Accounting Policies (Narrative) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($) reporting_units	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Property, Plant and Equipment [Line Items]
Liabilities and Equity	$ 25,734.3	$ 25,073.2
Fair value of recognized acquired assets	100.00%
Non controlling interest in acquisitions	100.00%
Recognized liabilities acquired	100.00%
Cash deposits with clearing organizations	$ 11.4	11.4
Reporting Units for Goodwill | reporting_units	1
Net Waivers	$ 7.2
Share Based Payment, Estimated Forfeitures, Percentage	3.00%
Percentage of Excess in Projected Benefit Obligation and Fair Value Assets To Be Amortized	10.00%
Advertising Costs	$ 32.7	35.4	$ 38.6
Debt issuance cost			9.0
Adjustments to Additional Paid in Capital, Income Tax Benefit from Share-based Compensation	$ 1.7	21.2	24.0
Building [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life, Minimum	50 years
Minimum [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life, Minimum	3 years
Intangible assets, useful life, years	7 years
Minimum [Member] | CLO [Member]
Property, Plant and Equipment [Line Items]
Reporting Period Lag Between Parent Company and Consolidated Entity	1 month
Minimum [Member] | Software Development [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life, Minimum	5 years
Maximum [Member]
Property, Plant and Equipment [Line Items]
Cash Equivalent, Maturity Period	3 months
Settlement Period for Fund and Investor Receivables	4 days
Amortization Period of Sales Commission of Share B Redemptions	6 years
Property, Plant and Equipment, Useful Life, Minimum	7 years
Intangible assets, useful life, years	12 years
Maximum [Member] | CLO [Member]
Property, Plant and Equipment [Line Items]
Reporting Period Lag Between Parent Company and Consolidated Entity	3 months
Maximum [Member] | Software Development [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life, Minimum	7 years
Waiver [Member]
Property, Plant and Equipment [Line Items]
Investment Advisory, Management and Administrative Fees	$ 16.2
Change in Third party distribution service and advisory fee	55.00%
Accounting Standards Update 2015-02 [Member]
Property, Plant and Equipment [Line Items]
Liabilities and Equity	$ 3,170.6
Accounting Standards Update 2015-02 [Member] | Consolidation, Eliminations [Member]
Property, Plant and Equipment [Line Items]
Liabilities and Equity	458.5
Accounting Standards Update 2016-09
Property, Plant and Equipment [Line Items]
Adjustments to Additional Paid in Capital, Income Tax Deficiency from Share-based Compensation	1.7
Adjustments to Additional Paid in Capital, Income Tax Benefit from Share-based Compensation		$ 21.2	$ 24.0
CLO [Member] | Accounting Standards Update 2015-02 [Member]
Property, Plant and Equipment [Line Items]
Liabilities and Equity	$ 2,366.5