Consolidated Statements of Shareholders' Equity (Unaudited) - 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, 2015	$ 1,116,260	$ 575	$ 542,041	$ (23,557)	$ 1,316,260	$ (719,059)
Beginning Balance (in shares) at Dec. 31, 2015		43,282,153
Increase (decrease) in shareholders' equity
Net Income	94,455				94,455
Other Comprehensive Income	14,133			14,133
Share-Based Compensation	3,314		3,314
Common Stock Issued under Purchase and Equity Compensation Plans and Related Tax Benefits	6,197	$ 1	1,573		(277)	4,900
Common Stock Issued under Purchase and Equity Compensation Plans and Related Tax Benefits (in shares)		201,445
Common Stock Repurchased	(37,010)					(37,010)
Common Stock Repurchased (in shares)		(567,435)
Cash Dividends Declared ($1.00 per share)	(40,130)				(40,130)
Balance at End of Period at Jun. 30, 2016	1,157,219	$ 576	546,928	(9,424)	1,370,308	(751,169)
Ending Balance (in shares) at Jun. 30, 2016		42,916,163
Balance at Beginning of Period at Mar. 31, 2016				(14,722)
Increase (decrease) in shareholders' equity
Net Income	44,245
Other Comprehensive Income	5,298			5,298
Balance at End of Period at Jun. 30, 2016	1,157,219	$ 576	546,928	(9,424)	1,370,308	(751,169)
Ending Balance (in shares) at Jun. 30, 2016		42,916,163
Balance at Beginning of Period at Dec. 31, 2016	$ 1,161,537	$ 576	551,628	(33,906)	1,415,440	(772,201)
Beginning Balance (in shares) at Dec. 31, 2016	42,635,978	42,635,978
Increase (decrease) in shareholders' equity
Net Income	$ 95,838				95,838
Other Comprehensive Income	8,293			8,293
Share-Based Compensation	3,726		3,726
Common Stock Issued under Purchase and Equity Compensation Plans and Related Tax Benefits	8,438	$ 0	1,055		(162)	7,545
Common Stock Issued under Purchase and Equity Compensation Plans and Related Tax Benefits (in shares)		275,605
Common Stock Repurchased	(21,287)					(21,287)
Common Stock Repurchased (in shares)		(255,629)
Cash Dividends Declared ($1.00 per share)	(42,788)				(42,788)
Balance at End of Period at Jun. 30, 2017	$ 1,213,757	$ 576	556,409	(25,613)	1,468,328	(785,943)
Ending Balance (in shares) at Jun. 30, 2017	42,655,954	42,655,954
Balance at Beginning of Period at Mar. 31, 2017				(28,866)
Increase (decrease) in shareholders' equity
Net Income	$ 44,662
Other Comprehensive Income	3,253			3,253
Balance at End of Period at Jun. 30, 2017	$ 1,213,757	$ 576	$ 556,409	$ (25,613)	$ 1,468,328	$ (785,943)
Ending Balance (in shares) at Jun. 30, 2017	42,655,954	42,655,954