Accounting Policies Impact of adoption ASU 2014-09 on the Condensed Consolidated Statements of Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Investment Advisory Fees	$ 1,043.7	$ 955.2
Distribution and Servicing Fees	246.1	206.4
Performance Fees	9.1	11.3
Other	56.9	19.7
Total operating revenues	1,355.8	1,192.6
Third party distribution service and advisory	419.1	349.3
Labor and Related Expense	390.4	397.5
Marketing Expense	28.1	24.4
Communications, Information Technology and Occupancy	102.2	85.5
General and Administrative Expense	94.9	78.0
Total operating expenses	1,034.7	934.7
Operating Income (Loss)	321.1	$ 257.9
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Investment Advisory Fees	1,097.5
Distribution and Servicing Fees	213.7
Performance Fees	9.1
Other	15.9
Total operating revenues	1,336.2
Third party distribution service and advisory	395.3
Labor and Related Expense	390.4
Marketing Expense	28.1
Communications, Information Technology and Occupancy	102.2
General and Administrative Expense	99.1
Total operating expenses	1,015.1
Operating Income (Loss)	321.1
Difference between Revenue Guidance in Effect before and after Topic 606 [Member] | Accounting Standards Update 2014-09 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Investment Advisory Fees	53.8
Distribution and Servicing Fees	(32.4)
Performance Fees	0.0
Other	(41.0)
Total operating revenues	(19.6)
Third party distribution service and advisory	(23.8)
Labor and Related Expense	0.0
Marketing Expense	0.0
Communications, Information Technology and Occupancy	0.0
General and Administrative Expense	4.2
Total operating expenses	(19.6)
Operating Income (Loss)	$ 0.0