Fair Value Measurements and the Fair Value Option - Unconsolidated Investments, Narrative (Details) (Details) $ in Millions	9 Months Ended
	Sep. 30, 2015 USD ($) Investment	Dec. 31, 2014 USD ($)
Equity Method Investments [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Unfunded capital commitments	$ 38.9
Equity Method Investments [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair value option		$ 61.0
Funds [Member] | Equity Method Investments [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Funds	33.3	24.9
Unfunded capital commitments	$ 38.1
Funds [Member] | Equity Method Investments [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Number of unconsolidated investments with fair value option election | Investment	5
Fair value option	$ 172.8	$ 61.0