UNCONSOLIDATED INVESTMENTS - Income from Unconsolidated Investments (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($) property	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Schedule of Equity Method Investments [Line Items]
Income from unconsolidated investments - operating performance	$ 35.5	$ 18.8	$ 16.4
Income from unconsolidated investments - realized gains	53.5	22.1	14.0
Income from unconsolidated investments - fair value	64.7	10.4	38.6
Income from unconsolidated investments - performance fees	36.3	27.4	8.8
Income from unconsolidated investments - impairment	(10.3)	0.0	0.0
Total interest income and foreign currency gain (loss)	$ 179.7	$ 78.7	$ 77.8
Western United States [Member] | Joint Venture [Member]
Schedule of Equity Method Investments [Line Items]
Number of real estate properties sold | property	3