Investments - Summary of Gross Unrealized Losses, AFS, Continuous Loss Position (Details) - USD ($) $ in Millions	Dec. 31, 2018	Sep. 30, 2018
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value		$ 59.7
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Gross Unrealized Losses		(3.9)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Greater, Fair Value		21.0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Greater, Gross Unrealized Losses		(0.5)
Available-for-sale Securities, Continuous Unrealized Loss Position, Total Fair Value		80.7
Available-for-sale Securities, Continuous Unrealized Loss Position, Total Gross Unrealized Losses		(4.4)
Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 10.7
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Gross Unrealized Losses	(2.0)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Greater, Fair Value	0.0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Greater, Gross Unrealized Losses	0.0
Available-for-sale Securities, Continuous Unrealized Loss Position, Total Fair Value	10.7
Available-for-sale Securities, Continuous Unrealized Loss Position, Total Gross Unrealized Losses	$ (2.0)
Sponsored Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value		48.8
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Gross Unrealized Losses		(2.1)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Greater, Fair Value		21.0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Greater, Gross Unrealized Losses		(0.5)
Available-for-sale Securities, Continuous Unrealized Loss Position, Total Fair Value		69.8
Available-for-sale Securities, Continuous Unrealized Loss Position, Total Gross Unrealized Losses		(2.6)
Debt and Other Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value		10.9
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Gross Unrealized Losses		(1.8)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Greater, Fair Value		0.0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Greater, Gross Unrealized Losses		0.0
Available-for-sale Securities, Continuous Unrealized Loss Position, Total Fair Value		10.9
Available-for-sale Securities, Continuous Unrealized Loss Position, Total Gross Unrealized Losses		$ (1.8)