Significant Accounting Policies (Details 2) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013
Investment in Commercial Real Estate Properties
Increase in rental revenue from amortization of acquired leases	$ 6,200,000	$ 6,200,000	$ 11,000,000	$ 10,100,000
Reduction in interest expense from amortization of above-market rate mortgages	(1,300,000)	(1,200,000)	(2,800,000)	(2,400,000)
Identified intangible assets (included in other assets):
Gross amount	702,189,000		702,189,000		746,704,000
Accumulated amortization	(382,231,000)		(382,231,000)		(343,339,000)
Net	319,958,000		319,958,000		403,365,000
Identified intangible liabilities (included in deferred revenue):
Gross amount	676,075,000		676,075,000		671,380,000
Accumulated amortization	(461,952,000)		(461,952,000)		(429,138,000)
Net	$ 214,123,000		$ 214,123,000		$ 242,242,000
Above and below market leases | Minimum
Investment in Commercial Real Estate Properties
Estimated useful life of other intangible assets			1 year
Above and below market leases | Maximum
Investment in Commercial Real Estate Properties
Estimated useful life of other intangible assets			14 years
In-place leases | Minimum
Investment in Commercial Real Estate Properties
Estimated useful life of other intangible assets			1 year
In-place leases | Maximum
Investment in Commercial Real Estate Properties
Estimated useful life of other intangible assets			14 years
Building | Minimum
Investment in Commercial Real Estate Properties
Estimated useful life			3 years
Building | Maximum
Investment in Commercial Real Estate Properties
Estimated useful life			40 years