Discontinued Operations (Details 1) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended									12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Mar. 31, 2010	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Revenues
Rental income										$ 301,055		$ 276,712		$ 239,037
Other operating										6,109		7,907		8,583
Total Revenue	81,260	79,528	78,551	77,011	75,259	74,259	71,358	70,716		316,350		291,592		249,997
Expenses
Property operating										117,683		113,083		98,101
General and administrative				400				200	200	20,908		20,990		16,886
Depreciation										85,122		75,292		64,016
Amortization										10,510		8,198		5,314
Bad debt, net of recoveries										243		(250)		(427)
Total Expenses										234,466		217,313		185,149
Other Income (Expense)
Interest expense										(75,053)		(76,038)		(65,710)
Interest and other income, net										981		819		2,402
Total other income (expense)										(74,072)		(77,205)		(63,788)
Income from discontinued operations										1,757		2,404		5,087
Impairments										(14,908)		(6,697)		(6,252)
Gain on sales of real estate properties										10,874		7,035		8,352
INCOME (LOSS) FROM DISCONTINUED OPERATIONS	(6,757)	3,328	102	1,050	1,178	604	522	438		(2,277)		2,742		7,187
Income (Loss) from Discontinued Operations per Common Share - Basic	$ (0.07)	$ 0.04	$ 0.00	$ 0.01	$ 0.02	$ 0.01	$ 0.01	$ 0.01		$ (0.03)		$ 0.04		$ 0.11
Income (Loss) from Discontinued Operations per Common Share - Diluted	$ (0.07)	$ 0.04	$ 0.00	$ 0.01	$ 0.02	$ 0.01	$ 0.01	$ 0.01		$ (0.03)		$ 0.04		$ 0.11
Discontinued Operations [Member]
Revenues
Rental income										5,679	[1]	10,656	[1]	14,835	[1]
Other operating										13	[1]	34	[1]	62	[1]
Total Revenue										5,692	[1]	10,690	[1]	14,897	[1]
Expenses
Property operating										2,500	[2]	4,503	[2]	5,487	[2]
General and administrative										6	[2]	8	[2]	19	[2]
Other operating										0	[2]	0	[2]	(135)	[2]
Depreciation										1,541	[2]	3,762	[2]	4,634	[2]
Amortization										54	[2]	28	[2]	28	[2]
Bad debt, net of recoveries										(3)	[2]	90	[2]	18	[2]
Total Expenses										4,098	[2]	8,391	[2]	10,051	[2]
Other Income (Expense)
Interest and other income, net										163	[3]	105	[3]	241	[3]
Total other income (expense)										163	[3]	105	[3]	241	[3]
Income from discontinued operations										1,757		2,404		5,087
Impairments										(14,908)	[4]	(6,697)	[4]	(6,252)	[4]
Gain on sales of real estate properties										10,874	[5]	7,035	[5]	8,352	[5]
INCOME (LOSS) FROM DISCONTINUED OPERATIONS										$ (2,277)		$ 2,742		$ 7,187
Income (Loss) from Discontinued Operations per Common Share - Basic										$ (0.03)		$ 0.04		$ 0.11
Income (Loss) from Discontinued Operations per Common Share - Diluted										$ (0.03)		$ 0.04		$ 0.11

[1] Total revenues for the years ended December 31, 2012, 2011 and 2010 included $5.4 million, $10.2 million and $13.8 million, respectively, related to properties sold; and $0.3 million, $0.5 million and $1.1 million, respectively, related to one property held for sale as of December 31, 2012.

[2] Total expenses for the years ended December 31, 2012, 2011 and 2010 included $3.4 million, $7.5 million and $3.4 million, respectively, related to properties sold; and $0.7 million, $0.9 million and $6.7 million, respectively, related to one property held for sale as of December 31, 2012.

[3] Other income (expense) for the years ended December 31, 2012, 2011, and 2010 included income related to properties sold.

[4] Impairments for the year ended December 31, 2012 included $11.1 million related to 12 properties sold and $3.8 million related to one property held for sale; December 31, 2011 included $1.7 million related to two properties sold and $5.0 million related to five properties held for sale; and December 31, 2010 included $1.0 million related to one property sold and $5.3 million related to four properties held for sale.

[5] Gain on sales of real estate properties for the years ended December 31, 2012, 2011 and 2010 included gains on the sale of seven, three and nine properties, respectively.