Discontinued Operations Discontinued Operations (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Revenue:
Rents	$ 116,479	$ 108,658	$ 347,680	$ 327,362
Other Real Estate Revenue	975	1,630	3,305	4,029
Tenant reimbursements	20,699	20,294	60,615	57,179
Total revenue	143,456	134,520	424,198	400,113
Expenses:
Property operating expenses	40,661	38,783	119,522	112,961
Real estate taxes	14,190	13,486	42,706	40,430
Depreciation and amortization	50,613	46,955	149,423	142,262
Total operating expenses	113,217	106,602	336,078	317,674
Interest income	268	311	448	2,631
Income from discontinued operations before gain on sale of interests in real estate	(70)	1,649	590	6,803
Net gain on disposition of discontinued operations	(16)	9,940	3,029	34,774
Income (Loss) from discontinued operations	(86)	11,589	3,619	41,577
Assets Held-for-sale [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Assets Held-for-sale, Long Lived	3,724		3,724
Real Estate Accumulated Depreciation Related to Properties Held for Sal	(1,399)		(1,399)
Operating Real Estate Investment Properties Held for Sale, Net	2,325		2,325
Development In Process Related to Properties Held-for-sale	0		0
Real Estate Investment Properties Held for Sale, Net	2,325		2,325
Other Assets Related to Assets Held-for-Sale	46		46
Assets Held-for-sale, Current	2,371		2,371
Discontinued Operations [Member]
Revenue:
Rents	0	5,602	4,129	22,282
Termination fees		4		11
Other Real Estate Revenue	0	129	123	318
Tenant reimbursements	3	386	334	1,872
Total revenue	3	6,121	4,586	24,483
Expenses:
Property operating expenses	38	1,977	1,672	6,863
Real estate taxes	17	707	557	2,717
Depreciation and amortization	18	1,788	1,769	8,106
Total operating expenses	73	4,472	3,998	17,686
Interest income	0	0	2	6
Income from discontinued operations before gain on sale of interests in real estate	(70)	1,649	590	6,803
Net gain on disposition of discontinued operations	(16)	9,940	3,029	34,774
Income (Loss) from discontinued operations	$ (86)	$ 11,589	$ 3,619	$ 41,577