Significant Accounting Policies (Details 2) (USD $)	3 Months Ended		6 Months Ended			1 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012	Apr. 30, 2013 16 Court Street	Jun. 30, 2013 300 Main Street	Jun. 30, 2013 Other intangible assets Minimum	Jun. 30, 2013 Other intangible assets Maximum	Jun. 30, 2013 Above-market leases Minimum	Jun. 30, 2013 Above-market leases Maximum	Jun. 30, 2013 Below-market leases Minimum	Jun. 30, 2013 Below-market leases Maximum	Jun. 30, 2013 In-place leases Minimum	Jun. 30, 2013 In-place leases Maximum	Jun. 30, 2013 Buildings Minimum	Jun. 30, 2013 Buildings Maximum
Investment in Commercial Real Estate Properties
Impairment charge for assets held for sale							$ 2,200,000
Purchase price fair value adjustment recognized						(2,300,000)
Estimated useful life																3 years	40 years
Estimated useful life of other intangible assets								1 year	14 years	1 year	14 years	1 year	14 years	1 year	14 years
Increase in rental revenue from amortization of acquired leases	6,200,000	2,800,000	10,100,000	4,800,000
Increase/(reduction) in interest expense from amortization of above-market rate mortgages	(1,300,000)	1,600,000	(2,600,000)	600,000
Identified intangible assets (included in other assets):
Gross amount	732,160,000		732,160,000		725,861,000
Accumulated amortization	(296,573,000)		(296,573,000)		(263,107,000)
Net	435,587,000		435,587,000		462,754,000
Identified intangible liabilities (included in deferred revenue):
Gross amount	667,495,000		667,495,000		651,921,000
Accumulated amortization	(391,631,000)		(391,631,000)		(357,225,000)
Net	$ 275,864,000		$ 275,864,000		$ 294,696,000