Significant Accounting Policies (Investments in Commercial Real Estate Properties)(Details) $ in Thousands			1 Months Ended		3 Months Ended		12 Months Ended
	Nov. 30, 2015 USD ($)	Apr. 30, 2014 USD ($)	Dec. 31, 2015 USD ($)	May. 31, 2014 USD ($)	Dec. 31, 2015 USD ($) property	Sep. 30, 2015 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Investment in Commercial Real Estate Properties
Depreciation expense (including amortization of the capital lease asset)							$ 523,800	$ 338,800	$ 296,500
Number of properties expected to be sold | property					2
Period from cessation of major construction to consider construction project as complete and available for occupancy, maximum (in years)							1 year
Weighted average amortization period for above-market leases (in years)							7 years 4 days
Weighted average amortization period for below-market leases (in years)							20 years 1 month 6 days
Weighted average amortization for in-place lease costs (in years)							21 years 8 months 12 days
Increase in rental revenue from amortization of acquired leases							$ 38,700	23,300	18,800
Write-off of above and in place market leases									6,800
Increase in rental revenue due to write off of balance related to a former tenant									25,600
Reduction in interest expense from amortization of above-market rate mortgages							2,300	5,000	$ 5,000
Identified intangible assets (included in other assets):
Gross amount			$ 939,518		$ 939,518		939,518	664,297
Accumulated amortization			(403,747)		(403,747)		(403,747)	(383,236)
Net			535,771		535,771		535,771	281,061
Identified intangible liabilities (included in deferred revenue):
Gross amount			866,561		866,561		866,561	655,755
Accumulated amortization			(486,928)		(486,928)		(486,928)	(483,948)
Net			379,633		379,633		379,633	$ 171,807
Net intangible assets reclassed to assets held for sale			200		200		200
Net intangible liabilities reclassed to liabilities relates to assets held for sale			100		100		100
Acquired above-market leases, net of acquired below-market leases
Estimated annual amortization
2016			4,002		4,002		4,002
2017			2,129		2,129		2,129
2018			439		439		439
2019			(376)		(376)		(376)
2020			(595)		(595)		(595)
All other identifiable assets
Estimated annual amortization
2016			9,931		9,931		9,931
2017			8,507		8,507		8,507
2018			7,207		7,207		7,207
2019			6,706		6,706		6,706
2020			6,453		6,453		$ 6,453
600 Lexington Avenue
Investment in Commercial Real Estate Properties
Purchase price fair value adjustment realized	$ 40,078		$ 40,100
388-390 Greenwich Street
Investment in Commercial Real Estate Properties
Purchase price fair value adjustment realized		$ 71,446		$ 71,400
Maximum | Above-market leases
Investment in Commercial Real Estate Properties
Estimated useful life of other intangible assets (in years)							14 years
Maximum | Below-market leases
Investment in Commercial Real Estate Properties
Estimated useful life of other intangible assets (in years)							14 years
Maximum | In-place leases
Investment in Commercial Real Estate Properties
Estimated useful life of other intangible assets (in years)							14 years
Minimum | Above-market leases
Investment in Commercial Real Estate Properties
Estimated useful life of other intangible assets (in years)							1 year
Minimum | Below-market leases
Investment in Commercial Real Estate Properties
Estimated useful life of other intangible assets (in years)							1 year
Minimum | In-place leases
Investment in Commercial Real Estate Properties
Estimated useful life of other intangible assets (in years)							1 year
Building (fee ownership)
Investment in Commercial Real Estate Properties
Estimated useful life (in years)							40 years
Building (leasehold interest) | Maximum
Investment in Commercial Real Estate Properties
Estimated useful life (in years)							40 years
Furniture and fixtures | Maximum
Investment in Commercial Real Estate Properties
Estimated useful life (in years)							7 years
Furniture and fixtures | Minimum
Investment in Commercial Real Estate Properties
Estimated useful life (in years)							4 years
Building | Maximum
Investment in Commercial Real Estate Properties
Estimated useful life (in years)							40 years
Building | Minimum
Investment in Commercial Real Estate Properties
Estimated useful life (in years)							3 years
140-150 Grand Street
Investment in Commercial Real Estate Properties
Impairment Charge						$ 19,200	$ 19,200
885 Third Avenue
Investment in Commercial Real Estate Properties
Impairment Charge					$ 6,600
One Vanderbilt
Investment in Commercial Real Estate Properties
Depreciation expense (including amortization of the capital lease asset)							$ 131,800