Intangible Assets (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Intangible assets
Total Cost	$ 138,750	$ 176,983
Accumulated Amortization	(86,175)	(106,468)
Intangible Assets, net	52,575	70,515
In-place lease value [Member]
Intangible assets
Total Cost	72,099	91,426
Accumulated Amortization	(45,704)	(55,498)
Intangible Assets, net	26,395	35,928
Tenant relationship value [Member]
Intangible assets
Total Cost	58,811	72,813
Accumulated Amortization	(38,716)	(45,114)
Intangible Assets, net	20,095	27,699
Above market leases acquired [Member]
Intangible assets
Total Cost	7,840	12,744
Accumulated Amortization	(1,755)	(5,856)
Intangible Assets, net	6,085	6,888
Below market leases acquired [Member]
Intangible assets
Total Cost	73,342	75,685
Accumulated Amortization	(43,518)	(40,579)
Intangible Assets, net	$ 29,824	$ 35,106