Corporate Indebtedness (Junior Subordinated Deferrable Interest Debentures and Principal Maturities)(Details) - USD ($)	1 Months Ended	12 Months Ended
	Jun. 30, 2005	Dec. 31, 2015
Scheduled Amortization and Principal Repayments
2016		$ 705,812,000
2017		1,290,385,000
2018		333,987,000
2019		1,031,726,000
2020		1,976,330,000
Thereafter		5,120,578,000
Total amortization of debt and principal repayments		10,458,818,000
Joint Venture Debt
Principal Repayments and Joint Venture Debt
2016		529,646,000
2017		615,085,000
2018		2,195,000
2019		104,687,000
2020		30,298,000
Thereafter		451,544,000
Total principal repayments		$ 1,733,455,000
Trust Preferred Securities
Debt Instrument [Line Items]
Proceeds from Issuance of Debt	$ 100,000,000.0
Coupon Rate (as a percent)		5.61%
Number of Years from Issuance of Debt for which Fixed Rate of Interest will be in Force		10 years
Scheduled Amortization and Principal Repayments
2016		$ 0
2017		0
2018		0
2019		0
2020		0
Thereafter		100,000,000
Total amortization of debt and principal repayments		$ 100,000,000
Trust Preferred Securities | LIBOR
Debt Instrument [Line Items]
Interest rate added to base rate (as a percent)		1.25%
Mortgages and other loans payable
Scheduled Amortization
2016		$ 51,018,000
2017		63,829,000
2018		64,462,000
2019		70,409,000
2020		52,799,000
Thereafter		147,604,000
Total amortization of debt		450,121,000
Principal Repayments and Joint Venture Debt
2016		399,486,000
2017		871,548,000
2018		19,525,000
2019		28,317,000
2020		679,531,000
Thereafter		4,572,974,000
Total principal repayments		6,571,381,000
Revolving Credit Facility
Scheduled Amortization and Principal Repayments
2016		0
2017		0
2018		0
2019		0
2020		994,000,000
Thereafter		0
Total amortization of debt and principal repayments		994,000,000
Unsecured Term Loan
Scheduled Amortization and Principal Repayments
2016		0
2017		0
2018		0
2019		933,000,000
2020		0
Thereafter		0
Total amortization of debt and principal repayments		$ 933,000,000
Unsecured Term Loan | LIBOR
Debt Instrument [Line Items]
Interest rate added to base rate (as a percent)		1.40%
Senior Unsecured Notes
Scheduled Amortization and Principal Repayments
2016		$ 255,308,000
2017		355,008,000
2018		250,000,000
2019		0
2020		250,000,000
Thereafter		300,000,000
Total amortization of debt and principal repayments		$ 1,410,316,000