Discontinued Operations (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012 Properties	Mar. 31, 2011
Revenue:
Rents	$ 116,296	$ 117,362
Tenant Reimbursements	19,332	22,532
Other	1,534	1,083
Total revenue	141,801	144,298
Expenses:
Property operating expenses	40,197	45,002
Real estate taxes	14,333	13,958
Depreciation and amortization	50,502	50,295
Total operating expenses	112,332	117,009
Income from discontinued operations before gain on sale of interests in real estate	557	1,077
Net gain on disposition of discontinued operations	14,668	0
Income from discontinued operations	15,225	1,077
Number of properties sold (in properties)	2
Discontinued Operations [Member]
Revenue:
Rents	2,053	3,643
Tenant Reimbursements	91	589
Other	1	(63)
Total revenue	2,145	4,169
Expenses:
Property operating expenses	367	1,176
Real estate taxes	270	490
Depreciation and amortization	951	1,426
Total operating expenses	1,588	3,092
Income from discontinued operations before gain on sale of interests in real estate	557
Net gain on disposition of discontinued operations	14,668
Income from discontinued operations	$ 15,225	$ 1,077
Number of properties sold (in properties)	2	6