Significant Accounting Policies (Details 2) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011 investment	Dec. 31, 2010 investment
Investment in Commercial Real Estate Properties
Depreciation expense (including amortization of the capital lease asset)	$ 306,800,000	$ 254,500,000	$ 207,100,000
Impairment charges recorded		5,800,000	2,800,000
Impairment charges recorded, number of investments		1	1
Period from cessation of major construction to consider construction project as complete and available for occupancy, maximum	1 year
Increase in rental revenue from amortization of acquired leases	10,500,000	19,800,000	22,700,000
Reduction in interest expense from amortization of above-market rate mortgages	(1,800,000)	(5,900,000)	(2,700,000)
Identified intangible assets (included in other assets):
Gross amount	725,861,000	673,495,000
Accumulated amortization	(263,107,000)	(193,442,000)
Net	462,754,000	480,053,000
Identified intangible liabilities (included in deferred revenue):
Gross amount	651,921,000	622,029,000
Accumulated amortization	(357,225,000)	(290,893,000)
Net	294,696,000	331,136,000
Other intangible assets | Minimum
Investment in Commercial Real Estate Properties
Estimated useful life of other intangible assets	1 year
Other intangible assets | Maximum
Investment in Commercial Real Estate Properties
Estimated useful life of other intangible assets	14 years
Above-market leases | Minimum
Investment in Commercial Real Estate Properties
Estimated useful life of other intangible assets	1 year
Above-market leases | Maximum
Investment in Commercial Real Estate Properties
Estimated useful life of other intangible assets	14 years
Below-market leases | Minimum
Investment in Commercial Real Estate Properties
Estimated useful life of other intangible assets	1 year
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
Acquired below-market leases, net of acquired above-market leases
Estimated annual amortization
2013	(268,000)
2014	390,000
2015	1,223,000
2016	3,415,000
2017	3,273,000
All other identifiable assets
Estimated annual amortization
2013	16,622,000
2014	12,887,000
2015	8,843,000
2016	5,556,000
2017	$ 4,445,000
Buildings | Minimum
Investment in Commercial Real Estate Properties
Estimated useful life of other intangible assets	3 years
Buildings | Maximum
Investment in Commercial Real Estate Properties
Estimated useful life of other intangible assets	40 years
Building (fee ownership)
Investment in Commercial Real Estate Properties
Estimated useful life	40 years
Building (leasehold interest) | Maximum
Investment in Commercial Real Estate Properties
Estimated useful life	40 years
Furniture and fixtures | Minimum
Investment in Commercial Real Estate Properties
Estimated useful life	4 years
Furniture and fixtures | Maximum
Investment in Commercial Real Estate Properties
Estimated useful life	7 years