Corporate Indebtedness - Junior Subordinated Deferrable Interest Debentures and Principal Maturities (Details) - USD ($)	1 Months Ended	9 Months Ended
	Jun. 30, 2005	Sep. 30, 2018
Principal Repayments and Joint Venture Debt
Remaining 2018		$ 9,881,000
2019		158,271,000
2020		977,997,000
2021		361,638,000
2022		1,007,985,000
Thereafter		3,113,258,000
Total principal repayments		$ 5,629,030,000
Trust Preferred Securities
Debt Instrument [Line Items]
Proceeds from issuance of debt	$ 100,000,000.0
LIBOR | Trust Preferred Securities
Debt Instrument [Line Items]
Interest rate added to base rate (as a percent)		1.25%
Joint venture
Principal Repayments and Joint Venture Debt
Remaining 2018		$ 2,568,000
2019		115,295,000
2020		276,822,000
2021		454,621,000
2022		220,810,000
Thereafter		2,879,344,000
Total principal repayments		3,949,460,000
Joint venture | Trust Preferred Securities
Principal Repayments and Joint Venture Debt
Remaining 2018		0
2019		0
2020		0
2021		0
2022		0
Thereafter		100,000,000
Total principal repayments		100,000,000
Joint venture | Mortgages and other loans payable
Scheduled Amortization
Remaining 2018		9,881,000
2019		42,271,000
2020		23,466,000
2021		11,638,000
2022		9,430,000
Thereafter		16,591,000
Total amortization of debt		113,277,000
Principal Repayments and Joint Venture Debt
Remaining 2018		0
2019		116,000,000
2020		704,531,000
2021		0
2022		198,555,000
Thereafter		1,251,667,000
Total principal repayments		2,270,753,000
Joint venture | Revolving Credit Facility
Principal Repayments and Joint Venture Debt
Remaining 2018		0
2019		0
2020		0
2021		0
2022		0
Thereafter		145,000,000
Total principal repayments		145,000,000
Joint venture | Unsecured Term Loans
Principal Repayments and Joint Venture Debt
Remaining 2018		0
2019		0
2020		0
2021		0
2022		0
Thereafter		1,500,000,000
Total principal repayments		1,500,000,000
Joint venture | Senior Unsecured Notes
Principal Repayments and Joint Venture Debt
Remaining 2018		0
2019		0
2020		250,000,000
2021		350,000,000
2022		800,000,000
Thereafter		100,000,000
Total principal repayments		$ 1,500,000,000