Significant Accounting Policies (Investments in Commercial Real Estate Properties)(Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Investment in Commercial Real Estate Properties
Rental revenue from amortization of acquired leases	$ (200,000)	$ 10,200,000	$ 191,400,000	$ 32,900,000
Reduction in interest expense from amortization of above-market rate mortgages	700,000	$ 600,000	2,000,000	$ 1,700,000
Identified intangible assets (included in other assets):
Gross amount	624,557,000		624,557,000		$ 939,518,000
Accumulated amortization	(397,148,000)		(397,148,000)		(403,747,000)
Net	227,409,000		227,409,000		535,771,000
Identified intangible liabilities (included in deferred revenue):
Gross amount	648,583,000		648,583,000		866,561,000
Accumulated amortization	(455,568,000)		(455,568,000)		(486,928,000)
Net	193,015,000		193,015,000		379,633,000
Net intangible assets reclassed to assets held for sale	300,000		300,000		200,000
Net intangible liabilities reclassed to liabilities relates to assets held for sale	800,000		$ 800,000		$ 100,000
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
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
Building | Minimum
Investment in Commercial Real Estate Properties
Estimated useful life (in years)			3 years
Building | Maximum
Investment in Commercial Real Estate Properties
Estimated useful life (in years)			40 years
388-390 Greenwich Street
Investment in Commercial Real Estate Properties
Rental revenue from amortization of acquired leases	$ 0		$ 172,400,000