Taxation (Income Before Taxes) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Schedule of Income Before Taxes, Domestic and Foreign [Line Items]
Income (Loss) from Continuing Operations before Income Taxes, Extraordinary Items, Noncontrolling Interest	$ 1,395.1	$ 1,255.2	$ 830.6
Domestic Tax Authority [Member]
Schedule of Income Before Taxes, Domestic and Foreign [Line Items]
Income (Loss) from Continuing Operations before Income Taxes, Extraordinary Items, Noncontrolling Interest	722.6	598.3	516.3
Foreign Tax Authority [Member]
Schedule of Income Before Taxes, Domestic and Foreign [Line Items]
Income (Loss) from Continuing Operations before Income Taxes, Extraordinary Items, Noncontrolling Interest	672.5	656.9	314.3
Separate Institutional Accounts [Member] | Domestic Tax Authority [Member]
Schedule of Income Before Taxes, Domestic and Foreign [Line Items]
Income (Loss) from Continuing Operations before Income Taxes, Extraordinary Items, Noncontrolling Interest	659.6	553.1	456.6
Separate Institutional Accounts [Member] | Foreign Tax Authority [Member]
Schedule of Income Before Taxes, Domestic and Foreign [Line Items]
Income (Loss) from Continuing Operations before Income Taxes, Extraordinary Items, Noncontrolling Interest	724.4	666.1	474.5
Consolidated Investment Products [Member]
Schedule of Income Before Taxes, Domestic and Foreign [Line Items]
Income (Loss) from Continuing Operations before Income Taxes, Extraordinary Items, Noncontrolling Interest	11.1	36.0	(100.5)
Consolidated Investment Products [Member] | Domestic Tax Authority [Member]
Schedule of Income Before Taxes, Domestic and Foreign [Line Items]
Income (Loss) from Continuing Operations before Income Taxes, Extraordinary Items, Noncontrolling Interest	63.0	45.2	59.7
Consolidated Investment Products [Member] | Foreign Tax Authority [Member]
Schedule of Income Before Taxes, Domestic and Foreign [Line Items]
Income (Loss) from Continuing Operations before Income Taxes, Extraordinary Items, Noncontrolling Interest	$ (51.9)	$ (9.2)	$ (160.2)