REAL ESTATE AND OTHER AFFILIATES (Narrative) (Details)		3 Months Ended					9 Months Ended	12 Months Ended
	May 23, 2019 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2015 USD ($) item	Dec. 31, 2018 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2020 USD ($)	Jul. 15, 2020
Investments in and Advances to Affiliates [Line Items]
Equity method investment, impairment									$ 0
Equity in earnings (losses) from real estate and other affiliates								$ 271,099,000	30,629,000	$ 39,954,000
Assets								9,140,332,000	8,413,766,000
Liabilities								5,396,195,000	5,080,778,000
Variable Interest Entity, Primary Beneficiary
Investments in and Advances to Affiliates [Line Items]
Assets									393,300,000
Liabilities									186,500,000
Other land, rental and property revenues
Investments in and Advances to Affiliates [Line Items]
Revenue								105,048,000	206,966,000	160,519,000
Equity Method Investments
Investments in and Advances to Affiliates [Line Items]
Carrying value of equity method investments								373,192,000	117,804,000
Equity in earnings (losses) from real estate and other affiliates								267,375,000	$ 27,006,000	36,599,000
Mr.C Seaport
Investments in and Advances to Affiliates [Line Items]
Equity method investment, impairment		$ 6,000,000.0						$ 6,000,000
Equity method investment, ownership percentage												35.00%
Mr.C Seaport | Seaport District
Investments in and Advances to Affiliates [Line Items]
Equity method investment, ownership percentage												35.00%
Mr.C Seaport | Equity Method Investments | Strategic Developments
Investments in and Advances to Affiliates [Line Items]
Equity method investment, ownership percentage								0.00%	35.00%
Carrying value of equity method investments								$ 0	$ 0
Equity in earnings (losses) from real estate and other affiliates								$ 0	$ 0	(240,000)
Mr.C Seaport | Equity Method Investments | Seaport District
Investments in and Advances to Affiliates [Line Items]
Equity method investment, ownership percentage								0.00%	35.00%
Carrying value of equity method investments								$ 0	$ 7,650,000
Equity in earnings (losses) from real estate and other affiliates								$ (6,900,000)	(1,980,000)	(465,000)
USAA Joint Venture
Investments in and Advances to Affiliates [Line Items]
Fair value of the land contributed to joint venture			$ 33,600,000
Contribution of property			$ 85,000,000.0
Capital contribution							$ 9,800,000
Joint venture, preferred return on capital			9.00%
Acquisition of 1% partnership interest in 110 North Wacker								90.00%
USAA Joint Venture | Local developer
Investments in and Advances to Affiliates [Line Items]
Capital contribution			$ 5,000,000.0				$ 1,100,000
USAA Joint Venture | USAA
Investments in and Advances to Affiliates [Line Items]
Capital contribution	$ 178,400,000		64,000,000.0
Joint venture capital obligations	$ 8,800,000		$ 105,600,000
Joint venture, preferred return on capital			9.00%
Joint venture, preferred return on capital, percent of cash distributions			11.11%
Acquisition of 1% partnership interest in 110 North Wacker								10.00%
110 North Wacker
Investments in and Advances to Affiliates [Line Items]
Carrying value of equity method investments											$ 273,600,000
110 North Wacker | Strategic Developments
Investments in and Advances to Affiliates [Line Items]
Equity in earnings (losses) from real estate and other affiliates								$ 267,500,000
110 North Wacker | Equity Method Investments | Strategic Developments
Investments in and Advances to Affiliates [Line Items]
Carrying value of equity method investments								0	0
Equity in earnings (losses) from real estate and other affiliates								267,518,000	0	0
110 North Wacker | Equity Method Investments | Strategic Developments | Other land, rental and property revenues
Investments in and Advances to Affiliates [Line Items]
Revenue								15,400,000
Bar Wayo
Investments in and Advances to Affiliates [Line Items]
Joint venture, preferred return on capital, percent of cash distributions					75.00%
Joint venture real estate, percentage funded				89.75%
Preferred return, on capital (as a percent)				10.00%
Join venture real estate, remaining cash distribution percentage					50.00%
Bar Wayo | Momofuku
Investments in and Advances to Affiliates [Line Items]
Joint venture, preferred return on capital, percent of cash distributions					25.00%
Joint venture real estate, percentage funded				10.25%
Bar Wayo | Equity Method Investments | Seaport District
Investments in and Advances to Affiliates [Line Items]
Carrying value of equity method investments								7,101,000	7,469,000
Equity in earnings (losses) from real estate and other affiliates								(2,392,000)	$ (612,000)	$ 0
Investment in real estate and other affiliates								7,100,000
The Summit
Investments in and Advances to Affiliates [Line Items]
Preferred return, on capital (as a percent)						5.00%
Entitlement of distribution by joint venture (in times) | item						2
The Summit | Equity Method Investments
Investments in and Advances to Affiliates [Line Items]
Fair value of the land contributed to joint venture						$ 13,400,000
SID Bonds transferred to a joint venture						1,300,000
Transaction value of the land contributed to joint venture						125,400,000
Value of land contributed to joint venture (price per acre)						226,000
Maximum capital contribution required						$ 30,000,000.0
110 North Wacker
Investments in and Advances to Affiliates [Line Items]
Recourse amount								100,600,000
Unconsolidated Properties
Investments in and Advances to Affiliates [Line Items]
Secured debt								545,000,000.0
Share of the entity in secured debt								$ 274,500,000