Fair Value of Financial Instruments - Assets and Liabilities Carrying at Fair Value on Recurring Basis (Detail) - USD ($) $ in Millions		12 Months Ended
		Sep. 30, 2016		Sep. 30, 2015		Sep. 30, 2014	Sep. 30, 2013
Fair Value Assets And Liabilities Measured On Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers, Net		$ 5.2	[1]	$ 19.1	[2]	$ 0.0
Assets
Cash and cash equivalents		497.3		661.2		728.8	$ 677.0
Fixed maturity securities, available-for-sale:
Asset Backed Loans And Other Invested Assets		35.0		226.7
Funds Withheld Receivable		1,650.4		1,710.1
Assets, Fair Value Disclosure		78.1		91.6
Corporate Debt Securities [Member]
Fair Value Assets And Liabilities Measured On Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers, Net						0.0
Other invested assets
Fair Value Assets And Liabilities Measured On Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers, Net		0.0	[1]	19.1	[2]
Estimate of Fair Value Measurement [Member]
Fixed maturity securities, available-for-sale:
Policy loans and other invested assets	[3]	8.5
Asset Backed Loans And Other Invested Assets	[3]	35.0		226.7
Liabilities
Debt Instrument, Fair Value Disclosure	[3],[4]	5,729.2		6,170.1
Derivatives, Liabilities
Liabilities Related to Investment Contracts, Fair Value Disclosure	[3]	922.9		960.3
Reported Value Measurement [Member]
Fixed maturity securities, available-for-sale:
Policy loans and other invested assets	[3]	8.5
Asset Backed Loans And Other Invested Assets	[3]	35.0		226.7
Liabilities
Debt Instrument, Fair Value Disclosure	[3],[4]	5,430.9		5,984.6
Derivatives, Liabilities
Liabilities Related to Investment Contracts, Fair Value Disclosure	[3]	988.3		1,084.5
Policy Loans [Member] | Estimate of Fair Value Measurement [Member]
Fixed maturity securities, available-for-sale:
Policy loans and other invested assets	[3]			9.0
Policy Loans [Member] | Reported Value Measurement [Member]
Fixed maturity securities, available-for-sale:
Policy loans and other invested assets	[3]			9.0
Other invested assets | Estimate of Fair Value Measurement [Member]
Fixed maturity securities, available-for-sale:
Policy loans and other invested assets	[3]			2.5
Other invested assets | Reported Value Measurement [Member]
Fixed maturity securities, available-for-sale:
Policy loans and other invested assets	[3]			2.5
Fair Value, Inputs, Level 2 [Member]
Liabilities
Debt Instrument, Fair Value Disclosure	[3],[4]	5,700.1		6,071.0
Fair Value, Inputs, Level 3 [Member]
Fixed maturity securities, available-for-sale:
Policy loans and other invested assets	[3]	8.5
Asset Backed Loans And Other Invested Assets	[3]	35.0		226.7
Liabilities
Debt Instrument, Fair Value Disclosure	[3],[4]	29.1		99.1
Derivatives, Liabilities
Liabilities Related to Investment Contracts, Fair Value Disclosure	[3]	922.9		960.3
Fair Value, Inputs, Level 3 [Member] | Policy Loans [Member]
Fixed maturity securities, available-for-sale:
Policy loans and other invested assets	[3]			9.0
Fair Value, Inputs, Level 3 [Member] | Other invested assets
Fixed maturity securities, available-for-sale:
Policy loans and other invested assets	[3]			2.5
Fair Value, Measurements, Recurring
Fixed maturity securities, available-for-sale:
Funds Withheld Receivable		1,535.1		1,638.1
Fair Value, Measurements, Recurring | Corporate Debt Securities [Member]
Fixed maturity securities, available-for-sale:
Available-for-sale Securities		0.0		14.1
Fair Value, Measurements, Recurring | Call Options Within Funds Withheld Receivable [Member]
Derivatives, Assets
Derivatives		11.3		5.4
Fair Value, Measurements, Recurring | Foreign exchange forward agreements
Derivatives, Liabilities
Derivative Liability		2.0		12.6
Fair Value, Measurements, Recurring | Commodity Option [Member]
Derivatives, Assets
Derivatives		2.9		0.0
Derivatives, Liabilities
Derivative Liability		0.1		5.6
Fair Value, Measurements, Recurring | Call options
Derivatives, Assets
Derivatives		5.9		1.0
Fair Value, Measurements, Recurring | Interest Rate Swap [Member]
Derivatives, Liabilities
Derivative Liability		1.1		2.6
Fair Value, Measurements, Recurring | Estimate of Fair Value Measurement [Member]
Fixed maturity securities, available-for-sale:
Assets, Fair Value Disclosure, Recurring		1,561.0		1,700.2
Derivatives, Liabilities
Liabilities, Fair Value Disclosure, Recurring		766.2		792.3
Fair Value, Measurements, Recurring | Estimate of Fair Value Measurement [Member] | Front Street Future Policyholder Benefits Reserve [Member]
Derivatives, Liabilities
Derivative Liability		631.8		629.2
Fair Value, Measurements, Recurring | Foreign exchange contracts | Foreign exchange forward agreements
Derivatives, Assets
Derivatives		5.8		6.0
Fair Value, Measurements, Recurring | Equity securities
Fixed maturity securities, available-for-sale:
Trading Securities		0.0		32.8
Fair Value, Measurements, Recurring | Other invested assets
Fixed maturity securities, available-for-sale:
Other invested assets		0.0		2.8
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fixed maturity securities, available-for-sale:
Funds Withheld Receivable		69.9		8.4
Assets, Fair Value Disclosure, Recurring		69.9		41.2
Derivatives, Liabilities
Liabilities, Fair Value Disclosure, Recurring		0.0		0.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Call Options Within Funds Withheld Receivable [Member]
Derivatives, Assets
Derivatives		0.0		0.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Call options
Derivatives, Assets
Derivatives		0.0		0.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | FIA embedded derivatives, included in contractholder funds
Derivatives, Liabilities
Derivative Liability		131.2		142.3
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Front Street Future Policyholder Benefits Reserve [Member]
Derivatives, Liabilities
Derivative Liability				0.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Equity securities
Fixed maturity securities, available-for-sale:
Trading Securities		0.0		32.8
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Other invested assets
Fixed maturity securities, available-for-sale:
Other invested assets		0.0		0.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 [Member]
Fixed maturity securities, available-for-sale:
Funds Withheld Receivable		1,387.1		1,555.0
Assets, Fair Value Disclosure, Recurring		1,413.0		1,567.4
Derivatives, Liabilities
Liabilities, Fair Value Disclosure, Recurring		3.2		20.8
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 [Member] | Corporate Debt Securities [Member]
Fixed maturity securities, available-for-sale:
Available-for-sale Securities		0.0		0.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 [Member] | Call Options Within Funds Withheld Receivable [Member]
Derivatives, Assets
Derivatives		11.3		5.4
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 [Member] | Foreign exchange forward agreements
Derivatives, Liabilities
Derivative Liability		2.0		12.6
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 [Member] | Commodity Option [Member]
Derivatives, Assets
Derivatives		2.9		0.0
Derivatives, Liabilities
Derivative Liability		0.1		5.6
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 [Member] | Call options
Derivatives, Assets
Derivatives		5.9		1.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 [Member] | Interest Rate Swap [Member]
Derivatives, Liabilities
Derivative Liability		1.1		2.6
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 [Member] | Foreign exchange contracts | Foreign exchange forward agreements
Derivatives, Assets
Derivatives		5.8		6.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 [Member]
Fixed maturity securities, available-for-sale:
Funds Withheld Receivable		78.1		74.7
Assets, Fair Value Disclosure, Recurring		78.1		91.6
Derivatives, Liabilities
Liabilities, Fair Value Disclosure, Recurring		763.0		771.5
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 [Member] | Corporate Debt Securities [Member]
Fixed maturity securities, available-for-sale:
Available-for-sale Securities		0.0		14.1
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 [Member] | Call Options Within Funds Withheld Receivable [Member]
Derivatives, Assets
Derivatives		0.0		0.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 [Member] | Call options
Derivatives, Assets
Derivatives		0.0		0.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 [Member] | FIA embedded derivatives, included in contractholder funds
Derivatives, Liabilities
Derivative Liability		131.2		142.3
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 [Member] | Foreign exchange contracts | Foreign exchange forward agreements
Derivatives, Assets
Derivatives				0.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 [Member] | Other invested assets | Other invested assets
Fixed maturity securities, available-for-sale:
Other invested assets		0.0
Discounted Cash Flow [Member] | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 [Member]
Fixed maturity securities, available-for-sale:
Other invested assets		0.0		2.8
Funds Withheld Receivable		5.4		19.2
Discounted Cash Flow [Member] | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 [Member] | Front Street Future Policyholder Benefits Reserve [Member]
Derivatives, Liabilities
Derivative Liability		631.8		629.2
Broker Quoted [Member] | Fair Value, Inputs, Level 3 [Member] | Corporate Debt Securities [Member]
Fixed maturity securities, available-for-sale:
Assets, Fair Value Disclosure		$ 0.0		$ 14.1
Contingent Purchase Price Reduction Receivable [Member]
Fair Value Assets And Liabilities Measured On Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers, Net						$ 0.0

[1] (a) During Fiscal 2016, the net transfer to Level 3 was exclusively from Level 2. The transfers into Level 3 were related to changes in the primary pricing source and changes in the observability of external information used in determining the fair value.

[2] (a) During Fiscal 2015, the net transfer to Level 3 was related to a loan receivable previously classified as a related party loan.

[3] The carrying amounts of cash and cash equivalents, trade receivables, accounts payable and accrued investment income approximate fair value due to their short duration and, accordingly, they are not presented in the tables above. (b) The fair values of debt set forth above are generally based on quoted or observed market prices.

[4] The fair values of debt set forth above are generally based on quoted or observed market prices.