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, 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
Increase (decrease) in shareholders' equity
Net Income	51,176				51,176
Other Comprehensive Loss	5,040			5,040
Share-Based Compensation	1,735		1,735
Common Stock Issued under Purchase and Equity Compensation Plans and Related Tax Benefits	6,577	$ 0	535		(702)	6,744
Common Stock Issued under Purchase and Equity Compensation Plans and Related Tax Benefits (in shares)		235,803
Common Stock Repurchased	(11,509)					(11,509)
Common Stock Repurchased (in shares)		(135,749)
Cash Dividends Declared (per share)	(21,419)				(21,419)
Balance at End of Period at Mar. 31, 2017	1,193,137	$ 576	553,898	(28,866)	1,444,495	(776,966)
Ending Balance (in shares) at Mar. 31, 2017		42,736,032
Balance at Beginning of Period at Dec. 31, 2017	$ 1,231,868	$ 576	561,161	(34,715)	1,512,218	(807,372)
Beginning Balance (in shares) at Dec. 31, 2017	42,401,443	42,401,443
Increase (decrease) in shareholders' equity
Net Income	$ 54,040				54,040
Other Comprehensive Loss	(8,905)			(8,905)
Reclassification of the Income Tax Effect of the Tax Reform from AOCI to RE	0			(7,477)	7,477
Share-Based Compensation	1,867		1,867
Common Stock Issued under Purchase and Equity Compensation Plans and Related Tax Benefits	1,951	$ 1	570		252	1,128
Common Stock Issued under Purchase and Equity Compensation Plans and Related Tax Benefits (in shares)		121,299
Common Stock Repurchased	(17,541)					(17,541)
Common Stock Repurchased (in shares)		(208,328)
Cash Dividends Declared (per share)	(22,087)				(22,087)
Balance at End of Period at Mar. 31, 2018	$ 1,241,193	$ 577	$ 563,598	$ (51,097)	$ 1,551,900	$ (823,785)
Ending Balance (in shares) at Mar. 31, 2018	42,314,414	42,314,414