Corporate Indebtedness - Junior Subordinated Deferrable Interest Debentures and Principal Maturities (Details) - USD ($)	1 Months Ended	3 Months Ended
	Jun. 30, 2005	Mar. 31, 2018
Principal Repayments and Joint Venture Debt
Remaining 2018		$ 352,555,000
2019		84,414,000
2020		1,142,880,000
2021		11,638,000
2022		1,007,985,000
Thereafter		2,856,708,000
Total principal repayments		$ 5,456,180,000
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		$ 45,477,000
2019		262,460,000
2020		471,714,000
2021		454,571,000
2022		220,759,000
Thereafter		2,878,381,000
Total principal repayments		4,333,362,000
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		28,555,000
2019		42,271,000
2020		23,466,000
2021		11,638,000
2022		9,430,000
Thereafter		16,591,000
Total amortization of debt		131,951,000
Principal Repayments and Joint Venture Debt
Remaining 2018		74,000,000
2019		42,143,000
2020		869,414,000
2021		0
2022		198,555,000
Thereafter		1,140,117,000
Total principal repayments		2,324,229,000
Joint venture | Revolving Credit Facility
Principal Repayments and Joint Venture Debt
Remaining 2018		0
2019		0
2020		0
2021		0
2022		0
Thereafter		0
Total principal repayments		0
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		250,000,000
2019		0
2020		250,000,000
2021		0
2022		800,000,000
Thereafter		100,000,000
Total principal repayments		$ 1,400,000,000