Discontinued Operations (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended				12 Months Ended			36 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	Dec. 31, 2012 Properties	Dec. 31, 2011	Dec. 31, 2010	Feb. 25, 2013 sqft	Dec. 31, 2012 Discontinued Operations [Member] sqft	Dec. 31, 2011 Discontinued Operations [Member]	Dec. 31, 2010 Discontinued Operations [Member]	Dec. 31, 2012 Discontinued Operations [Member] Properties sqft
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Income from discontinued operations									$ 36,683	$ 14,250	$ 19,719		$ 36,683	$ 14,250	$ 19,719
Revenue:
Rents									459,855	462,771	443,485		6,384	21,608	26,979
Tenant Reimbursements									78,613	79,108	75,330		800	3,326	5,540
Gain (Loss) on Contract Termination									3,233	2,951	5,516		11	75	250
Other									6,016	5,387	4,271		72	3	109
Total revenue	142,463	140,383	137,576	139,411	141,486	139,401	139,249	141,617	559,833	561,753	540,432		7,267	25,012	32,878
Expenses:
Property operating expenses									159,296	167,077	163,294		1,857	6,631	9,258
Real estate taxes									55,969	54,171	51,927		753	2,618	3,453
Depreciation and amortization									195,841	210,334	203,345		2,750	8,777	11,432
Total operating expenses									441,646	461,774	447,738		5,360	18,026	24,143
Investment Income, Interest									3,012	1,813	3,218		2	0	4
Income from discontinued operations before gain on sale of interests in real estate									1,909	6,986	8,739		1,909	6,986	8,739
Net gain on disposition of discontinued operations									$ 34,774	$ 7,264	$ 10,980		$ 34,774	$ 7,264	$ 10,980
Net Rentable Space Sold												800,556	1,800,000			1,800,000
Number of properties sold (in properties)									2							26