SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Narrative) (Details)		1 Months Ended	3 Months Ended	12 Months Ended
		Sep. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2020 USD ($) segment	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Number of business segments | segment				4
Impairment charge for real estate				$ 48,738,000	$ 0	$ 0
ASC 842 provision for operating leases					6,300,000
Accounts receivable, allowance			$ 15,600,000	24,000,000.0	15,600,000
Straight-line rent asset, allowance			1,000,000.0	9,000,000.0	1,000,000.0
Special Improvement District receivable			42,996,000	54,770,000	42,996,000
TIF receivable			3,931,000	893,000	3,931,000
Deferred leasing costs, accumulated amortization			31,700,000	39,700,000	31,700,000
Cumulative effect decrease from adopting ASU 2016-13			(3,332,988,000)	$ (3,715,023,000)	(3,332,988,000)	(3,238,126,000)	$ (3,188,551,000)
Financing receivable threshold period past due				30 days
Project in the west side of Manhattan
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Development value of project overseen by president in prior role				$ 20,000,000,000
MetLife Stadium
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Development value of project overseen by president in prior role				1,300,000,000
COVID-19
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total provision for operating leases				27,800,000
ASC 842 provision for operating leases				21,800,000
ASC 450 provision for operating leases				6,000,000.0
MUD Receivables
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accrued interest			17,300,000	15,700,000	17,300,000
Current-period provision for expected credit losses				0
Retained earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect decrease from adopting ASU 2016-13			46,385,000	72,556,000	46,385,000	$ 120,341,000	$ 109,508,000
Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect decrease from adopting ASU 2016-13	[1]		18,000		18,000
Cumulative Effect, Period of Adoption, Adjustment | Retained earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect decrease from adopting ASU 2016-13	[1]		18,000.0		18,000.0
Chief Executive Officer
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Salary and benefit related expense		$ 1,400,000
2019 Restructuring Plan
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Value of non-core assets expected to sell			2,000,000,000.0		$ 2,000,000,000.0
2019 Restructuring Plan | Employee Severance
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Restructuring charges			$ 34,300,000	2,600,000
Minimum | 2019 Restructuring Plan | Employee Severance
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Restructuring charges, expected cost remaining				$ 200,000
Minimum | Master Planned Communities
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Estimated useful life of MPC assets				20 years
Maximum | 2019 Restructuring Plan | Employee Severance
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Restructuring charges, expected cost remaining				$ 500,000
Maximum | Master Planned Communities
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Estimated useful life of MPC assets				40 years

[1] Related to the adoption of ASU 2016-13, Financial Instruments-Credit Losses (Topic 326) and all its related amendments as of January 1, 2020.