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	Treasury Stock	Total
Balance at Dec. 31, 2016	$ 174	$ 184,549	$ 15,843	$ 553,995	$ (83,835)	$ 670,726
Net income				62,387		62,387
Unrealized gains (losses) on securities available for sale, net of deferred federal income taxes			8,426			8,426
Restricted share awards issued	1	362				363
Recognition of employee share-based compensation, net of deferred federal income taxes		4,803				4,803
Dividends paid and accrued				(45,689)		(45,689)
Balance at Dec. 31, 2017	175	189,714	24,269	570,693	(83,835)	701,016
Reclassification of certain tax effects from accumulated other comprehensive income at January 1, 2018			4,736	(4,736)
Net income				83,195		83,195
Unrealized gains (losses) on securities available for sale, net of deferred federal income taxes			(22,816)			(22,816)
Restricted share awards issued	1	375				376
Recognition of employee share-based compensation, net of deferred federal income taxes		6,203				6,203
Dividends paid and accrued				(49,330)		(49,330)
Balance at Dec. 31, 2018	176	196,292	(10,706)	616,717	(83,835)	718,644
Cumulative effect of adoption of updated accounting guidance for equity financial instruments at January 1, 2018			(16,895)	16,895
Net income				99,601		99,601
Unrealized gains (losses) on securities available for sale, net of deferred federal income taxes			38,896			38,896
Restricted share awards issued	1	462				463
Recognition of employee share-based compensation, net of deferred federal income taxes		5,567				5,567
Dividends paid and accrued				(52,392)		(52,392)
Balance at Dec. 31, 2019	$ 177	$ 202,321	$ 28,190	661,553	$ (83,835)	808,406
Cumulative effect of adoption of updated accounting guidance for callable debt securities at January 1, 2019				$ (2,373)		$ (2,373)