Condensed Consolidated Statements of Changes in Equity - USD ($) shares in Thousands, $ 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, 2018	$ 198,610	$ 137	$ 266,031	$ 642,316	$ (710,006)	$ 132
Balance, Shares at Dec. 31, 2018		13,747
Shares of Beneficial Interest:
Issued, net	(56)	$ 1	(57)
Issued (in shares)		10
Restricted stock-based compensation expense	504		504
Dividends	(27,988)				(27,988)
Comprehensive income:
Net income	13,126			13,126
Unrealized (loss)/gain on cash flow hedges	429					429
Subtotal - comprehensive income	13,555			13,126		429
Balance at Sep. 30, 2019	184,625	$ 138	266,478	655,442	(737,994)	561
Balance, Shares at Sep. 30, 2019		13,757
Balance at Jun. 30, 2019	188,540	$ 138	266,252	650,789	(728,639)
Balance, Shares at Jun. 30, 2019		13,757
Shares of Beneficial Interest:
Issued, net	53		53
Restricted stock-based compensation expense	173		173
Dividends	(9,355)				(9,355)
Comprehensive income:
Net income	4,653			4,653
Unrealized (loss)/gain on cash flow hedges	561					561
Subtotal - comprehensive income	5,214			4,653		561
Balance at Sep. 30, 2019	184,625	$ 138	266,478	655,442	(737,994)	561
Balance, Shares at Sep. 30, 2019		13,757
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 (loss)/gain 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 (loss)/gain 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