Investments (Realized Gains Losses Available-For-Sale Securities) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Proceeds from Sales	$ 43.8	$ 3.3
Gross Realized Gains	1.9	0.3
Gross Realized Losses	(1.3)	0.0
Seed Money [Member]
Schedule of Available-for-sale Securities [Line Items]
Proceeds from Sales	34.2	1.2
Gross Realized Gains	1.6	0.3
Gross Realized Losses	(1.3)	0.0
Collateralized Loan Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Proceeds from Sales	2.0	0.5
Gross Realized Gains	0.3	0.0
Gross Realized Losses	0.0	0.0
Other Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Proceeds from Sales	7.6	1.6
Gross Realized Gains	0.0	0.0
Gross Realized Losses	$ 0.0	$ 0.0