Consolidated Statements of Changes in Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss)	Treasury Stock
Balance at Dec. 31, 2015	$ 352,880	$ 5,173	$ 30,739	$ 304,340	$ 13,002	$ (374)
Balance (in shares) at Dec. 31, 2015		5,173,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	86,477			86,477
Dividends paid	(81,822)			(81,822)
Change in fair value of marketable securities	(5,273)				(5,273)
Change in value of interest rate cap	133				133
Deferred stock unit grants	450		450
Balance (in shares) at Dec. 31, 2016		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 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
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)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of change in accounting principle (see Note 2)	0			5,156	(5,156)
Net income	32,844			32,844
Dividends paid	(92,100)			(92,100)
Change in fair value of marketable securities	0
Change in value of interest rate cap	(1)				(1)
Deferred stock unit grants	$ 394		394
Balance (in shares) at Dec. 31, 2018	5,173,450	5,173,000
Balance at Dec. 31, 2018	$ 285,092	$ 5,173	$ 31,971	$ 248,443	$ (127)	$ (368)