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, 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)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	7,870			7,870
Dividends paid	(46,044)			(46,044)
Cumulative effect of change in accounting principle	0			5,156	(5,156)
Change in fair value of interest rate cap	(4)				(4)
Deferred stock unit grants	394		394
Ending Balance, Shares at Jun. 30, 2018		5,173,000
Ending Balance, Value at Jun. 30, 2018	306,171	$ 5,173	31,971	269,525	(130)	(368)
Beginning Balance, Shares at Mar. 31, 2018		5,173,000
Beginning Balance, Value at Mar. 31, 2018	311,281	$ 5,173	31,577	274,977	(78)	(368)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	17,570			17,570
Dividends paid	(23,022)			(23,022)
Change in fair value of interest rate cap	(52)				(52)
Deferred stock unit grants	394		394
Ending Balance, Shares at Jun. 30, 2018		5,173,000
Ending Balance, Value at Jun. 30, 2018	$ 306,171	$ 5,173	31,971	269,525	(130)	(368)
Beginning Balance, Shares at Dec. 31, 2018	5,173,450	5,173,000
Beginning 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	29,148			29,148
Dividends paid	(46,056)			(46,056)
Change in fair value of interest rate cap	32				32
Deferred stock unit grants	$ 394		394
Ending Balance, Shares at Jun. 30, 2019	5,173,450	5,173,000
Ending Balance, Value at Jun. 30, 2019	$ 268,610	$ 5,173	32,365	231,535	(95)	(368)
Beginning Balance, Shares at Mar. 31, 2019		5,173,000
Beginning Balance, Value at Mar. 31, 2019	279,942	$ 5,173	31,971	243,280	(114)	(368)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	11,283			11,283
Dividends paid	(23,028)			(23,028)
Change in fair value of interest rate cap	19				19
Deferred stock unit grants	$ 394		394
Ending Balance, Shares at Jun. 30, 2019	5,173,450	5,173,000
Ending Balance, Value at Jun. 30, 2019	$ 268,610	$ 5,173	$ 32,365	$ 231,535	$ (95)	$ (368)