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, 2012	$ 332,153	$ 5,173	$ 29,352	$ 296,797	$ 1,206	$ (375)
Opening Balance (in shares) at Dec. 31, 2012		5,173,000
Net income	56,915			56,915
Dividends paid	(56,197)			(56,197)
Change in unrealized net gain on available-for-sale securities	316				316
Change in value of interest rate cap	0
Deferred stock unit grant	394		394
Other			(1)			1
Closing Balance (in shares) at Dec. 31, 2013		5,173,000
Closing Balance, at Dec. 31, 2013	333,581	$ 5,173	29,745	297,515	1,522	(374)
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	5,173,450	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)