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, 2017	$ 210,561	$ 137	$ 265,335	$ 618,120	$ (673,175)	$ 144
Balance, Shares at Dec. 31, 2017		13,735
Shares of Beneficial Interest:
Issued, net	(45)		(45)
Issued, net (in shares)		9
Restricted stock-based compensation expense	254		254
Dividends ($1.355/share) for six months ended June 30, 2019, ($.68/share) for three months ended June 30, 2019, ($1.335/share) for six months ended June 30, 2018 and ($.67/share) for three months ended June 30, 2018	(18,343)				(18,343)
Comprehensive income:
Net income	15,409			15,409
Unrealized loss on interest rate cap	154					154
Subtotal - comprehensive income	15,563			15,409		154
Balance at Jun. 30, 2018	207,990	$ 137	265,544	633,529	(691,518)	298
Balance, Shares at Jun. 30, 2018		13,744
Balance at Mar. 31, 2018	211,356	$ 137	265,511	627,724	(682,309)	293
Balance, Shares at Mar. 31, 2018		13,736
Shares of Beneficial Interest:
Issued, net	(102)		(102)
Issued, net (in shares)		8
Restricted stock-based compensation expense	135		135
Dividends ($1.355/share) for six months ended June 30, 2019, ($.68/share) for three months ended June 30, 2019, ($1.335/share) for six months ended June 30, 2018 and ($.67/share) for three months ended June 30, 2018	(9,209)				(9,209)
Comprehensive income:
Net income	5,805			5,805
Unrealized loss on interest rate cap	5					5
Subtotal - comprehensive income	5,810			5,805		5
Balance at Jun. 30, 2018	207,990	$ 137	265,544	633,529	(691,518)	298
Balance, Shares at Jun. 30, 2018		13,744
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	(109)	$ 1	(110)
Issued, net (in shares)		10
Restricted stock-based compensation expense	331		331
Dividends ($1.355/share) for six months ended June 30, 2019, ($.68/share) for three months ended June 30, 2019, ($1.335/share) for six months ended June 30, 2018 and ($.67/share) for three months ended June 30, 2018	(18,633)				(18,633)
Comprehensive income:
Net income	8,473			8,473
Unrealized loss on interest rate cap	(132)					(132)
Subtotal - comprehensive income	8,341			8,473		$ (132)
Balance at Jun. 30, 2019	188,540	$ 138	266,252	650,789	(728,639)
Balance, Shares at Jun. 30, 2019		13,757
Balance at Mar. 31, 2019	193,627	$ 137	266,247	646,528	(719,285)
Balance, Shares at Mar. 31, 2019		13,748
Shares of Beneficial Interest:
Issued, net	(166)	$ 1	(167)
Issued, net (in shares)		9
Restricted stock-based compensation expense	172		172
Dividends ($1.355/share) for six months ended June 30, 2019, ($.68/share) for three months ended June 30, 2019, ($1.335/share) for six months ended June 30, 2018 and ($.67/share) for three months ended June 30, 2018	(9,354)				(9,354)
Comprehensive income:
Net income	4,261			4,261
Subtotal - comprehensive income	4,261			4,261
Balance at Jun. 30, 2019	$ 188,540	$ 138	$ 266,252	$ 650,789	$ (728,639)
Balance, Shares at Jun. 30, 2019		13,757