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 ($)	May 31, 2014 USD ($)	Jun. 30, 2016 USD ($) property	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($) property	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Investment in Commercial Real Estate Properties
Depreciation expense (including amortization of the capital lease asset)								$ 783,500	$ 523,800	$ 338,800
Number of properties expected to be sold | property					1		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)								5 years 1 month 13 days
Weighted average amortization period for below-market leases (in years)								24 years 10 days
Weighted average amortization for in-place lease costs (in years)								9 years 11 months
Increase in rental revenue from amortization of acquired leases								$ 196,200	38,700	23,300
Reduction in interest expense from amortization of above-market rate mortgages								2,800	2,300	$ 5,000
Identified intangible assets (included in other assets):
Gross amount			$ 939,518			$ 939,518		651,099	939,518
Accumulated amortization			(403,747)			(403,747)		(410,930)	(403,747)
Net			535,771			535,771		240,169	535,771
Identified intangible liabilities (included in deferred revenue):
Gross amount			866,561			866,561		655,930	866,561
Accumulated amortization			(486,928)			(486,928)		(464,749)	(486,928)
Net			379,633			379,633		191,181	$ 379,633
Net intangible assets reclassed to assets held for sale								0
Net intangible liabilities reclassed to liabilities relates to assets held for sale								0
Other Identifiable Assets [Member]
Estimated annual amortization
2017								24,801
2018								17,600
2019								14,780
2020								12,742
2021								8,986
Acquired above-market leases, net of acquired below-market leases
Below Market Lease, Net, Amortization Income, Fiscal Year Maturity [Abstract]
2017								(19,286)
2018								(17,022)
2019								(16,300)
2020								(15,279)
2021								$ (6,356)
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					$ 10,400		$ 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
388-390 Greenwich Street
Investment in Commercial Real Estate Properties
Increase in rental revenue from amortization of acquired leases								$ 172,400