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		13,757,000
Shares of Beneficial Interest:
Issued, net	(297)		(297)
Issued (in shares)		13
Restricted stock-based compensation expense	417		417
Dividends and dividend equivalents	(18,925)				(18,925)
Comprehensive income:
Net income	9,254			9,254
Unrealized net gain (loss) on cash flow hedges	(5,943)					(5,943)
Subtotal - comprehensive income	3,311			9,254		(5,943)
Balance at Jun. 30, 2020	166,240	$ 138	266,843	670,534	(766,342)	(4,933)
Balance, Shares at Jun. 30, 2020		13,770,000
Balance at Mar. 31, 2020	172,440	$ 138	266,970	665,834	(756,841)	(3,661)
Balance, Shares at Mar. 31, 2020		13,758,000
Shares of Beneficial Interest:
Issued, net	(347)		(347)
Issued (in shares)		12
Restricted stock-based compensation expense	220		220
Dividends and dividend equivalents	(9,501)				(9,501)
Comprehensive income:
Net income	4,700			4,700
Unrealized net gain (loss) on cash flow hedges	(1,272)					(1,272)
Subtotal - comprehensive income	3,428			4,700		(1,272)
Balance at Jun. 30, 2020	166,240	$ 138	266,843	670,534	(766,342)	(4,933)
Balance, Shares at Jun. 30, 2020		13,770,000
Balance at Dec. 31, 2020	159,005	$ 138	267,368	680,727	(785,413)	(3,815)
Balance, Shares at Dec. 31, 2020		13,771,000
Shares of Beneficial Interest:
Issued, net	106		106
Issued (in shares)		12
Repurchased	(16)		(16)
Restricted stock-based compensation expense	493		493
Dividends and dividend equivalents	(19,219)				(19,219)
Comprehensive income:
Net income	12,207			12,207
Unrealized net gain (loss) on cash flow hedges	2,788					2,788
Subtotal - comprehensive income	14,995			12,207		2,788
Balance at Jun. 30, 2021	155,364	$ 138	267,951	692,934	(804,632)	(1,027)
Balance, Shares at Jun. 30, 2021		13,783,000
Balance at Mar. 31, 2021	158,667	$ 138	267,667	686,313	(794,984)	(467)
Balance, Shares at Mar. 31, 2021		13,772,000
Shares of Beneficial Interest:
Issued, net	49		49
Issued (in shares)		11
Repurchased	(16)		(16)
Restricted stock-based compensation expense	251		251
Dividends and dividend equivalents	(9,648)				(9,648)
Comprehensive income:
Net income	6,621			6,621
Unrealized net gain (loss) on cash flow hedges	(560)					(560)
Subtotal - comprehensive income	6,061			6,621		(560)
Balance at Jun. 30, 2021	$ 155,364	$ 138	$ 267,951	$ 692,934	$ (804,632)	$ (1,027)
Balance, Shares at Jun. 30, 2021		13,783,000