Company Quarterly Financial Information (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Company Quarterly Financial Information [Abstract]
Revenues	$ 77,749	$ 78,612	$ 75,424	$ 73,531	$ 72,269	$ 72,895	$ 70,831	$ 69,143	$ 305,316	$ 285,138	$ 276,323
Net income (loss)	3,920	8,828	9,834	3,013	4,294	23,595	(33,753)	(1,127)	25,595	(6,991)	(11,489)
Noncontrolling interests	(19)	(34)	(88)	12	18	(62)	125	(50)
Dividends to preferred shareholders	(922)	(922)	(922)	(1,689)	(1,871)	(1,864)	(1,878)	(1,890)	(4,455)	(7,503)	(7,637)
Preferred stock redemption costs				(1,757)		1	(37)	(8)	1,757	44
Net income (loss) available to common shareholders	$ 2,979	$ 7,872	$ 8,824	$ (421)	$ 2,441	$ 21,670	$ (35,543)	$ (3,075)	$ 19,254	$ (14,507)	$ (10,860)
Net income (loss) available to common shareholders/unitholders - basic	$ (0.06)	$ 0.15	$ 0.18	$ (0.01)	$ 0.05	$ 0.44	$ (0.73)	$ (0.06)	$ 0.38	$ (0.30)	$ (0.24)
Net income (loss) available to common shareholders/unitholders - diluted	$ (0.06)	$ 0.15	$ 0.17	$ (0.01)	$ 0.05	$ 0.44	$ (0.73)	$ (0.06)	$ 0.38	$ (0.30)	$ (0.24)