UNCONSOLIDATED INVESTMENTS - Distributions from Joint Ventures (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($)
Schedule of Equity Method Investments [Line Items]
Operating distributions from unconsolidated investments	$ 85.7
Investing distributions from unconsolidated investments	133.4
Multifamily [Member]
Schedule of Equity Method Investments [Line Items]
Operating distributions from unconsolidated investments	26.3
Investing distributions from unconsolidated investments	31.0
Commercial [Member]
Schedule of Equity Method Investments [Line Items]
Operating distributions from unconsolidated investments	8.8
Investing distributions from unconsolidated investments	15.3
Funds [Member]
Schedule of Equity Method Investments [Line Items]
Operating distributions from unconsolidated investments	4.0
Investing distributions from unconsolidated investments	5.7
Residential, Hotel and Other [Member]
Schedule of Equity Method Investments [Line Items]
Operating distributions from unconsolidated investments	46.6
Investing distributions from unconsolidated investments	81.4
Western United States [Member]
Schedule of Equity Method Investments [Line Items]
Operating distributions from unconsolidated investments	79.2
Investing distributions from unconsolidated investments	111.2
Western United States [Member] | Multifamily [Member]
Schedule of Equity Method Investments [Line Items]
Operating distributions from unconsolidated investments	26.1
Investing distributions from unconsolidated investments	26.1
Western United States [Member] | Commercial [Member]
Schedule of Equity Method Investments [Line Items]
Operating distributions from unconsolidated investments	6.5
Investing distributions from unconsolidated investments	15.3
Western United States [Member] | Funds [Member]
Schedule of Equity Method Investments [Line Items]
Operating distributions from unconsolidated investments	4.0
Investing distributions from unconsolidated investments	5.7
Western United States [Member] | Residential, Hotel and Other [Member]
Schedule of Equity Method Investments [Line Items]
Operating distributions from unconsolidated investments	42.6
Investing distributions from unconsolidated investments	64.1
United Kingdom [Member]
Schedule of Equity Method Investments [Line Items]
Operating distributions from unconsolidated investments	2.3
Investing distributions from unconsolidated investments	0.0
United Kingdom [Member] | Multifamily [Member]
Schedule of Equity Method Investments [Line Items]
Operating distributions from unconsolidated investments	0.0
Investing distributions from unconsolidated investments	0.0
United Kingdom [Member] | Commercial [Member]
Schedule of Equity Method Investments [Line Items]
Operating distributions from unconsolidated investments	2.3
Investing distributions from unconsolidated investments	0.0
United Kingdom [Member] | Funds [Member]
Schedule of Equity Method Investments [Line Items]
Operating distributions from unconsolidated investments	0.0
Investing distributions from unconsolidated investments	0.0
United Kingdom [Member] | Residential, Hotel and Other [Member]
Schedule of Equity Method Investments [Line Items]
Operating distributions from unconsolidated investments	0.0
Investing distributions from unconsolidated investments	0.0
Spain [Member]
Schedule of Equity Method Investments [Line Items]
Operating distributions from unconsolidated investments	4.0
Investing distributions from unconsolidated investments	17.3
Spain [Member] | Multifamily [Member]
Schedule of Equity Method Investments [Line Items]
Operating distributions from unconsolidated investments	0.0
Investing distributions from unconsolidated investments	0.0
Spain [Member] | Commercial [Member]
Schedule of Equity Method Investments [Line Items]
Operating distributions from unconsolidated investments	0.0
Investing distributions from unconsolidated investments	0.0
Spain [Member] | Funds [Member]
Schedule of Equity Method Investments [Line Items]
Operating distributions from unconsolidated investments	0.0
Investing distributions from unconsolidated investments	0.0
Spain [Member] | Residential, Hotel and Other [Member]
Schedule of Equity Method Investments [Line Items]
Operating distributions from unconsolidated investments	4.0
Investing distributions from unconsolidated investments	17.3
Japan [Member]
Schedule of Equity Method Investments [Line Items]
Operating distributions from unconsolidated investments	0.2
Investing distributions from unconsolidated investments	4.9
Japan [Member] | Multifamily [Member]
Schedule of Equity Method Investments [Line Items]
Operating distributions from unconsolidated investments	0.2
Investing distributions from unconsolidated investments	4.9
Japan [Member] | Commercial [Member]
Schedule of Equity Method Investments [Line Items]
Operating distributions from unconsolidated investments	0.0
Investing distributions from unconsolidated investments	0.0
Japan [Member] | Funds [Member]
Schedule of Equity Method Investments [Line Items]
Operating distributions from unconsolidated investments	0.0
Investing distributions from unconsolidated investments	0.0
Japan [Member] | Residential, Hotel and Other [Member]
Schedule of Equity Method Investments [Line Items]
Operating distributions from unconsolidated investments	0.0
Investing distributions from unconsolidated investments	$ 0.0