Consolidated Statements of Changes in Shareholders' Equity - USD ($) $ in Thousands	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss), Net of Taxes	Retained Earnings Revision of Prior Period, Accounting Standards Update, Adjustment	Retained Earnings	Treasury Stock	Revision of Prior Period, Accounting Standards Update, Adjustment	Total
Balance at Dec. 31, 2018	$ 176	$ 196,292	$ (10,706)		$ 616,717	$ (83,835)		$ 718,644
Net income (loss)					29,946			29,946
Unrealized gains (losses) on securities available for sale, net of deferred federal income taxes			20,053					20,053
Restricted share awards issued	1	462						463
Recognition of employee share-based compensation, net of deferred federal income taxes		1,260						1,260
Dividends paid and accrued					(12,300)			(12,300)
Balance at Mar. 31, 2019	177	198,014	9,347		631,990	(83,835)		755,693
Balance at Dec. 31, 2018	176	196,292	(10,706)		616,717	(83,835)		718,644
Net income (loss)								55,880
Acquisition of treasury stock								0
Balance at Jun. 30, 2019	177	199,644	22,141		645,553	(83,835)		783,680
Cumulative effect of adoption of updated accounting guidance for callable debt securities at January 1, 2019, net of taxes				$ (2,373)			$ (2,373)
Balance at Mar. 31, 2019	177	198,014	9,347		631,990	(83,835)		755,693
Net income (loss)					25,934			25,934
Unrealized gains (losses) on securities available for sale, net of deferred federal income taxes			12,794					12,794
Recognition of employee share-based compensation, net of deferred federal income taxes		1,630						1,630
Dividends paid and accrued					(12,371)			(12,371)
Balance at Jun. 30, 2019	177	199,644	22,141		645,553	(83,835)		783,680
Cumulative effect of adoption of updated accounting guidance for callable debt securities at January 1, 2019, net of taxes								661,553
Balance at Dec. 31, 2019	177	202,321	28,190		661,553	(83,835)		808,406
Net income (loss)					(1,990)			(1,990)
Unrealized gains (losses) on securities available for sale, net of deferred federal income taxes			(21,786)					(21,786)
Restricted share awards issued		528						528
Recognition of employee share-based compensation, net of deferred federal income taxes		1,215						1,215
Dividends paid and accrued					(13,872)			(13,872)
Acquisition of treasury stock						(10,392)		(10,392)
Balance at Mar. 31, 2020	177	204,064	6,404		645,691	(94,227)		762,109
Balance at Dec. 31, 2019	177	202,321	28,190		661,553	(83,835)		808,406
Net income (loss)								40,504
Acquisition of treasury stock								(22,145)
Balance at Jun. 30, 2020	177	206,130	42,302		674,349	(105,980)		816,978
Balance at Mar. 31, 2020	177	204,064	6,404		645,691	(94,227)		762,109
Net income (loss)					42,494			42,494
Unrealized gains (losses) on securities available for sale, net of deferred federal income taxes			35,898					35,898
Recognition of employee share-based compensation, net of deferred federal income taxes		2,066						2,066
Dividends paid and accrued					(13,836)			(13,836)
Acquisition of treasury stock						(11,753)		(11,753)
Balance at Jun. 30, 2020	$ 177	$ 206,130	$ 42,302		$ 674,349	$ (105,980)		816,978
Cumulative effect of adoption of updated accounting guidance for callable debt securities at January 1, 2019, net of taxes								$ 674,349