Consolidated Statements of Shareholders' Equity - USD ($) shares in Thousands, $ in Thousands		Total	Common Stock [Member]	Additional Paid-In Capital [Member]	Retained Earnings [Member]	Total	Treasury Stock [Member]
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 loss		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
Beginning Balance at Dec. 31, 2017		2,229,857	$ 552,232	1,478,389	821,619	(32,974)	(589,409)
Beginning Balance (in shares) at Dec. 31, 2017			175,170
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income		49,480			49,480
Other comprehensive loss		(27,097)				(27,097)
Stock issued		2,790	$ 450	1,646			694
Stock issued, including related tax benefits (in shares)			234
Stock-based compensation awards		1,510		1,510
Reclassification of stranded tax effects	[1]	0			7,101	(7,101)
Common stock cash dividends		(21,047)			(21,047)
Ending Balance at Mar. 31, 2018		$ 2,235,493	$ 552,682	$ 1,481,545	$ 857,153	$ (67,172)	$ (588,715)
Ending Balance (in shares) at Mar. 31, 2018			175,404

[1] Result of adoption of ASU 2018-02, See Note 1 to Consolidated Financial Statements for further details.