Mortgage Notes Payable (Narrative) (Details) (USD $)	3 Months Ended		9 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	Sep. 30, 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								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
Gains (losse) from early extinguishments of debt	(30,000)	(5,494,000)	122,000	(4,453,000)					300,000
Asssumed debt												1,300,000,000	306,000,000
Amount of debt extinguished							25,000,000
Gain on forgiveness of debt from discontinued operations	0	0	20,182,000	0		20,200,000	17,800,000
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%
Accrued interest payable	181,310,000		181,310,000		72,461,000						175,800,000
Related party notes payable	$ 180,000,000		$ 180,000,000		$ 0					$ 450,000,000				$ 270,000,000	$ 180,000,000