Consolidated Statements of Changes in Equity (Unaudited) - USD ($) $ in Thousands	Total	Common Stock	Additional Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss)	Treasury Stock
Beginning Balance, Value at Dec. 31, 2016	$ 352,845	$ 5,173	$ 31,189	$ 308,995	$ 7,862	$ (374)
Beginning Balance, Shares at Dec. 31, 2016		5,173,000
Net income	62,626			62,626
Dividends paid	(65,218)			(65,218)
Change in fair value of marketable securities	(8,494)				(8,494)
Change in fair value of interest rate cap	(67)				(67)
Deferred stock unit grants	394		394
Other	0		(6)			6
Ending Balance, Value at Sep. 30, 2017	342,086	$ 5,173	31,577	306,403	(699)	(368)
Ending Balance, Shares at Sep. 30, 2017		5,173,000
Beginning Balance, Value at Dec. 31, 2017	$ 343,955	$ 5,173	31,577	302,543	5,030	(368)
Beginning Balance, Shares at Dec. 31, 2017	5,173,450	5,173,000
Net income	$ 22,873			22,873
Dividends paid	(69,072)			(69,072)
Cumulative effect of change in accounting principle (see Note 3)	0			5,156	(5,156)
Change in fair value of marketable securities	0
Change in fair value of interest rate cap	(5)				(5)
Deferred stock unit grants	394		394
Ending Balance, Value at Sep. 30, 2018	$ 298,145	$ 5,173	$ 31,971	$ 261,500	$ (131)	$ (368)
Ending Balance, Shares at Sep. 30, 2018	5,173,450	5,173,000