Summary of Significant Accounting Policies (Details) $ in Millions	3 Months Ended	12 Months Ended
	Sep. 30, 2015 USD ($)	Dec. 31, 2015 USD ($) item	Dec. 31, 2014 USD ($)
Summary of Significant Accounting Policies
Number of operating business segments | item		1
Ownership interest of seed investment in new product (as a percent)		100.00%
Investment Securities and Investments in Sponsored Funds
Transfers to equity method securities		$ 160.2
Unrealized losses	$ 2.1
Summary of property and equipment
Internal costs capitalized		$ 13.9	$ 13.5
Maximum
Summary of property and equipment
Ownership interest in LLCs (as a percent)		3.00%
Furniture and fixtures | Minimum
Summary of property and equipment
Estimated useful lives		3 years
Furniture and fixtures | Maximum
Summary of property and equipment
Estimated useful lives		10 years
Computer software | Minimum
Summary of property and equipment
Estimated useful lives		1 year
Computer software | Maximum
Summary of property and equipment
Estimated useful lives		10 years
Data processing equipment | Minimum
Summary of property and equipment
Estimated useful lives		1 year
Data processing equipment | Maximum
Summary of property and equipment
Estimated useful lives		5 years
Buildings | Minimum
Summary of property and equipment
Estimated useful lives		1 year
Buildings | Maximum
Summary of property and equipment
Estimated useful lives		40 years
Other equipment | Minimum
Summary of property and equipment
Estimated useful lives		3 years
Other equipment | Maximum
Summary of property and equipment
Estimated useful lives		26 years
Leasehold improvements | Minimum
Summary of property and equipment
Estimated useful lives		1 year
Leasehold improvements | Maximum
Summary of property and equipment
Estimated useful lives		15 years