Consolidated Statements of Shareholders' Equity - USD ($) shares in Thousands, $ in Thousands	Common Stock	Additional Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss)	Treasury Stock	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			197,396			197,396
Dividends:
Payable/Cash			(72,839)			(72,839)
Purchase of treasury stock					(704)	(704)
Exercise of stock options	$ 102	2,130				2,232
Exercise of stock options (in shares)	102
Stock compensation expense recognized in earnings		391				391
Other comprehensive loss, net of tax:
Net change in unrealized gains and losses on available for sale securities, net of reclassification adjustments				(10,102)		(10,102)
Balance at Sep. 30, 2021	$ 96,343	151,855	2,414,183	10,723	(378,732)	2,294,372
Balance (in shares) at Sep. 30, 2021	96,343
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
Increase (decrease) in shareholders' equity
Net income			54,599			54,599
Dividends:
Payable/Cash			(38,026)			(38,026)
Purchase of treasury stock					(656)	(656)
Exercise of stock options	$ 11	225				236
Exercise of stock options (in shares)	11
Stock compensation expense recognized in earnings		118				118
Other comprehensive loss, net of tax:
Net change in unrealized gains and losses on available for sale securities, net of reclassification adjustments				(7,569)		(7,569)
Balance at Sep. 30, 2021	$ 96,343	151,855	2,414,183	10,723	(378,732)	2,294,372
Balance (in shares) at Sep. 30, 2021	96,343
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			194,820			194,820
Dividends:
Payable/Cash			(75,375)			(75,375)
Purchase of treasury stock					(51,939)	(51,939)
Exercise of stock options	$ 34	698				732
Exercise of stock options (in shares)	34
Stock compensation expense recognized in earnings		347				347
Other comprehensive loss, net of tax:
Net change in unrealized gains and losses on available for sale securities, net of reclassification adjustments				(422,792)		(422,792)
Balance at Sep. 30, 2022	$ 96,385	153,189	2,590,155	(454,772)	(430,683)	1,954,274
Balance (in shares) at Sep. 30, 2022	96,385
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
Increase (decrease) in shareholders' equity
Net income			83,356			83,356
Dividends:
Payable/Cash			(37,349)			(37,349)
Purchase of treasury stock					(11,390)	(11,390)
Exercise of stock options	$ 17	328				345
Exercise of stock options (in shares)	17
Stock compensation expense recognized in earnings		118				118
Other comprehensive loss, net of tax:
Net change in unrealized gains and losses on available for sale securities, net of reclassification adjustments				(178,037)		(178,037)
Balance at Sep. 30, 2022	$ 96,385	$ 153,189	$ 2,590,155	$ (454,772)	$ (430,683)	$ 1,954,274
Balance (in shares) at Sep. 30, 2022	96,385