Corporate Indebtedness (Junior Subordinated Deferrable Interest Debentures and Principal Maturities)(Details) - USD ($)	1 Months Ended	3 Months Ended
	Jun. 30, 2005	Mar. 31, 2017
Scheduled Amortization and Principal Repayments
Remaining 2017		$ 650,980,000
2017		603,101,000
2018		1,242,618,000
2019		970,958,000
2020		130,418,000
Thereafter		3,066,470,000
Total amortization of debt and principal repayments		6,664,545,000
Joint Venture Debt
Principal Repayments and Joint Venture Debt
Remaining 2017		82,368,000
2017		282,958,000
2018		555,545,000
2019		320,884,000
2020		184,134,000
Thereafter		1,349,340,000
Total principal repayments		2,775,229,000
Trust Preferred Securities
Debt Instrument [Line Items]
Proceeds from Issuance of Debt	$ 100,000,000.0
Scheduled Amortization and Principal Repayments
Remaining 2017		0
2017		0
2018		0
2019		0
2020		0
Thereafter		100,000,000
Total amortization of debt and principal repayments		$ 100,000,000
Debt Instrument, Interest Rate, Stated Percentage		5.61%
Number of Years from Issuance of Debt for which Fixed Rate of Interest will be in Force		10 years
Mortgages and other loans payable
Scheduled Amortization
Remaining 2017		$ 38,330,000
2017		54,938,000
2018		59,618,000
2019		41,427,000
2020		30,418,000
Thereafter		90,533,000
Total amortization of debt		315,264,000
Principal Repayments and Joint Venture Debt
Remaining 2017		267,650,000
2017		298,163,000
2018		0
2019		679,531,000
2020		100,000,000
Thereafter		2,575,937,000
Total principal repayments		3,921,281,000
Revolving Credit Facility
Scheduled Amortization and Principal Repayments
Remaining 2017		0
2017		0
2018		0
2019		0
2020		0
Thereafter		0
Total amortization of debt and principal repayments		0
Unsecured Term Loan
Scheduled Amortization and Principal Repayments
Remaining 2017		0
2017		0
2018		1,183,000,000
2019		0
2020		0
Thereafter		0
Total amortization of debt and principal repayments		1,183,000,000
Senior Unsecured Notes
Scheduled Amortization and Principal Repayments
Remaining 2017		345,000,000
2017		250,000,000
2018		0
2019		250,000,000
2020		0
Thereafter		300,000,000
Total amortization of debt and principal repayments		$ 1,145,000,000
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%