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	119,475			119,475
Other comprehensive income	26,508				26,508
Stock issued	5,526	$ 594	2,099			2,833
Stock issued, including related tax benefits (in shares)		454
Stock-based compensation awards	4,808		4,808
Acquisition of treasury stock (in shares)		(1,486)
Acquisition of treasury stock	(18,545)					(18,545)
Common stock cash dividends	(50,230)			(50,230)
Ending Balance at Sep. 30, 2016	2,129,436	$ 547,735	1,457,597	710,833	4,491	(591,220)
Ending Balance (in shares) at Sep. 30, 2016		173,144
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	137,752
Other comprehensive income	14,246				14,246
Stock issued	7,037	$ 2,446	5,209			(618)
Stock issued, including related tax benefits (in shares)		1,017
Stock-based compensation awards	3,339		3,339
Common stock cash dividends	(57,703)			(57,703)
Ending Balance at Sep. 30, 2017	$ 2,225,786	$ 552,153	$ 1,476,150	$ 812,148	$ (24,203)	$ (590,462)
Ending Balance (in shares) at Sep. 30, 2017		175,057