The current period expense charged against earnings on long-lived, physical assets not used in production, and which are not intended for resale, to allocate or recognize the cost of such assets over their useful lives; or to record the reduction in book value of an intangible asset over the benefit period of such asset; or to reflect consumption during the period of an asset that is not used in production related to Commercial Real Estate Properties.

No definition available.

Significant Accounting Policies (Details) $ in Thousands	1 Months Ended	3 Months Ended					12 Months Ended
Significant Accounting Policies (Details) $ in Thousands	Mar. 31, 2014 USD ($)	Dec. 31, 2015 USD ($) property	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Investment in Commercial Real Estate Properties
Depreciation expense (including amortization of capital lease asset)							$ 187,200	$ 184,300	$ 170,000
Impairment charge		$ 0	$ (9,998)	$ 0	$ 0		$ (9,998)	$ 0	$ 0
Number of properties expected to be sold \| property		1
Maximum period of cessation of major construction activity to completion of construction project (in years)							1 year
Weighted average amortization period (in years)							4 years 4 months 24 days	13 years 3 months 18 days	11 years 10 months 24 days
Rental revenue from amortization of acquired leases		$ 24,900				$ 21,000	$ 19,000
Write-off of balances associated with former tenant									$ 6,800
Increase (decrease) in rental revenue									25,800
Increase (reduction) in interest expense from amortization of above-market rate mortgages							400	$ (2,100)	$ (2,500)
Identified intangible assets (included in other assets):
Gross amount		307,824				297,238	307,824	297,238
Accumulated amortization		(235,040)				(216,841)	(235,040)	(216,841)
Net		72,784				80,397	72,784	80,397
Identified intangible liabilities (included in deferred revenue):
Gross amount		523,228				450,326	523,228	450,326
Accumulated amortization		(346,857)				(322,694)	(346,857)	(322,694)
Net		$ 176,371				$ 127,632	$ 176,371	$ 127,632
16 Court Street
Investment in Commercial Real Estate Properties
Income from amortization of remaining value of below-market mortgage	$ 300
Above-market leases \| Minimum
Investment in Commercial Real Estate Properties
Estimated useful life of intangible assets (years)							1 year
Above-market leases \| Maximum
Investment in Commercial Real Estate Properties
Estimated useful life of intangible assets (years)							14 years
Below-market leases \| Minimum
Investment in Commercial Real Estate Properties
Estimated useful life of intangible assets (years)							1 year
Below-market leases \| Maximum
Investment in Commercial Real Estate Properties
Estimated useful life of intangible assets (years)							14 years
In-place leases \| Minimum
Investment in Commercial Real Estate Properties
Estimated useful life of intangible assets (years)							1 year
In-place leases \| Maximum
Investment in Commercial Real Estate Properties
Estimated useful life of intangible assets (years)							14 years
Building \| Minimum
Investment in Commercial Real Estate Properties
Estimated useful life (years)							3 years
Building \| Maximum
Investment in Commercial Real Estate Properties
Estimated useful life (years)							40 years
Building (leasehold interest) \| Maximum
Investment in Commercial Real Estate Properties
Estimated useful life (years)							40 years
Furniture and fixtures \| Minimum
Investment in Commercial Real Estate Properties
Estimated useful life (years)							4 years
Furniture and fixtures \| Maximum
Investment in Commercial Real Estate Properties
Estimated useful life (years)							7 years
140 Grand Street
Investment in Commercial Real Estate Properties
Impairment charge							$ (10,000)