Significant Accounting Policies (Details 2) (USD $)	12 Months Ended			1 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Jun. 30, 2013	Apr. 30, 2013	May 31, 2014
Investment in Commercial Real Estate Properties
Depreciation expense (including amortization of the capital lease asset)	$ 338,800,000	$ 296,500,000	$ 287,500,000
Period from cessation of major construction to consider construction project as complete and available for occupancy, maximum	1 year
Weighted average amortization period for above-market leases	7 years 11 days
Weighted average amortization period for below-market leases	5 years 7 months 13 days
Weighted average amortization period for in-place lease costs	5 years 11 months 16 days
Increase in rental revenue from amortization of acquired leases	23,300,000	18,800,000	16,500,000
Write-off of above and in place market leases		6,800,000
Increase in rental revenue		25,600,000
Reduction in interest expense from amortization of above-market rate mortgages	(5,000,000)	(5,000,000)	(1,400,000)
Identified intangible assets (included in other assets):
Gross amount	664,297,000	746,704,000
Accumulated amortization	(383,236,000)	(343,339,000)
Net	281,061,000	403,365,000
Identified intangible liabilities (included in deferred revenue):
Gross amount	655,755,000	671,380,000
Accumulated amortization	(483,948,000)	(429,138,000)
Net	171,807,000	242,242,000
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
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
2015	1,978,000
2016	(592,000)
2017	(1,303,000)
2018	(3,135,000)
2019	(3,994,000)
All other identifiable assets
Estimated annual amortization
2015	9,150,000
2016	5,919,000
2017	4,442,000
2018	3,005,000
2019	2,467,000
300 Main Street
Investment in Commercial Real Estate Properties
Impairment charges recorded				2,200,000
16 Court Street
Investment in Commercial Real Estate Properties
Purchase price fair value adjustment					(2,300,000)
Buildings (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
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
388-390 Greenwich Street
Investment in Commercial Real Estate Properties
Purchase price fair value adjustment recognized						$ 71,400,000