MORTGAGE DEBT - Schedule of Mortgage Debt (Details) - Mortgage debt [Member] - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Debt, net of discount (premium)	$ 2,655.3	$ 2,968.3
Unamortized loan fees	(14.3)	(18.0)
Total Debt	2,641.0	2,950.3
Unamortized premium (discount)	4.0	1.9
Mortgage Loans Secured by Multi-Family Properties [Member] \| Western U.S. [Member]
Debt Instrument [Line Items]
Debt, net of discount (premium)	1,324.7	1,286.2
Mortgage Loans Secured by Multi-Family Properties [Member] \| Ireland [Member]
Debt Instrument [Line Items]
Debt, net of discount (premium)		147.8
Mortgage Loans Secured by Commercial [Member] \| Western U.S. [Member]
Debt Instrument [Line Items]
Debt, net of discount (premium)	405.4	385.3
Mortgage Loans Secured by Commercial [Member] \| United Kingdom [Member]
Debt Instrument [Line Items]
Debt, net of discount (premium)	514.5	499.6
Mortgage Loans Secured by Commercial [Member] \| Ireland [Member]
Debt Instrument [Line Items]
Debt, net of discount (premium)	289.6	438.9
Mortgage Loans Secured by Commercial [Member] \| Spain [Member]
Debt Instrument [Line Items]
Debt, net of discount (premium)	40.3	90.1
Mortgage Loans Secured by Hotel [Member] \| Western U.S. [Member]
Debt Instrument [Line Items]
Debt, net of discount (premium)		37.9
Mortgage Loans Secured by Hotel [Member] \| Ireland [Member]
Debt Instrument [Line Items]
Debt, net of discount (premium)	$ 80.8	$ 82.5

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Amount, after accumulated amortization, of debt discount (premium).

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Amount, after accumulated amortization, of debt issuance costs. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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