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, 2013	$ 291,034	$ 1,541,248	$ 628,166	$ (35,379)	$ 2,425,069
Balance, Shares Issued at Dec. 31, 2013	87,398
Net income			175,722		175,722
Other comprehensive income				(14,695)	(14,695)
Comprehensive income			175,722	(14,695)	161,027
Cash dividends declared ($0.96 per common share)			(80,392)		(80,392)
Common stock activity, long-term incentive plan	$ 273	14,043			14,316
Common stock activity, long-term incentive plan, Shares	82
Purchase of common stock under stock buyback program		(47,618)			(47,618)
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 income				(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	11
Purchase of common stock under stock buyback program		(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				87,491
Net income			149,296		$ 149,296
Other comprehensive income				(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	4
Issuance of stock from dividend reinvestment and stock purchase plan		1,515			1,515
Common stock issued in public stock offering (6,325,000 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