Mortgage Notes Payable (Narrative) (Details) (USD $)	3 Months Ended		9 Months Ended			0 Months Ended	9 Months Ended	0 Months Ended	3 Months Ended	0 Months Ended		0 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012	Feb. 20, 2013 Montvale Center [Member]	Sep. 30, 2012 Montvale Center [Member]	Feb. 05, 2013 Kingstowne One [Member]	Sep. 30, 2013 140 Kendrick [Member]	May 31, 2013 767 Fifth Avenue (The GM Building) [Member]	Sep. 30, 2013 767 Fifth Avenue (The GM Building) [Member]	May 31, 2013 First Mortgage [Member] 767 Fifth Avenue (The GM Building) [Member]	May 31, 2013 Subordinated Debt [Member] 767 Fifth Avenue (The GM Building) [Member]	May 31, 2013 Consolidation, Eliminations [Member] 767 Fifth Avenue (The GM Building) [Member]	May 31, 2013 Company [Member] 767 Fifth Avenue (The GM Building) [Member]
Mortgage Notes Payable [Line Items]
Repayments of Secured Debt			$ 96,750,000	$ 248,304,000				$ 17,000,000	$ 47,600,000
Coupon/Stated Rate (percent)								5.96%	7.51%	11.00%		5.95%	6.02%
Maturity date								May 05, 2013	Jul. 01, 2013	Jun. 09, 2017		Oct. 07, 2017	Oct. 07, 2017
Penalty For Early Repayment Of Debt								0.0	0
Amount of debt extinguished							25,000,000
Gain on forgiveness of debt from discontinued operations	0	0	20,736,000	0		20,700,000	18,400,000
Gains from early extinguishments of debt	(30,000)	(5,494,000)	122,000	(4,453,000)					300,000
Indebtedness assumed												1,300,000,000	306,000,000
MortgageLoans Accounted For At Fair Value Upon Consolidation										1,500,000,000			311,700,000
Interest Rate Used To Account For Mortgage Loan At Fai rValue Upon Consolidation												2.44%	5.53%
Related party notes payable	180,000,000		180,000,000		0					450,000,000				270,000,000	180,000,000
Accrued interest payable	$ 181,310,000		$ 181,310,000		$ 72,461,000						$ 175,800,000