Unconsolidated Investments - Schedule of Joint Ventures (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	$ 613.7	$ 519.3
Western U.S. [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	517.2	506.9
Ireland [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	89.6
United Kingdom [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	6.9	9.9
Japan [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures		2.5
Multifamily [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	274.3	202.2
Multifamily [Member] | Western U.S. [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	184.7	199.7
Multifamily [Member] | Ireland [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	89.6
Multifamily [Member] | United Kingdom [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	0.0	0.0
Multifamily [Member] | Japan [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures		2.5
Commercial [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	46.8	52.0
Commercial [Member] | Western U.S. [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	39.9	42.1
Commercial [Member] | Ireland [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	0.0
Commercial [Member] | United Kingdom [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	6.9	9.9
Commercial [Member] | Japan [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures		0.0
Funds [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	108.5	85.3
Funds [Member] | Western U.S. [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	108.5	85.3
Funds [Member] | Ireland [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	0.0
Funds [Member] | United Kingdom [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	0.0	0.0
Funds [Member] | Japan [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures		0.0
Residential and Other [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	184.1	179.8
Residential and Other [Member] | Western U.S. [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	184.1	179.8
Residential and Other [Member] | Ireland [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	0.0
Residential and Other [Member] | United Kingdom [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	$ 0.0	0.0
Residential and Other [Member] | Japan [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures		$ 0.0