Condensed Consolidated Statements of Changes in Equity - USD ($) $ in Thousands	Total	Common Stock	Capital in excess of par value	Cumulative net income	Cumulative dividends	Accumulated other comprehensive income/(loss)
Balance at Dec. 31, 2019	$ 181,734	$ 138	$ 266,723	$ 661,280	$ (747,417)	$ 1,010
Balance, Shares at Dec. 31, 2019		13,757
Shares of Beneficial Interest:
Issued, net	(317)		(317)
Issued (in shares)		14
Restricted stock-based compensation expense	665		665
Dividends	(28,426)				(28,426)
Comprehensive income:
Net income	14,447			14,447
Unrealized net gain (loss) on cash flow hedges	(5,709)					(5,709)
Subtotal - comprehensive income	8,738			14,447		(5,709)
Balance at Sep. 30, 2020	162,394	$ 138	267,071	675,727	(775,843)	(4,699)
Balance, Shares at Sep. 30, 2020		13,771
Balance at Jun. 30, 2020	166,240	$ 138	266,843	670,534	(766,342)	(4,933)
Balance, Shares at Jun. 30, 2020		13,770
Shares of Beneficial Interest:
Issued, net	(20)		(20)
Issued (in shares)		1
Restricted stock-based compensation expense	248		248
Dividends	(9,501)				(9,501)
Comprehensive income:
Net income	5,193			5,193
Unrealized net gain (loss) on cash flow hedges	234					234
Subtotal - comprehensive income	5,427			5,193		234
Balance at Sep. 30, 2020	162,394	$ 138	267,071	675,727	(775,843)	(4,699)
Balance, Shares at Sep. 30, 2020		13,771
Balance at Dec. 31, 2020	159,005	$ 138	267,368	680,727	(785,413)	(3,815)
Balance, Shares at Dec. 31, 2020		13,771
Shares of Beneficial Interest:
Issued, net	161		161
Issued (in shares)		13
Repurchased	(16)		(16)
Restricted stock-based compensation expense	719		719
Dividends	(28,868)				(28,868)
Comprehensive income:
Net income	17,551			17,551
Unrealized net gain (loss) on cash flow hedges	3,259					3,259
Subtotal - comprehensive income	20,810			17,551		3,259
Balance at Sep. 30, 2021	151,811	$ 138	268,232	698,278	(814,281)	(556)
Balance, Shares at Sep. 30, 2021		13,784
Balance at Jun. 30, 2021	155,364	$ 138	267,951	692,934	(804,632)	(1,027)
Balance, Shares at Jun. 30, 2021		13,783
Shares of Beneficial Interest:
Issued, net	55		55
Issued (in shares)		1
Restricted stock-based compensation expense	226		226
Dividends	(9,649)				(9,649)
Comprehensive income:
Net income	5,344			5,344
Unrealized net gain (loss) on cash flow hedges	471					471
Subtotal - comprehensive income	5,815			5,344		471
Balance at Sep. 30, 2021	$ 151,811	$ 138	$ 268,232	$ 698,278	$ (814,281)	$ (556)
Balance, Shares at Sep. 30, 2021		13,784