Consolidated Statements of Shareholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock [Member]	Additional Paid-In Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income [Member]	Treasury Stock [Member]
Beginning Balance at Dec. 31, 2015	$ 2,041,894	$ 547,141	$ 1,450,690	$ 641,588	$ (22,017)	$ (575,508)
Beginning Balance (in shares) at Dec. 31, 2015		174,176
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	38,257			38,257
Other comprehensive income	16,880				16,880
Stock issued	1,647	$ 121	345			1,181
Stock issued, including related tax benefits (in shares)		134
Stock-based compensation awards	1,436		1,436
Acquisition of treasury stock (in shares)		(917)
Acquisition of treasury stock	(11,196)					(11,196)
Common stock cash dividends	(15,609)			(15,609)
Ending Balance at Mar. 31, 2016	2,073,309	$ 547,262	1,452,471	664,236	(5,137)	(585,523)
Ending Balance (in shares) at Mar. 31, 2016		173,393
Beginning Balance at Dec. 31, 2016	2,121,115	$ 549,707	1,467,602	732,099	(38,449)	(589,844)
Beginning Balance (in shares) at Dec. 31, 2016		174,040
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	43,380			43,380
Other comprehensive income	3,897				3,897
Stock issued	4,731	$ 585	3,265			881
Stock issued, including related tax benefits (in shares)		303
Stock-based compensation awards	734		734
Common stock cash dividends	(19,174)			(19,174)
Ending Balance at Mar. 31, 2017	$ 2,154,683	$ 550,292	$ 1,471,601	$ 756,305	$ (34,552)	$ (588,963)
Ending Balance (in shares) at Mar. 31, 2017		174,343