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		137,752			137,752
Other comprehensive loss		14,246				14,246
Stock issued, including related tax benefits (in shares)			1,017
Stock issued		7,037	$ 2,446	5,209			(618)
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
Beginning Balance at Jun. 30, 2017						(24,875)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income		48,905
Other comprehensive loss		672
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
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		150,310
Other comprehensive loss		(44,565)				(44,565)
Stock issued, including related tax benefits (in shares)			833
Stock issued		4,748	$ 1,936	2,773			39
Stock-based compensation awards (in shares)			16
Stock-based compensation awards		6,007	$ 40	5,967
Reclassification of stranded tax effects	[1]	0			7,101	(7,101)
Common stock cash dividends		(63,343)			(63,343)
Ending Balance at Sep. 30, 2018		2,283,014	$ 554,208	1,487,129	915,687	(84,640)	(589,370)
Ending Balance (in shares) at Sep. 30, 2018			176,019
Beginning Balance at Jun. 30, 2018						(73,258)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income		65,633
Other comprehensive loss		(11,382)
Ending Balance at Sep. 30, 2018		$ 2,283,014	$ 554,208	$ 1,487,129	$ 915,687	$ (84,640)	$ (589,370)
Ending Balance (in shares) at Sep. 30, 2018			176,019

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