Consolidated Statements of Changes in Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss)	Treasury Stock	Cumulative Effect, Period of Adoption, Adjustment	Cumulative Effect, Period of Adoption, Adjustment Retained Earnings	Cumulative Effect, Period of Adoption, Adjustment Accumulated Other Comprehensive Income (Loss)
Beginning Balance (in shares) at Dec. 31, 2017		5,173,000
Beginning Balance, Value at Dec. 31, 2017	$ 343,955	$ 5,173	$ 31,577	$ 302,543	$ 5,030	$ (368)	$ 0	$ 5,156	$ (5,156)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	32,844			32,844
Dividends paid	(92,100)			(92,100)
Change in fair value of interest rate derivatives	(1)				(1)
Deferred stock unit grants	394		394
Ending Balance (in shares) at Dec. 31, 2018		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 interest rate derivatives	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)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	41,939			41,939
Dividends paid	(92,168)			(92,168)
Change in fair value of interest rate derivatives	(658)				(658)
Deferred stock unit grants	$ 600		600
Ending Balance (in shares) at Dec. 31, 2020	5,173,450	5,173,000
Ending Balance, Value at Dec. 31, 2020	$ 203,228	$ 5,173	$ 32,965	$ 166,165	$ (707)	$ (368)