Consolidated Statements of Changes in Shareholders' Equity - USD ($) $ in Thousands	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive (Loss) Income, Net of Taxes	Retained Earnings	Treasury Stock	Total
Balance at Dec. 31, 2020	$ 178	$ 209,779	$ 53,527	$ 745,029	$ (123,834)	$ 884,679
Net income				36,174		36,174
Unrealized gains (losses) on securities available for sale, net of deferred federal income taxes			(19,000)			(19,000)
Restricted share awards issued		475				475
Recognition of employee share-based compensation, net of deferred federal income taxes		1,384				1,384
Dividends paid and accrued				(13,459)		(13,459)
Balance at Mar. 31, 2021	178	211,638	34,527	767,744	(123,834)	890,253
Balance at Dec. 31, 2020	178	209,779	53,527	745,029	(123,834)	884,679
Net income						98,661
Balance at Sep. 30, 2021	178	214,576	33,909	803,248	(123,834)	928,077
Balance at Mar. 31, 2021	178	211,638	34,527	767,744	(123,834)	890,253
Net income				37,667		37,667
Unrealized gains (losses) on securities available for sale, net of deferred federal income taxes			4,428			4,428
Recognition of employee share-based compensation, net of deferred federal income taxes		1,441				1,441
Dividends paid and accrued				(13,490)		(13,490)
Balance at Jun. 30, 2021	178	213,079	38,955	791,921	(123,834)	920,299
Net income				24,820		24,820
Unrealized gains (losses) on securities available for sale, net of deferred federal income taxes			(5,046)			(5,046)
Recognition of employee share-based compensation, net of deferred federal income taxes		1,497				1,497
Dividends paid and accrued				(13,493)		(13,493)
Balance at Sep. 30, 2021	178	214,576	33,909	803,248	(123,834)	928,077
Balance at Dec. 31, 2021	178	216,070	24,579	821,743	(135,397)	927,173
Net income				7,838		7,838
Unrealized gains (losses) on securities available for sale, net of deferred federal income taxes			(47,357)			(47,357)
Restricted share awards issued	1	603				604
Recognition of employee share-based compensation, net of deferred federal income taxes		1,479				1,479
Dividends paid and accrued				(13,246)		(13,246)
Acquisition of treasury stock					(14,603)	(14,603)
Balance at Mar. 31, 2022	179	218,152	(22,778)	816,335	(150,000)	861,888
Balance at Dec. 31, 2021	178	216,070	24,579	821,743	(135,397)	927,173
Net income						21,931
Acquisition of treasury stock						(14,603)
Balance at Sep. 30, 2022	179	220,755	(90,457)	803,888	(150,000)	784,365
Balance at Mar. 31, 2022	179	218,152	(22,778)	816,335	(150,000)	861,888
Net income				7,901		7,901
Unrealized gains (losses) on securities available for sale, net of deferred federal income taxes			(39,186)			(39,186)
Recognition of employee share-based compensation, net of deferred federal income taxes		1,301				1,301
Dividends paid and accrued				(13,281)		(13,281)
Balance at Jun. 30, 2022	179	219,453	(61,964)	810,955	(150,000)	818,623
Net income				6,192		6,192
Unrealized gains (losses) on securities available for sale, net of deferred federal income taxes			(28,493)			(28,493)
Recognition of employee share-based compensation, net of deferred federal income taxes		1,302				1,302
Dividends paid and accrued				(13,259)		(13,259)
Balance at Sep. 30, 2022	$ 179	$ 220,755	$ (90,457)	$ 803,888	$ (150,000)	$ 784,365