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		88,847			88,847
Other comprehensive loss		13,574				13,574
Stock issued, including related tax benefits (in shares)			877
Stock issued		4,870	$ 2,229	4,178			(1,537)
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
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		84,677			84,677
Other comprehensive loss		(33,183)				(33,183)
Stock issued, including related tax benefits (in shares)			661
Stock issued		2,455	$ 1,686	1,652			(883)
Stock-based compensation awards (in shares)			16
Stock-based compensation awards		4,184	$ 40	4,144
Reclassification of stranded tax effects	[1]	0			7,101	(7,101)
Common stock cash dividends		(42,205)			(42,205)
Ending Balance at Jun. 30, 2018		$ 2,245,785	$ 553,958	$ 1,484,185	$ 871,192	$ (73,258)	$ (590,292)
Ending Balance (in shares) at Jun. 30, 2018			175,847

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