Consolidated Statements of Changes in Equity (Unaudited) - USD ($) $ in Thousands	Total	Common Stock	Additional Capital	Retained   Earnings	Accumulated  Other Comprehensive (Loss) Income	Treasury Stock
Beginning Balance, Shares at Dec. 31, 2019		5,173,000
Beginning 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	16,903			16,903
Dividends paid	(46,068)			(46,068)
Change in fair value of interest rate derivatives	21				21
Deferred stock unit grants	600		600
Ending Balance, Shares at Jun. 30, 2020		5,173,000
Ending Balance, Value at Jun. 30, 2020	224,971	$ 5,173	32,965	187,229	(28)	(368)
Beginning Balance, Shares at Mar. 31, 2020		5,173,000
Beginning Balance, Value at Mar. 31, 2020	235,078	$ 5,173	32,365	197,932	(24)	(368)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	12,331			12,331
Dividends paid	(23,034)			(23,034)
Change in fair value of interest rate derivatives	(4)				(4)
Deferred stock unit grants	600		600
Ending Balance, Shares at Jun. 30, 2020		5,173,000
Ending Balance, Value at Jun. 30, 2020	$ 224,971	$ 5,173	32,965	187,229	(28)	(368)
Beginning Balance, Shares at Dec. 31, 2020	5,173,450	5,173,000
Beginning Balance, Value at Dec. 31, 2020	$ 203,228	$ 5,173	32,965	166,165	(707)	(368)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	43,780			43,780
Dividends paid	(46,100)			(46,100)
Change in fair value of interest rate derivatives	4,447				4,447
Deferred stock unit grants	$ 450		450
Ending Balance, Shares at Jun. 30, 2021	5,173,450	5,173,000
Ending Balance, Value at Jun. 30, 2021	$ 205,805	$ 5,173	33,415	163,845	3,740	(368)
Beginning Balance, Shares at Mar. 31, 2021		5,173,000
Beginning Balance, Value at Mar. 31, 2021	203,258	$ 5,173	32,965	160,997	4,491	(368)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	25,898			25,898
Dividends paid	(23,050)			(23,050)
Change in fair value of interest rate derivatives	(751)				(751)
Deferred stock unit grants	$ 450		450
Ending Balance, Shares at Jun. 30, 2021	5,173,450	5,173,000
Ending Balance, Value at Jun. 30, 2021	$ 205,805	$ 5,173	$ 33,415	$ 163,845	$ 3,740	$ (368)