Unconsolidated Investments - Schedule of Joint Ventures (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	$ 1,277.0	$ 1,326.5
Multifamily [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	601.5	609.2
Commercial [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	272.5	266.2
Hotel [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	77.1	72.8
Funds [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	142.3	139.6
Residential and Other [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	183.6	238.7
Western U.S. [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	688.6	759.7
Western U.S. [Member] | Multifamily [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	219.3	230.5
Western U.S. [Member] | Commercial [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	80.1	78.1
Western U.S. [Member] | Hotel [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	77.1	72.8
Western U.S. [Member] | Funds [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	128.5	139.6
Western U.S. [Member] | Residential and Other [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	183.6	238.7
Ireland [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	535.7	518.1
Ireland [Member] | Multifamily [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	382.2	378.7
Ireland [Member] | Commercial [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	150.8	139.4
Ireland [Member] | Hotel [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	0.0	0.0
Ireland [Member] | Funds [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	2.7	0.0
Ireland [Member] | Residential and Other [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	0.0	0.0
United Kingdom [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	52.7	48.7
United Kingdom [Member] | Multifamily [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	0.0	0.0
United Kingdom [Member] | Commercial [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	41.6	48.7
United Kingdom [Member] | Hotel [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	0.0	0.0
United Kingdom [Member] | Funds [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	11.1	0.0
United Kingdom [Member] | Residential and Other [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	$ 0.0	$ 0.0