Corporate Indebtedness - Junior Subordinated Deferrable Interest Debentures and Principal Maturities (Details) - USD ($)	1 Months Ended	12 Months Ended
	Jun. 30, 2005	Dec. 31, 2017
Principal Repayments and Joint Venture Debt
2018		$ 494,402,000
2018		42,289,000
2019		953,018,000
2020		11,656,000
2021		1,008,003,000
Thereafter		3,396,623,000
Scheduled Amortization and Principal Repayments
Total fixed rate and floating rate debt		$ 5,905,991,000
Trust Preferred Securities
Debt Instrument [Line Items]
Proceeds from issuance of debt	$ 100,000,000.0
Trust Preferred Securities | LIBOR
Debt Instrument [Line Items]
Interest rate added to base rate (as a percent)		1.25%
Joint venture
Principal Repayments and Joint Venture Debt
2018		$ 200,250,000
2018		717,682,000
2019		473,809,000
2020		449,740,000
2021		223,330,000
Thereafter		2,119,481,000
Long Term Debt, Maturities, Repayments of Principal		4,184,292,000
Joint venture | Trust Preferred Securities
Principal Repayments and Joint Venture Debt
2018		0
2018		0
2019		0
2020		0
2021		0
Thereafter		100,000,000
Scheduled Amortization and Principal Repayments
Total fixed rate and floating rate debt		100,000,000
Joint venture | Mortgages and other loans payable
Scheduled Amortization
2018		37,971,000
2018		42,289,000
2019		23,487,000
2020		11,656,000
2021		9,448,000
Thereafter		16,675,000
Principal Repayments and Joint Venture Debt
2018		206,431,000
2018		0
2019		679,531,000
2020		0
2021		198,555,000
Thereafter		1,639,948,000
Future Amortization of Debt		141,526,000
Long Term Debt, Maturities, Repayments of Principal		2,724,465,000
Joint venture | Revolving Credit Facility
Principal Repayments and Joint Venture Debt
2018		0
2018		0
2019		0
2020		0
2021		0
Thereafter		40,000,000
Scheduled Amortization and Principal Repayments
Total fixed rate and floating rate debt		40,000,000
Joint venture | Unsecured Term Loans
Principal Repayments and Joint Venture Debt
2018		0
2018		0
2019		0
2020		0
2021		0
Thereafter		1,500,000,000
Scheduled Amortization and Principal Repayments
Total fixed rate and floating rate debt		1,500,000,000
Joint venture | Senior Unsecured Notes
Principal Repayments and Joint Venture Debt
2018		250,000,000
2018		0
2019		250,000,000
2020		0
2021		800,000,000
Thereafter		100,000,000
Scheduled Amortization and Principal Repayments
Total fixed rate and floating rate debt		$ 1,400,000,000