Consolidated Statements of Changes in Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Capital	Retained Earnings	Accumulated Other Comprehensive Income	Treasury Stock
Balance at Dec. 31, 2014	$ 348,399	$ 5,173	$ 30,139	$ 299,004	$ 14,457	$ (374)
Balance (in shares) at Dec. 31, 2014		5,173,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
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
Balance (in shares) at Dec. 31, 2015		5,173,000
Balance at Dec. 31, 2015	352,880	$ 5,173	30,739	304,340	13,002	(374)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
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
Balance (in shares) at Dec. 31, 2016	5,173,450	5,173,000
Balance at Dec. 31, 2016	$ 352,845	$ 5,173	31,189	308,995	7,862	(374)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	80,509			80,509
Dividends paid	(86,961)			(86,961)
Change in unrealized net gain on available-for-sale securities	(2,762)				(2,762)
Change in value of interest rate cap	(70)				(70)
Deferred stock unit grant	394		394
Other	$ 0		(6)			6
Balance (in shares) at Dec. 31, 2017	5,173,450	5,173,000
Balance at Dec. 31, 2017	$ 343,955	$ 5,173	$ 31,577	$ 302,543	$ 5,030	$ (368)