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, 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
Increase (decrease) in shareholders' equity
Net Income	54,040				54,040
Other Comprehensive Income	(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	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
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
Increase (decrease) in shareholders' equity
Net Income	108,758
Other Comprehensive Income	(11,663)			(11,663)
Reclassification of the Income Tax Effect of the Tax Reform from AOCI to RE				(7,477)
Balance at End of Period at Jun. 30, 2018	1,247,717	$ 577	566,436	(53,855)	1,581,168	(846,609)
Ending Balance (in shares) at Jun. 30, 2018		42,084,066
Balance at Beginning of Period at Mar. 31, 2018	1,241,193	$ 577	563,598	(51,097)	1,551,900	(823,785)
Beginning Balance (in shares) at Mar. 31, 2018		42,314,414
Increase (decrease) in shareholders' equity
Net Income	54,718				54,718
Other Comprehensive Income	(2,758)			(2,758)
Reclassification of the Income Tax Effect of the Tax Reform from AOCI to RE	0			0	0
Share-Based Compensation	2,188		2,188
Common Stock Issued under Purchase and Equity Compensation Plans	2,428	$ 0	650		(86)	1,864
Common Stock Issued under Purchase and Equity Compensation Plans and Related Tax Benefits (in shares)		58,345
Common Stock Repurchased	(24,688)					24,688
Common Stock Repurchased (in shares)		(288,693)
Cash Dividends Declared (per share)	(25,364)				(25,364)
Balance at End of Period at Jun. 30, 2018	1,247,717	$ 577	566,436	(53,855)	1,581,168	(846,609)
Ending Balance (in shares) at Jun. 30, 2018		42,084,066
Balance at Beginning of Period at Dec. 31, 2018	$ 1,268,200	$ 577	571,704	(51,043)	1,641,314	(894,352)
Beginning Balance (in shares) at Dec. 31, 2018	41,499,898	41,499,898
Increase (decrease) in shareholders' equity
Net Income	$ 58,799				58,799
Other Comprehensive Income	7,165			7,165
Share-Based Compensation	2,274		2,274
Common Stock Issued under Purchase and Equity Compensation Plans	2,087	$ 1	616		(203)	1,673
Common Stock Issued under Purchase and Equity Compensation Plans and Related Tax Benefits (in shares)		131,529
Common Stock Repurchased	(43,189)					(43,189)
Common Stock Repurchased (in shares)		(552,739)
Cash Dividends Declared (per share)	(25,646)				(25,646)
Balance at End of Period at Mar. 31, 2019	1,269,690	$ 578	574,594	(43,878)	1,674,264	(935,868)
Ending Balance (in shares) at Mar. 31, 2019		41,078,688
Balance at Beginning of Period at Dec. 31, 2018	$ 1,268,200	$ 577	571,704	(51,043)	1,641,314	(894,352)
Beginning Balance (in shares) at Dec. 31, 2018	41,499,898	41,499,898
Increase (decrease) in shareholders' equity
Net Income	$ 115,718
Other Comprehensive Income	23,619			23,619
Balance at End of Period at Jun. 30, 2019	$ 1,285,948	$ 579	577,346	(27,424)	1,704,993	(969,546)
Ending Balance (in shares) at Jun. 30, 2019	40,687,719	40,687,719
Balance at Beginning of Period at Mar. 31, 2019	$ 1,269,690	$ 578	574,594	(43,878)	1,674,264	(935,868)
Beginning Balance (in shares) at Mar. 31, 2019		41,078,688
Increase (decrease) in shareholders' equity
Net Income	56,919				56,919
Other Comprehensive Income	16,454			16,454
Share-Based Compensation	2,164		2,164
Common Stock Issued under Purchase and Equity Compensation Plans	2,262	$ 1	588		365	1,308
Common Stock Issued under Purchase and Equity Compensation Plans and Related Tax Benefits (in shares)		43,180
Common Stock Repurchased	(34,986)					(34,986)
Common Stock Repurchased (in shares)		(434,149)
Cash Dividends Declared (per share)	(26,555)				(26,555)
Balance at End of Period at Jun. 30, 2019	$ 1,285,948	$ 579	$ 577,346	$ (27,424)	$ 1,704,993	$ (969,546)
Ending Balance (in shares) at Jun. 30, 2019	40,687,719	40,687,719