Consolidated Statements of Shareholders' Equity - USD ($) shares in Thousands, $ in Thousands	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Treasury Stock [Member]	Total
Balance at Dec. 31, 2020	$ 96,241	$ 149,334	$ 2,289,626	$ 20,825	$ (378,028)	$ 2,177,998
Balance (in shares) at Dec. 31, 2020	96,241
Increase (decrease) in shareholders' equity
Net income			142,797			142,797
Dividends:
Cash			(34,813)			(34,813)
Purchase of treasury stock					(48)	(48)
Exercise of stock options	$ 91	1,905				1,996
Exercise of stock options (in shares)	91
Stock compensation expense recognized in earnings		273				273
Other comprehensive loss, net of tax:
Net change in unrealized gains and losses on available for sale securities, net of reclassification adjustments				(2,533)		(2,533)
Balance at Jun. 30, 2021	$ 96,332	151,512	2,397,610	18,292	(378,076)	2,285,670
Balance (in shares) at Jun. 30, 2021	96,332
Balance at Mar. 31, 2021	$ 96,312	150,972	2,305,584	17,813	(378,076)	2,192,605
Balance (in shares) at Mar. 31, 2021	96,312
Increase (decrease) in shareholders' equity
Net income			92,026			92,026
Dividends:
Exercise of stock options	$ 20	426				446
Exercise of stock options (in shares)	20
Stock compensation expense recognized in earnings		114				114
Other comprehensive loss, net of tax:
Net change in unrealized gains and losses on available for sale securities, net of reclassification adjustments				479		479
Balance at Jun. 30, 2021	$ 96,332	151,512	2,397,610	18,292	(378,076)	2,285,670
Balance (in shares) at Jun. 30, 2021	96,332
Balance at Dec. 31, 2021	$ 96,351	152,144	2,470,710	(31,980)	(378,744)	2,308,481
Balance (in shares) at Dec. 31, 2021	96,351
Increase (decrease) in shareholders' equity
Net income			111,464			111,464
Dividends:
Cash			(38,026)			(38,026)
Purchase of treasury stock					(40,549)	(40,549)
Exercise of stock options	$ 17	370				387
Exercise of stock options (in shares)	17
Stock compensation expense recognized in earnings		229				229
Other comprehensive loss, net of tax:
Net change in unrealized gains and losses on available for sale securities, net of reclassification adjustments				(244,755)		(244,755)
Balance at Jun. 30, 2022	$ 96,368	152,743	2,544,148	(276,735)	(419,293)	2,097,231
Balance (in shares) at Jun. 30, 2022	96,368
Balance at Mar. 31, 2022	$ 96,357	152,405	2,486,177	(205,136)	(382,124)	2,147,679
Balance (in shares) at Mar. 31, 2022	96,357
Increase (decrease) in shareholders' equity
Net income			57,971			57,971
Dividends:
Purchase of treasury stock					(37,169)	(37,169)
Exercise of stock options	$ 11	230				241
Exercise of stock options (in shares)	11
Stock compensation expense recognized in earnings		108				108
Other comprehensive loss, net of tax:
Net change in unrealized gains and losses on available for sale securities, net of reclassification adjustments				(71,599)		(71,599)
Balance at Jun. 30, 2022	$ 96,368	$ 152,743	$ 2,544,148	$ (276,735)	$ (419,293)	$ 2,097,231
Balance (in shares) at Jun. 30, 2022	96,368