Significant Accounting Policies (Details 2) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Investment in Commercial Real Estate Properties
Increase in rental revenue from amortization of acquired leases	$ 8,300	$ 6,400	$ 22,600	$ 12,500
Reduction in interest expense from amortization of above-market rate mortgages	600	$ 1,300	1,200	$ 2,800
Identified intangible assets (included in other assets):
Gross amount	941,064		941,064		$ 664,297
Accumulated amortization	(399,103)		(399,103)		(383,236)
Net	541,961		541,961		281,061
Identified intangible liabilities (included in deferred revenue):
Gross amount	804,550		804,550		655,755
Accumulated amortization	(484,673)		(484,673)		(483,948)
Net	319,877		319,877		$ 171,807
Net intangible assets reclassed to assets held for sale	3,600		3,600
Net intangible liabilities reclassed to liabilities relates to assets held for sale	$ 8,300		$ 8,300
Above-market leases | Minimum
Investment in Commercial Real Estate Properties
Estimated useful life of other intangible assets (in years)			1 year
Above-market leases | Maximum
Investment in Commercial Real Estate Properties
Estimated useful life of other intangible assets (in years)			14 years
In-place leases | Minimum
Investment in Commercial Real Estate Properties
Estimated useful life of other intangible assets (in years)			1 year
In-place leases | Maximum
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