Consolidated Statements of Shareholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	Cumulative Effect, Period of Adoption, Adjustment	Common Stock	Capital Surplus	Accumulated Other Comprehensive Income (Loss)	Retained Earnings	Retained Earnings Cumulative Effect, Period of Adoption, Adjustment	Treasury Stock
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
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
Increase (decrease) in shareholders' equity
Net Income	167,770
Other Comprehensive Income	29,269				29,269
Balance at End of Period at Sep. 30, 2019	1,291,490		$ 579	580,200	(21,774)	1,730,437		(997,952)
Ending Balance (in shares) at Sep. 30, 2019			40,359,259
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
Increase (decrease) in shareholders' equity
Net Income	52,052					52,052
Other Comprehensive Income	5,650				5,650
Share-Based Compensation	2,224			2,224
Common Stock Issued under Purchase and Equity Compensation Plans	1,653			630		(271)		1,294
Common Stock Issued under Purchase and Equity Compensation Plans and Related Tax Benefits (in shares)			26,275
Common Stock Repurchased	(29,700)							(29,700)
Common Stock Repurchased (in shares)			(354,735)
Cash Dividends Declared (per share)	(26,337)					(26,337)
Balance at End of Period at Sep. 30, 2019	1,291,490		$ 579	580,200	(21,774)	1,730,437		(997,952)
Ending Balance (in shares) at Sep. 30, 2019			40,359,259
Balance at Beginning of Period at Dec. 31, 2019	$ 1,286,832	$ 3,632	$ 579	582,566	(31,112)	1,761,415	$ 3,632	(1,026,616)
Beginning Balance (in shares) at Dec. 31, 2019	40,039,695		40,039,695
Increase (decrease) in shareholders' equity
Net Income	$ 34,742					34,742
Other Comprehensive Income	$ 41,933				41,933
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member
Share-Based Compensation	$ 1,497			1,497
Common Stock Issued under Purchase and Equity Compensation Plans	3,761			329		653		2,779
Common Stock Issued under Purchase and Equity Compensation Plans and Related Tax Benefits (in shares)			154,091
Common Stock Repurchased	(17,633)							(17,633)
Common Stock Repurchased (in shares)			(197,276)
Cash Dividends Declared (per share)	(26,835)					(26,835)
Balance at End of Period at Mar. 31, 2020	1,327,929		$ 579	584,392	10,821	1,773,607		(1,041,470)
Ending Balance (in shares) at Mar. 31, 2020			39,996,510
Balance at Beginning of Period at Dec. 31, 2019	$ 1,286,832	$ 3,632	$ 579	582,566	(31,112)	1,761,415	$ 3,632	(1,026,616)
Beginning Balance (in shares) at Dec. 31, 2019	40,039,695		40,039,695
Increase (decrease) in shareholders' equity
Net Income	$ 111,490
Other Comprehensive Income	45,511				45,511
Balance at End of Period at Sep. 30, 2020	$ 1,361,739		$ 580	588,632	14,399	1,797,763		(1,039,635)
Ending Balance (in shares) at Sep. 30, 2020	40,060,675		40,060,675
Balance at Beginning of Period at Mar. 31, 2020	$ 1,327,929		$ 579	584,392	10,821	1,773,607		(1,041,470)
Beginning Balance (in shares) at Mar. 31, 2020			39,996,510
Increase (decrease) in shareholders' equity
Net Income	38,908					38,908
Other Comprehensive Income	8,104				8,104
Share-Based Compensation	2,207			2,207
Common Stock Issued under Purchase and Equity Compensation Plans	1,926		$ 1	347		680		898
Common Stock Issued under Purchase and Equity Compensation Plans and Related Tax Benefits (in shares)			53,672
Common Stock Repurchased	(148)							(148)
Common Stock Repurchased (in shares)			(2,488)
Cash Dividends Declared (per share)	(26,844)					(26,844)
Balance at End of Period at Jun. 30, 2020	1,352,082		$ 580	586,946	18,925	1,786,351		(1,040,720)
Ending Balance (in shares) at Jun. 30, 2020			40,047,694
Increase (decrease) in shareholders' equity
Net Income	37,840					37,840
Other Comprehensive Income	(4,526)				(4,526)
Share-Based Compensation	1,643			1,643
Common Stock Issued under Purchase and Equity Compensation Plans	1,673			43		429		1,201
Common Stock Issued under Purchase and Equity Compensation Plans and Related Tax Benefits (in shares)			14,919
Common Stock Repurchased	(116)							(116)
Common Stock Repurchased (in shares)			(1,938)
Cash Dividends Declared (per share)	(26,857)					(26,857)
Balance at End of Period at Sep. 30, 2020	$ 1,361,739		$ 580	$ 588,632	$ 14,399	$ 1,797,763		$ (1,039,635)
Ending Balance (in shares) at Sep. 30, 2020	40,060,675		40,060,675