937)		109,359
Cash dividends declared ($0.96 per common share)			(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 plan		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					87,495
Net income			215,632			$ 215,632
Other comprehensive loss, net of tax				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 ($0.96 per common share)			(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 plan		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

[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.