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	78,007			78,007
Other comprehensive income	29,706				29,706
Stock issued	3,348	$ 389	1,405			1,554
Stock issued, including related tax benefits (in shares)		273
Stock-based compensation awards	3,256		3,256
Acquisition of treasury stock (in shares)		(1,310)
Acquisition of treasury stock	(16,254)					(16,254)
Common stock cash dividends	(32,960)			(32,960)
Ending Balance at Jun. 30, 2016	2,106,997	$ 547,530	1,455,351	686,635	7,689	(590,208)
Ending Balance (in shares) at Jun. 30, 2016		173,139
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	88,847
Other comprehensive income	13,574				13,574
Stock issued	4,870	$ 2,229	4,178			(1,537)
Stock issued, including related tax benefits (in shares)		877
Stock-based compensation awards	1,769		1,769
Common stock cash dividends	(38,405)			(38,405)
Ending Balance at Jun. 30, 2017	$ 2,191,770	$ 551,936	$ 1,473,549	$ 782,541	$ (24,875)	$ (591,381)
Ending Balance (in shares) at Jun. 30, 2017		174,917