Assets and Liabilities, at Fair Value - Rollforward of Level 3 (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Rollforward of Level 3 Liabilities [Roll Forward]
Business Combination, Contingent Consideration Arrangements, Change in Amount of Contingent Consideration, Liability	$ 0.0	$ (0.2)	$ (0.4)
Contingent Consideration [Member]
Rollforward of Level 3 Liabilities [Roll Forward]
Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Value	1.0	0.8
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings	0.0	0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Period Increase (Decrease)	0.0	0.2
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Purchases	0.0	0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements	(1.0)	0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Transfers, Net	0.0	0.0
Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Value	0.0	1.0	0.8
Equity Securities [Member]
Rollforward of Level 3 Assets [Roll Forward]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	0.1	0.2
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	0.0	0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Period Increase (Decrease)	(0.1)	(0.1)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	0.0	0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	0.0	0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers, Net	0.0	0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	0.0	0.1	0.2
Corporate Debt Securities [Member]
Rollforward of Level 3 Assets [Roll Forward]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	$ 0.0	3.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings		0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Period Increase (Decrease)		0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases		0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements		(3.0)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers, Net		0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value		$ 0.0	$ 3.0