Unconsolidated Investments - Schedule of Joint Ventures (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	$ 498.3	$ 435.8
Western United States [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	437.3	375.4
Japan [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	6.2
United Kingdom [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	25.4	31.5
Spain [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	29.4	28.9
Multifamily [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	175.6	134.5
Multifamily [Member] | Western United States [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	169.4	134.5
Multifamily [Member] | Japan [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	6.2
Multifamily [Member] | United Kingdom [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	0.0	0.0
Multifamily [Member] | Spain [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	0.0	0.0
Commercial [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	85.9	141.8
Commercial [Member] | Western United States [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	60.5	110.3
Commercial [Member] | Japan [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	25.4	31.5
Commercial [Member] | Spain [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	0.0	0.0
Loan [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	12.5	50.3
Loan [Member] | Western United States [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	12.5	50.3
Loan [Member] | Japan [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	0.0
Loan [Member] | United Kingdom [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	0.0	0.0
Loan [Member] | Spain [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	0.0	0.0
Residential [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	182.6	71.0
Residential [Member] | Western United States [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	182.6	71.0
Residential [Member] | Japan [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	0.0
Residential [Member] | United Kingdom [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	0.0	0.0
Residential [Member] | Spain [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	0.0	0.0
Other [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	41.7	38.2
Other [Member] | Western United States [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	12.3	9.3
Other [Member] | Japan [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	0.0
Other [Member] | United Kingdom [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	0.0	0.0
Other [Member] | Spain [Member]
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures	$ 29.4	$ 28.9