Consolidated Statements of Changes in Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Capital	Retained Earnings	Accumulated Other Comprehensive Income	Treasury Stock
Opening Balance, at Dec. 31, 2013	$ 333,581	$ 5,173	$ 29,745	$ 297,515	$ 1,522	$ (374)
Opening Balance (in shares) at Dec. 31, 2013		5,173,000
Net income	67,925			67,925
Dividends paid	(66,436)			(66,436)
Change in unrealized net gain on available-for-sale securities	13,124				13,124
Change in value of interest rate cap	(189)				(189)
Deferred stock unit grant	394		394
Closing Balance (in shares) at Dec. 31, 2014		5,173,000
Closing Balance, at Dec. 31, 2014	348,399	$ 5,173	30,139	299,004	14,457	(374)
Net income	76,907			76,907
Dividends paid	(71,571)			(71,571)
Change in unrealized net gain on available-for-sale securities	(1,455)				(1,455)
Change in value of interest rate cap	0
Deferred stock unit grant	$ 600		600
Closing Balance (in shares) at Dec. 31, 2015	5,173,450	5,173,000
Closing Balance, at Dec. 31, 2015	$ 352,880	$ 5,173	30,739	304,340	13,002	(374)
Net income	86,477			86,477
Dividends paid	(81,822)			(81,822)
Change in unrealized net gain on available-for-sale securities	(5,273)				(5,273)
Change in value of interest rate cap	133				133
Deferred stock unit grant	$ 450		450
Closing Balance (in shares) at Dec. 31, 2016	5,173,450	5,173,000
Closing Balance, at Dec. 31, 2016	$ 352,845	$ 5,173	$ 31,189	$ 308,995	$ 7,862	$ (374)