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, Value at Dec. 31, 2016	$ 352,845	$ 5,173	$ 31,189	$ 308,995	$ 7,862	$ (374)
Beginning Balance, Shares at Dec. 31, 2016		5,173,000
Net income	42,327			42,327
Dividends paid	(43,474)			(43,474)
Change in fair value of marketable securities	(6,841)				(6,841)
Change in fair value of interest rate cap	(56)				(56)
Deferred stock unit grants	394		394
Other	0		(6)			6
Ending Balance, Value at Jun. 30, 2017	345,195	$ 5,173	31,577	307,848	965	(368)
Ending Balance, Shares at Jun. 30, 2017		5,173,000
Beginning Balance, Value at Dec. 31, 2017	$ 343,955	$ 5,173	31,577	302,543	5,030	(368)
Beginning Balance, Shares at Dec. 31, 2017	5,173,450	5,173,000
Net income	$ 7,870			7,870
Dividends paid	(46,044)			(46,044)
Cumulative effect of change in accounting principle (see Note 3)	0			5,156	(5,156)
Change in fair value of marketable securities	0
Change in fair value of interest rate cap	(4)				(4)
Deferred stock unit grants	394		394
Ending Balance, Value at Jun. 30, 2018	$ 306,171	$ 5,173	$ 31,971	$ 269,525	$ (130)	$ (368)
Ending Balance, Shares at Jun. 30, 2018	5,173,450	5,173,000