Consolidated Statements of Changes in Shareholders' Equity - USD ($) $ in Thousands	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income, Net of Taxes	Retained Earnings	Treasury Stock	Total
Balance at Dec. 31, 2013	$ 172	$ 170,391	$ 17,200	$ 567,792	$ (60,368)	$ 695,187
Net income (loss)				59,354		59,354
Unrealized gains (losses) on securities available for sale, net of deferred federal income taxes			11,515			11,515
Restricted share awards issued	1	216				217
Recognition of employee share-based compensation, net of deferred federal income taxes		4,677				4,677
Exercise of options, net of federal income taxes		299				299
Dividends paid and accrued				(39,499)		(39,499)
Acquisition of treasury stock					(23,467)	(23,467)
Balance at Dec. 31, 2014	173	175,583	28,715	587,647	(83,835)	708,283
Net income (loss)				(13,853)		(13,853)
Unrealized gains (losses) on securities available for sale, net of deferred federal income taxes			(12,251)			(12,251)
Restricted share awards issued	1	246				247
Recognition of employee share-based compensation, net of deferred federal income taxes		3,787				3,787
Exercise of options, net of federal income taxes		280				280
Dividends paid and accrued				(41,994)		(41,994)
Balance at Dec. 31, 2015	174	179,896	16,464	531,800	(83,835)	644,499
Net income (loss)				64,585		64,585
Unrealized gains (losses) on securities available for sale, net of deferred federal income taxes			(621)			(621)
Restricted share awards issued		280				280
Recognition of employee share-based compensation, net of deferred federal income taxes		4,122				4,122
Exercise of options, net of federal income taxes		251				251
Dividends paid and accrued				(42,390)		(42,390)
Balance at Dec. 31, 2016	$ 174	$ 184,549	$ 15,843	$ 553,995	$ (83,835)	$ 670,726