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, 2015	$ 174	$ 179,896	$ 16,464	$ 531,800	$ (83,835)	$ 644,499
Net income				52,632		52,632
Unrealized gains on securities available for sale, net of deferred federal income taxes			18,446			18,446
Restricted share awards issued		280				280
Recognition of employee share-based compensation, net of deferred federal income taxes		2,970				2,970
Exercise of options, net of federal income taxes		251				251
Dividends paid and accrued				(31,775)		(31,775)
Balance at Sep. 30, 2016	174	183,397	34,910	552,657	(83,835)	687,303
Balance at Dec. 31, 2016	174	184,549	15,843	553,995	(83,835)	670,726
Net income				51,078		51,078
Unrealized gains on securities available for sale, net of deferred federal income taxes			10,528			10,528
Restricted share awards issued	1	362				363
Recognition of employee share-based compensation, net of deferred federal income taxes		3,665				3,665
Dividends paid and accrued				(33,521)		(33,521)
Balance at Sep. 30, 2017	$ 175	$ 188,576	$ 26,371	$ 571,552	$ (83,835)	$ 702,839