Investments - Available-for-Sale Securities (Detail) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Sep. 30, 2016	Sep. 30, 2015
Schedule of Investments [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	$ 46.4	$ 65.1	$ 78.1	$ 91.6
Total Gains (Losses) Included in Earnings	(1.3)	0.6
Purchases	3.0	5.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	6.3	26.6
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements		5.5
Net transfer In (Out) of Level 3 (a)	(27.1)
Corporate Debt Securities [Member]
Schedule of Investments [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value				$ 14.1
Total Gains (Losses) Included in Earnings		(0.5)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales		$ 13.6
Fixed-maturity securities, available-for-sale [Member]
Schedule of Investments [Line Items]
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	18.9		46.2
Available-for-sale Securities, Debt Securities	1,532.8		1,535.1
Total investments, Carrying Value	1,532.8		1,535.1
Available-for-sale Debt Securities, Amortized Cost Basis	$ 1,570.1		$ 1,531.8