Intangible Assets (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Intangible assets
Total Cost	$ 168,867	$ 176,983
Accumulated Amortization	(104,898)	(106,468)
Intangible Assets, net	63,969	70,515
In-place lease value [Member]
Intangible assets
Total Cost	88,796	91,426
Accumulated Amortization	(56,520)	(55,498)
Intangible Assets, net	32,276	35,928
Tenant relationship value [Member]
Intangible assets
Total Cost	70,712	72,813
Accumulated Amortization	(45,493)	(45,114)
Intangible Assets, net	25,219	27,699
Above market leases acquired [Member]
Intangible assets
Total Cost	9,359	12,744
Accumulated Amortization	(2,885)	(5,856)
Intangible Assets, net	6,474	6,888
Below market leases acquired [Member]
Intangible assets
Total Cost	75,550	75,685
Accumulated Amortization	(42,272)	(40,579)
Intangible Assets, net	$ 33,278	$ 35,106