Consolidated Statements of Changes in Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss)	Treasury Stock
Begining Balance (in shares) at Dec. 31, 2016		5,173,000
Beginning Balance, Value 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 fair value of marketable securities	(2,762)				(2,762)
Change in value of interest rate cap	(70)				(70)
Deferred stock unit grants	394		394
Other	0		(6)			6
Ending Balance (in shares) at Dec. 31, 2017		5,173,000
Ending Balance, Value at Dec. 31, 2017	343,955	$ 5,173	31,577	302,543	5,030	(368)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	32,844			32,844
Dividends paid	(92,100)			(92,100)
Cumulative effect of change in accounting principle	0			5,156	(5,156)
Change in fair value of marketable securities	0
Change in value of interest rate cap	(1)				(1)
Deferred stock unit grants	$ 394		394
Ending Balance (in shares) at Dec. 31, 2018	5,173,450	5,173,000
Ending Balance, Value at Dec. 31, 2018	$ 285,092	$ 5,173	31,971	248,443	(127)	(368)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	60,075			60,075
Dividends paid	(92,124)			(92,124)
Change in fair value of marketable securities	0
Change in value of interest rate cap	78				78
Deferred stock unit grants	$ 394		394
Ending Balance (in shares) at Dec. 31, 2019	5,173,450	5,173,000
Ending Balance, Value at Dec. 31, 2019	$ 253,515	$ 5,173	$ 32,365	$ 216,394	$ (49)	$ (368)