Intangible Assets (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Intangible assets
Total Cost	$ 159,004	$ 176,983
Accumulated Amortization	(101,077)	(106,468)
Intangible Assets, net	57,927	70,515
In-place lease value [Member]
Intangible assets
Total Cost	82,861	91,426
Accumulated Amortization	(54,017)	(55,498)
Intangible Assets, net	28,844	35,928
Tenant relationship value [Member]
Intangible assets
Total Cost	67,414	72,813
Accumulated Amortization	(44,560)	(45,114)
Intangible Assets, net	22,854	27,699
Above market leases acquired [Member]
Intangible assets
Total Cost	8,729	12,744
Accumulated Amortization	(2,500)	(5,856)
Intangible Assets, net	6,229	6,888
Below market leases acquired [Member]
Intangible assets
Total Cost	74,100	75,685
Accumulated Amortization	(42,574)	(40,579)
Intangible Assets, net	$ 31,526	$ 35,106