Mortgage Debt - Schedule of Mortgage Debt (Details) - Mortgage Debt [Member] - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Mortgage debt (excluding loan fees)	$ 2,463.3	$ 2,655.3
Unamortized loan fees	(12.4)	(14.3)
Total debt	2,450.9	2,641.0
Unamortized discount (premium)	(3.9)	(4.0)
Mortgage Loans Secured by Multi-Family Properties [Member] \| Western U.S. [Member]
Debt Instrument [Line Items]
Mortgage debt (excluding loan fees)	1,332.1	1,324.7
Mortgage Loans Secured by Commercial [Member] \| Western U.S. [Member]
Debt Instrument [Line Items]
Mortgage debt (excluding loan fees)	272.6	405.4
Mortgage Loans Secured by Commercial [Member] \| United Kingdom [Member]
Debt Instrument [Line Items]
Mortgage debt (excluding loan fees)	385.0	514.5
Mortgage Loans Secured by Commercial [Member] \| Ireland [Member]
Debt Instrument [Line Items]
Mortgage debt (excluding loan fees)	355.3	289.6
Mortgage Loans Secured by Commercial [Member] \| Spain [Member]
Debt Instrument [Line Items]
Mortgage debt (excluding loan fees)	79.0	40.3
Mortgage Loans Secured by Hotel [Member] \| Ireland [Member]
Debt Instrument [Line Items]
Mortgage debt (excluding loan fees)	$ 39.3	$ 80.8

Long-Term Debt, Net of (Premium) Discount

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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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