Discontinued Operations (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended		12 Months Ended	3 Months Ended		9 Months Ended		21 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011 Properties	Sep. 30, 2012 Discontinued Operations [Member]	Sep. 30, 2011 Discontinued Operations [Member]	Sep. 30, 2012 Discontinued Operations [Member] Properties	Sep. 30, 2011 Discontinued Operations [Member]	Sep. 30, 2012 Discontinued Operations [Member] Properties
Revenue:
Rents	$ 113,989	$ 116,251	$ 343,260	$ 346,579		$ 271	$ 5,450	$ 6,384	$ 16,742
Tenant Reimbursements	20,695	19,010	58,308	59,187		(16)	797	742	2,728
Gain (Loss) on Contract Termination	931	157	2,522	2,673		4	33	11	33
Other	1,761	955	4,259	3,314		(1)	(2)	88	0
Total revenue	140,383	139,401	417,370	420,267		258	6,278	7,225	19,503
Expenses:
Property operating expenses	40,705	40,788	118,068	124,023		154	1,573	1,855	5,212
Real estate taxes	14,064	13,204	42,292	40,699		29	659	755	2,105
Depreciation and amortization	48,738	51,943	147,618	157,157		6	2,296	2,750	6,857
Total operating expenses	110,885	113,491	329,999	344,585		189	4,528	5,360	14,174
Investment Income, Interest	311	523	2,635	1,385		0		2
Income from discontinued operations before gain on sale of interests in real estate	69	1,750	1,867	5,329		69	1,750	1,867	5,329
Net gain on disposition of discontinued operations	9,940	0	34,774	3,836		9,940	0	34,774	3,836
Income from discontinued operations	$ 10,009	$ 1,750	$ 36,641	$ 9,165		$ 10,009	$ 1,750	$ 36,641	$ 9,165
Number of properties sold (in properties)					2			14		18