Unconsolidated Investments - Additional Information (Details) $ in Millions	3 Months Ended	9 Months Ended
	Jun. 30, 2015 USD ($) limited_partnership entity	Sep. 30, 2015 USD ($) JointVenture	Sep. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)
Schedule of Equity Method Investments [Line Items]
Investments in joint ventures		$ 498.3		$ 435.8
Payments to acquire equity method investments		155.2	$ 139.6
Equity method investment distribution received		50.8
Contributions to acquire interest in joint venture		88.2
Contributions to existing joint ventures		67.0
Proceeds from equity method investments		$ 142.8
Number of joint ventures with unfulfilled capital commitments | JointVenture		4
Guarantor obligations, maximum exposure, percentage of property level debt		1.00%
Investments in loan pool participations		$ 1.3		$ 56.4
Performance Guarantee [Member] | Loans Payable [Member]
Schedule of Equity Method Investments [Line Items]
Guarantor obligations, maximum exposure		59.9
Equity Method Investments [Member]
Schedule of Equity Method Investments [Line Items]
Unfunded capital commitments		38.9
Vintage Housing Holdings [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage	61.00%
Payments to acquire equity method investments	$ 78.7	78.7
Number of syndicated limited partnerships | limited_partnership	30
Number of non-affiliated entities | entity	1
Gain on fair value through equity income		12.9
Equity method investment distribution received		$ 5.2
Vintage Housing Holdings [Member] | Non-Affiliated Entity [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage	39.00%
Minimum [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		5.00%
Maximum [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investment, ownership percentage		50.00%