Intangible Assets (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Finite-Lived Intangible Assets [Line Items]
Intangible liabilities written off through accelerated amortization	$ 200,000		$ 400,000
Intangible assets
Total Cost	176,983,000	222,160,000
Accumulated Amortization	(106,468,000)	(124,698,000)
Intangibles, net	70,515,000	97,462,000
Below market leases acquired	35,106,000	29,233,000
Leases, Acquired-in-Place [Member]
Intangible assets
Total Cost	91,426,000	108,456,000
Accumulated Amortization	(55,498,000)	(63,010,000)
Intangibles, net	35,928,000	45,446,000
Customer Relationships [Member]
Intangible assets
Total Cost	72,813,000	95,385,000
Accumulated Amortization	(45,114,000)	(52,113,000)
Intangibles, net	27,699,000	43,272,000
Above market tenant leases acquired [Member]
Intangible assets
Total Cost	12,744,000	18,319,000
Accumulated Amortization	(5,856,000)	(9,575,000)
Intangibles, net	6,888,000	8,744,000
Below market ground lease acquired [Member]
Intangible assets
Total Cost	75,685,000	67,198,000
Accumulated Amortization	(40,579,000)	(37,965,000)
Intangibles, net	35,106,000	29,233,000
Tenant Move-Outs Prior to End of the Lease Term [Member]
Finite-Lived Intangible Assets [Line Items]
Intangible assets written off through accelerated amortization	$ 7,400,000	$ 1,500,000	$ 2,400,000