Corporate Indebtedness (Junior Subordinated Deferrable Interest Debentures and Principal Maturities)(Details) - USD ($)	1 Months Ended	3 Months Ended
	Jun. 30, 2005	Mar. 31, 2016
Scheduled Amortization and Principal Repayments
Remaining 2016		$ 351,810,000
2017		1,495,199,000
2018		356,536,000
2019		1,003,255,000
2020		1,757,330,000
Thereafter		5,120,578,000
Total amortization of debt and principal repayments		10,084,708,000
Joint Venture Debt
Principal Repayments and Joint Venture Debt
Remaining 2016		526,982,000
2017		591,284,000
2018		447,000
2019		104,687,000
2020		30,298,000
Thereafter		451,544,000
Total principal repayments		$ 1,705,242,000
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
Remaining 2016		$ 0
2017		0
2018		0
2019		0
2020		0
Thereafter		100,000,000
Total amortization of debt and principal repayments		100,000,000
Mortgages and other loans payable
Scheduled Amortization
Remaining 2016		42,226,000
2017		63,643,000
2018		64,119,000
2019		70,255,000
2020		52,799,000
Thereafter		147,604,000
Total amortization of debt		440,646,000
Principal Repayments and Joint Venture Debt
Remaining 2016		309,584,000
2017		1,076,548,000
2018		42,417,000
2019		0
2020		679,531,000
Thereafter		4,572,974,000
Total principal repayments		6,681,054,000
Revolving Credit Facility
Scheduled Amortization and Principal Repayments
Remaining 2016		0
2017		0
2018		0
2019		0
2020		775,000,000
Thereafter		0
Total amortization of debt and principal repayments		775,000,000
Unsecured Term Loan
Scheduled Amortization and Principal Repayments
Remaining 2016		0
2017		0
2018		0
2019		933,000,000
2020		0
Thereafter		0
Total amortization of debt and principal repayments		933,000,000
Senior Unsecured Notes
Scheduled Amortization and Principal Repayments
Remaining 2016		0
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		$ 1,155,008,000
LIBOR | Trust Preferred Securities
Debt Instrument [Line Items]
Interest rate added to base rate (as a percent)		1.25%
LIBOR | Unsecured Term Loan
Debt Instrument [Line Items]
Interest rate added to base rate (as a percent)		1.40%