Consolidated Statements of Shareholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Share Repurchase Program [Member]	Common Stock [Member]	Common Stock [Member] Share Repurchase Program [Member]	Additional Paid-In Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income [Member]	Treasury Stock [Member]	Treasury Stock [Member] Share Repurchase Program [Member]
Beginning Balance at Dec. 31, 2014	$ 1,996,665		$ 545,555		$ 1,420,523	$ 558,810	$ (17,722)	$ (510,501)
Beginning Balance (in shares) at Dec. 31, 2014			178,924
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	40,036					40,036
Other comprehensive income	7,922						7,922
Stock issued, including related tax benefits	1,941		$ 179		418			1,344
Stock issued, including related tax benefits (in shares)			174
Stock-based compensation awards	1,071				1,071
Common stock cash dividends	(16,122)					(16,122)
Ending Balance at Mar. 31, 2015	2,031,513		$ 545,734		1,422,012	582,724	(9,800)	(509,157)
Ending Balance (in shares) at Mar. 31, 2015			179,098
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, including related tax benefits	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