Taxation (Income Before Taxes) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Schedule of Income Before Taxes, Domestic and Foreign [Line Items]
Income before income taxes	$ 1,138.1	$ 1,429.2	$ 1,206.6
Domestic tax authority
Schedule of Income Before Taxes, Domestic and Foreign [Line Items]
Income before income taxes	491.8	638.5	539.9
Foreign tax authority
Schedule of Income Before Taxes, Domestic and Foreign [Line Items]
Income before income taxes	646.3	790.7	666.7
Separate Institutional Accounts | Domestic tax authority
Schedule of Income Before Taxes, Domestic and Foreign [Line Items]
Income before income taxes	472.9	638.4	537.5
Separate Institutional Accounts | Foreign tax authority
Schedule of Income Before Taxes, Domestic and Foreign [Line Items]
Income before income taxes	656.1	754.8	652.0
Consolidated Investment Products
Schedule of Income Before Taxes, Domestic and Foreign [Line Items]
Income before income taxes	9.1	36.0	17.1
Consolidated Investment Products | Domestic tax authority
Schedule of Income Before Taxes, Domestic and Foreign [Line Items]
Income before income taxes	18.9	0.1	2.4
Consolidated Investment Products | Foreign tax authority
Schedule of Income Before Taxes, Domestic and Foreign [Line Items]
Income before income taxes	$ (9.8)	$ 35.9	$ 14.7