Debt (Parentheticals) (Details) (USD $)	0 Months Ended		6 Months Ended									6 Months Ended			6 Months Ended
	Jun. 10, 2011	Jun. 30, 2011 Unsecured Revolving Credit Facilities [Member]	Jun. 30, 2011 Unsecured Revolving Credit Facilities [Member] Unsecured Revolving Credit Facility 1 [Member]	Jun. 30, 2011 Unsecured Revolving Credit Facilities [Member] Unsecured Revolving Credit Facility 2 [Member]	Jun. 30, 2011 Convertible Senior Debentures [Member]	Jun. 30, 2011 Convertible Senior Debentures [Member] Convertible Senior Debenture 3.63% Due 2026 [Member]	Jun. 30, 2011 Convertible Senior Debentures [Member] Convertible Senior Debenture 2.85% Due 2027 [Member]	Jun. 30, 2011 Exchangeable Senior Debentures [Member] Exchangeable Senior Debenture 3.88% Due 2025 [Member]	Jun. 30, 2011 Senior Unsecured Notes [Member]	Jun. 30, 2011 Senior Unsecured Notes [Member] Senior Unsecured Notes Due 2039 [Member]	Jun. 30, 2011 Notes And Mortgages Payable Variable Rate [Member]	Jun. 30, 2011 Notes And Mortgages Payable Variable Rate [Member] Retail 435 Seventh Avenue [Member]	Jun. 30, 2011 Notes And Mortgages Payable Variable Rate [Member] Retail Beverly Connection [Member]	Jul. 31, 2011 Notes And Mortgages Payable Variable Rate [Member] Retail Beverly Connection [Member]	Jun. 30, 2011 Notes And Mortgages Payable Variable Rate [Member] Retail Cross Collateralized Mortgages On 40 Strip Shopping Centers [Member]	Jun. 30, 2011 Notes And Mortgages Payable Variable Rate [Member] Washington DC Office River House Apartments [Member]	Jun. 30, 2011 Notes And Mortgages Payable Variable Rate [Member] New York Office 866 UN Plaza [Member]	Jun. 30, 2011 Notes And Mortgages Payable Fixed Rate [Member]	Jun. 30, 2011 Notes And Mortgages Payable Fixed Rate [Member] Retail North Bergen Tonnelle Avenue [Member]	Jan. 10, 2011 Notes And Mortgages Payable Fixed Rate [Member] Retail North Bergen Tonnelle Avenue [Member]	Jun. 30, 2011 Notes And Mortgages Payable Fixed Rate [Member] Retail Cross Collateralized Mortgages On 40 Strip Shopping Centers [Member]	Jun. 30, 2011 Notes And Mortgages Payable Fixed Rate [Member] Other Borgata Land [Member]	Jan. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] Other Borgata Land [Member]	Jun. 30, 2011 Notes And Mortgages Payable Fixed Rate [Member] Washington DC Office River House Apartments [Member]	Jun. 30, 2011 Notes And Mortgages Payable Fixed Rate [Member] Washington Dc Office 220 20th Street [Member]	Jan. 18, 2011 Notes And Mortgages Payable Fixed Rate [Member] Washington Dc Office 220 20th Street [Member]	Jun. 30, 2011 Notes And Mortgages Payable Fixed Rate [Member] Washington DC Office West End 25 [Member]	May 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] Washington DC Office West End 25 [Member]	Jun. 30, 2011 Notes And Mortgages Payable Fixed Rate [Member] Washington Dc Office2121 Crystal Drive [Member]	Feb. 10, 2011 Notes And Mortgages Payable Fixed Rate [Member] Washington Dc Office2121 Crystal Drive [Member]	Jun. 30, 2011 Notes And Mortgages Payable Fixed Rate [Member] New York Office Two Penn Plaza [Member]	Feb. 11, 2011 Notes And Mortgages Payable Fixed Rate [Member] New York Office Two Penn Plaza [Member]
Spread Over LIBOR	170.00%		55.00%	135.00%								300.00%	350.00%		136.00%		125.00%
LIBOR Floor												200.00%	150.00%		100.00%
Spread over Freddie Mac Interest Rate																153.00%
Debt Instrument, Interest Rate at Period End		72.00%	72.00%		533.00%	532.00%	545.00%	532.00%	590.00%	788.00%	217.00%	500.00%	500.00%		236.00%	153.00%	152.00%	559.00%	459.00%		419.00%	514.00%		543.00%	461.00%		488.00%		551.00%		513.00%
Unsecured Revolving Credit Facility Reserved For Outstanding Letters Of Credit				$ 21,534,000
Future Increased Decreased Spread Over Libor														500.00%
Swapped Loan Spread Over Libor																																200.00%
Notes And Loans Payable Refinanced Amount																																425,000,000
Notes And Loans Payable Financing Amount																				75,000,000			60,000,000			76,100,000		101,671,000		150,000,000
Retained Proceeds From Refinancing																																$ 139,000,000
Debt Amortization Period																				25			30			30		30		30		30