Discontinued Operations (Details) (USD $) In Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Revenue:
Rents	$ 121,701	$ 116,520	$ 362,846	$ 343,568
Tenant reimbursements	19,806	19,784	61,831	57,834
Other	939	1,227	3,378	3,170
Total revenue	145,664	141,492	439,275	417,989
Expenses:
Property operating expenses	42,363	41,851	128,977	124,187
Real estate taxes	13,863	14,096	42,734	40,217
Depreciation and amortization	54,252	51,644	163,940	154,962
Total operating expenses	118,034	114,872	358,357	342,297
Income from discontinued operations before gain on sale of interests in real estate	30	385	93	1,060
Net gain on disposition of discontinued operations		(3)	3,836	6,346
Income from discontinued operations	30	382	3,929	7,406
Discontinued Operations [Member]
Revenue:
Rents		1,423	475	4,416
Tenant reimbursements	1	518	84	1,875
Other	14	37	(64)	39
Total revenue	15	1,978	495	6,330
Expenses:
Property operating expenses	(2)	751	258	2,357
Real estate taxes		293	70	924
Depreciation and amortization	(13)	549	74	1,989
Total operating expenses	$ (15)	$ 1,593	$ 402	$ 5,270