Debt (Parentheticals) (Details) (USD $)		9 Months Ended			9 Months Ended								9 Months Ended
	Sep. 30, 2011 Unsecured Revolving Credit Facilities [Member]	Sep. 30, 2011 Unsecured Revolving Credit Facilities [Member] Unsecured Revolving Credit Facility $1.595 Billion [Member]	Sep. 30, 2011 Unsecured Revolving Credit Facilities [Member] Unsecured Revolving Credit Facility $1.25 Billion Maturing 06/16 [Member]	Jun. 07, 2011 Unsecured Revolving Credit Facilities [Member] Unsecured Revolving Credit Facility $1.25 Billion Maturing 06/16 [Member]	Sep. 30, 2011 Unsecured Revolving Credit Facilities [Member] New Unsecured Revolving Credit Facility $1.25 Billion	Sep. 30, 2011 Convertible Senior Debentures [Member]	Sep. 30, 2011 Convertible Senior Debentures [Member] Convertible Senior Debenture 3.63% Due 2026 [Member]	Sep. 30, 2011 Convertible Senior Debentures [Member] Convertible Senior Debenture 2.85% Due 2027 [Member]	Sep. 30, 2011 Exchangeable Senior Debentures [Member] Exchangeable Senior Debenture 3.88% Due 2025 [Member]	Sep. 30, 2011 Senior Unsecured Notes [Member]	Sep. 30, 2011 Senior Unsecured Notes [Member] Senior Unsecured Notes Due 2039 [Member]	Sep. 30, 2011 Notes And Mortgages Payable Variable Rate [Member]	Sep. 30, 2011 Notes And Mortgages Payable Variable Rate [Member] Retail 435 Seventh Avenue [Member]	Sep. 30, 2011 Notes And Mortgages Payable Variable Rate [Member] Retail Beverly Connection [Member]	Sep. 30, 2011 Notes And Mortgages Payable Variable Rate [Member] Retail Beverly Connection New Loan [Member]	Sep. 30, 2011 Notes And Mortgages Payable Variable Rate [Member] Retail Cross Collateralized Mortgages On 40 Strip Shopping Centers [Member]	Sep. 30, 2011 Notes And Mortgages Payable Variable Rate [Member] Washington DC Office River House Apartments [Member]	Sep. 30, 2011 Notes And Mortgages Payable Variable Rate [Member] New York Office 866 UN Plaza [Member]	Sep. 30, 2011 Notes And Mortgages Payable Fixed Rate [Member]	Sep. 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]	Sep. 30, 2011 Notes And Mortgages Payable Fixed Rate [Member] Retail Cross Collateralized Mortgages On 40 Strip Shopping Centers [Member]	Sep. 30, 2011 Notes And Mortgages Payable Fixed Rate [Member] Other 555 California Street [Member]	Sep. 01, 2011 Notes And Mortgages Payable Fixed Rate [Member] Other 555 California Street [Member]	Sep. 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]	Sep. 30, 2011 Notes And Mortgages Payable Fixed Rate [Member] Washington DC Office River House Apartments [Member]	Sep. 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]	Sep. 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]	Sep. 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]	Sep. 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		0.55%	1.35%		1.25%								3.00%	3.50%	4.25%	1.36%		1.25%
LIBOR Floor													2.00%		0.50%	1.00%
Spread over Freddie Mac Interest Rate																	1.53%
Debt Instrument, Interest Rate at Period End	0.75%	0.75%				5.33%	5.32%	5.45%	5.32%	5.99%	7.88%	2.19%	5.00%	5.00%		2.36%	1.53%	1.52%	5.52%	4.59%		4.20%	5.10%		5.14%		5.43%	4.61%		4.88%		5.51%		5.13%
Swapped Loan Spread Over Libor																																			2.00%
Notes And Loans Payable Refinanced Amount																								$ 600,000,000											$ 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		30
Line of Credit Facility, Maximum Borrowing Capacity		$ 1,595,000,000	$ 1,250,000,000	$ 1,000,000,000	$ 1,250,000,000
Line of Credit Facility, Commitment Fee Percentage			0.30%		0.25%