Subsequent Events (Details 1) ft² in Thousands			3 Months Ended			9 Months Ended
	Oct. 30, 2015 USD ($) Extensions	Oct. 19, 2015 USD ($) ft² $ / ft²	Sep. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)
Subsequent Events [Line Items]
Gain on sale of interest in leases			$ 0		$ 0	$ 21,376,000	$ 0
Real Estate Fund Nondepreciable Real Estate			555,414,000			555,414,000		$ 513,973,000
Proceeds from sales of real estate						375,850,000	335,489,000
Net gains on sale of real estate			0	$ 3,675,000	$ 57,796,000	10,867,000	$ 57,796,000
Unsecured term loan			$ 847,594,000			847,594,000		$ 1,347,159,000
Proceeds from sale of real estate						$ 10,731,000
Subsequent Event [Member] | Unsecured Term Loan [Member]
Subsequent Events [Line Items]
Unsecured term loan maturity	October 2018
Number Of Extensions Available | Extensions	2
Length Of Extension Available	1 year
Debt Instrument, Description of Variable Rate Basis	LIBOR
Debt Instrument, Basis Spread on Variable Rate	1.15%
Unsecured Term Loan Unused Fee	0.20%
Unsecured Term Loan Borrowing	$ 187,500,000
Unsecured term loan, description	The facility provides that the maximum amount available is twice the amount outstanding on April 29, 2016, limited to $750,000,000, and all draws must be made by October 2017.
Unsecured term loan final withdrawal date	October 2017
Subsequent Event [Member] | Unsecured Term Loan [Member] | Maximum [Member]
Subsequent Events [Line Items]
Unsecured term loan	$ 750,000,000
New York Office 20 Broad Street [Member] | Subsequent Event [Member]
Subsequent Events [Line Items]
Square Footage Of Real Estate Property | ft²		473
Aggregare consideration for sale of leasehold and early termination of lease		$ 200,000,000
Aggregare consideration for sale of leasehold and early termination of lease per square foot | $ / ft²		423
Income from sell of leashold interest in property		$ 156,000,000
Gain on sale of interest in leases		141,000,000
Lease termination income		$ 15,000,000