Consolidated Statements of Shareholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Capital Surplus	Accumulated Other Comprehensive Income (Loss)	Retained Earnings	Treasury Stock
Balance at Beginning of Period at Dec. 31, 2014	$ 1,055,086	$ 574	$ 531,932	$ (26,686)	$ 1,234,801	$ (685,535)
Beginning Balance (in shares) at Dec. 31, 2014		43,724,208
Increase (decrease) in shareholders' equity
Net Income	160,704				160,704
Other Comprehensive Income (Loss)	3,129			3,129
Share-Based Compensation	7,689		7,689
Common Stock Issued under Purchase and Equity Compensation Plans and Related Tax Benefits	21,000	$ 1	2,420		(878)	19,457
Common Stock Issued under Purchase and Equity Compensation Plans and Related Tax Benefits (in shares)		401,904
Common Stock Repurchased	(52,981)					(52,981)
Common Stock Repurchased (in shares)		(843,959)
Cash Dividends Declared ($1.80 per share)	(78,367)				(78,367)
Balance at End of Period at Dec. 31, 2015	1,116,260	$ 575	542,041	(23,557)	1,316,260	(719,059)
Ending Balance (in shares) at Dec. 31, 2015		43,282,153
Increase (decrease) in shareholders' equity
Net Income	181,461				181,461
Other Comprehensive Income (Loss)	(10,349)			(10,349)
Share-Based Compensation	6,786		6,786
Common Stock Issued under Purchase and Equity Compensation Plans and Related Tax Benefits	10,343	$ 1	2,801		(1,124)	8,665
Common Stock Issued under Purchase and Equity Compensation Plans and Related Tax Benefits (in shares)		259,985
Common Stock Repurchased	(61,807)					(61,807)
Common Stock Repurchased (in shares)		(906,160)
Cash Dividends Declared ($1.80 per share)	(81,157)				(81,157)
Balance at End of Period at Dec. 31, 2016	$ 1,161,537	$ 576	551,628	(33,906)	1,415,440	(772,201)
Ending Balance (in shares) at Dec. 31, 2016	42,635,978	42,635,978
Increase (decrease) in shareholders' equity
Net Income	$ 184,672				184,672
Other Comprehensive Income (Loss)	(809)			(809)
Share-Based Compensation	7,369		7,369
Common Stock Issued under Purchase and Equity Compensation Plans and Related Tax Benefits	13,241	$ 0	2,164		(828)	11,905
Common Stock Issued under Purchase and Equity Compensation Plans and Related Tax Benefits (in shares)		337,091
Common Stock Repurchased	$ (47,076)					(47,076)
Common Stock Repurchased (in shares)	(549,199)	(571,626)
Cash Dividends Declared ($1.80 per share)	$ (87,066)				(87,066)
Balance at End of Period at Dec. 31, 2017	$ 1,231,868	$ 576	$ 561,161	$ (34,715)	$ 1,512,218	$ (807,372)
Ending Balance (in shares) at Dec. 31, 2017	42,401,443	42,401,443