Fair Value of Financial Instruments - Assets and Liabilities Carrying at Fair Value on Recurring Basis (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2012		Sep. 30, 2011
Fair Value Assets And Liabilities Measured On Nonrecurring Basis [Line Items]
Document Period End Date	Sep. 30, 2014
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers, Net	$ (23.9)	$ (53.0)	$ (731.8)	[1]
Assets
Cash and cash equivalents	1,319.2	1,899.7	1,470.7		1,137.4
Derivatives, Assets
Derivatives	296.3	221.8
Fixed maturity securities, available-for-sale:
Asset Backed Loans And Other Invested Assets	811.6	560.4
Other invested assets	165.0	31.2
Assets, Fair Value Disclosure	1,066.0		523.5
Derivatives, Liabilities
Equity Conversion Option Of Preferred Stock		330.8
Asset-backed securities
Fair Value Assets And Liabilities Measured On Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers, Net	(3.8)	(10.5)	(737.9)	[1]
Commercial mortgage-backed securities
Fair Value Assets And Liabilities Measured On Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers, Net	(6.0)
Corporates
Fair Value Assets And Liabilities Measured On Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers, Net	(14.1)	(33.8)	18.9	[1]
Hybrids
Fair Value Assets And Liabilities Measured On Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers, Net		(8.7)	3.7	[1]
Non-agency residential mortgage-backed securities
Fair Value Assets And Liabilities Measured On Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers, Net			(2.9)	[1]
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
Estimate of Fair Value Measurement [Member]
Fair Value Assets And Liabilities Measured On Nonrecurring Basis [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	1,319.2	1,899.7
Fixed maturity securities, available-for-sale:
Asset Backed Loans And Other Invested Assets	811.6	560.4
Liabilities
Debt Instrument, Fair Value Disclosure	5,308.5	4,773.2
Derivatives, Liabilities
Redeemable preferred stock, excluding equity conversion feature		377.1
Liabilities Related to Investment Contracts, Fair Value Disclosure	13,108.8	12,378.6
Reported Value Measurement [Member]
Fair Value Assets And Liabilities Measured On Nonrecurring Basis [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	1,319.2	1,899.7
Fixed maturity securities, available-for-sale:
Asset Backed Loans And Other Invested Assets	811.6	560.4
Liabilities
Debt Instrument, Fair Value Disclosure	5,157.8	4,896.1
Derivatives, Liabilities
Redeemable preferred stock, excluding equity conversion feature		329.4
Liabilities Related to Investment Contracts, Fair Value Disclosure	14,555.4	13,703.8
Policy loans | Estimate of Fair Value Measurement [Member]
Fixed maturity securities, available-for-sale:
Policy loans and other invested assets	151.7	31.2
Policy loans | Reported Value Measurement [Member]
Fixed maturity securities, available-for-sale:
Policy loans and other invested assets	151.7	31.2
Other invested assets
Fixed maturity securities, available-for-sale:
Trading Securities	165.0	31.2
Fair Value, Inputs, Level 1
Fair Value Assets And Liabilities Measured On Nonrecurring Basis [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	1,319.2	1,899.7
Fixed maturity securities, available-for-sale:
Asset Backed Loans And Other Invested Assets
Liabilities
Debt Instrument, Fair Value Disclosure
Derivatives, Liabilities
Redeemable preferred stock, excluding equity conversion feature
Liabilities Related to Investment Contracts, Fair Value Disclosure
Fair Value, Inputs, Level 1 | Policy loans
Fixed maturity securities, available-for-sale:
Policy loans and other invested assets
Fair Value, Inputs, Level 2 [Member]
Fair Value Assets And Liabilities Measured On Nonrecurring Basis [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	0	0
Fixed maturity securities, available-for-sale:
Asset Backed Loans And Other Invested Assets
Liabilities
Debt Instrument, Fair Value Disclosure	5,308.5	4,773.2
Derivatives, Liabilities
Redeemable preferred stock, excluding equity conversion feature
Liabilities Related to Investment Contracts, Fair Value Disclosure
Fair Value, Inputs, Level 2 [Member] | Policy loans
Fixed maturity securities, available-for-sale:
Policy loans and other invested assets
Fair Value, Inputs, Level 3 [Member]
Fair Value Assets And Liabilities Measured On Nonrecurring Basis [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	0	0
Fixed maturity securities, available-for-sale:
Asset Backed Loans And Other Invested Assets	811.6	560.4
Liabilities
Debt Instrument, Fair Value Disclosure
Derivatives, Liabilities
Redeemable preferred stock, excluding equity conversion feature		377.1
Liabilities Related to Investment Contracts, Fair Value Disclosure	13,108.8	12,378.6
Fair Value, Inputs, Level 3 [Member] | Policy loans
Fixed maturity securities, available-for-sale:
Policy loans and other invested assets	151.7	31.2
Fair Value, Measurements, Recurring
Assets
Contingent purchase price reduction receivable	41.5	41.0
Fixed maturity securities, available-for-sale:
Funds Withheld Receivable	154.4
Derivatives, Liabilities
Equity Conversion Option Of Preferred Stock		330.8
Fair Value, Measurements, Recurring | Asset-backed securities
Fixed maturity securities, available-for-sale:
Available-for-sale Securities	1,792.9	1,523.1
Fair Value, Measurements, Recurring | Commercial mortgage-backed securities
Fixed maturity securities, available-for-sale:
Available-for-sale Securities	636.9	454.4
Fair Value, Measurements, Recurring | Corporates
Fixed maturity securities, available-for-sale:
Available-for-sale Securities	9,795.8	9,418.3
Fair Value, Measurements, Recurring | Hybrids
Fixed maturity securities, available-for-sale:
Available-for-sale Securities	1,316.1	428.8
Fair Value, Measurements, Recurring | Municipals
Fixed maturity securities, available-for-sale:
Available-for-sale Securities	1,259.8	1,007.0
Fair Value, Measurements, Recurring | Agency residential mortgage-backed securities
Fixed maturity securities, available-for-sale:
Available-for-sale Securities	107.3	98.6
Fair Value, Measurements, Recurring | Non-agency residential mortgage-backed securities
Fixed maturity securities, available-for-sale:
Available-for-sale Securities	2,006.7	1,368.0
Fair Value, Measurements, Recurring | Foreign exchange forward agreements
Derivatives, Assets
Derivatives	0.6	1.8
Derivatives, Liabilities
Derivative Liability	0.1	10.0
Fair Value, Measurements, Recurring | Commodity swap and option agreements
Derivatives, Assets
Derivatives	3.2	4.1
Derivatives, Liabilities
Derivative Liability	0.4	2.4
Fair Value, Measurements, Recurring | Call options
Derivatives, Assets
Derivatives	296.3	221.8
Fair Value, Measurements, Recurring | Available-for-sale
Fixed maturity securities, available-for-sale:
Equity securities - available-for-sale	663.6	271.0
Fair Value, Measurements, Recurring | FIA embedded derivatives, included in contractholder funds
Derivatives, Liabilities
Derivative Liability	1,908.1	1,544.4
Fair Value, Measurements, Recurring | Future contracts
Derivatives, Liabilities
Derivative Liability	0.5	1.0
Fair Value, Measurements, Recurring | Interest Rate Swap [Member]
Derivatives, Liabilities
Derivative Liability	1.8
Fair Value, Measurements, Recurring | Estimate of Fair Value Measurement [Member]
Fixed maturity securities, available-for-sale:
Assets, Fair Value Disclosure, Recurring	18,501.7	15,921.2
Derivatives, Liabilities
Liabilities, Fair Value Disclosure, Recurring	2,062.2	1,888.6
Fair Value, Measurements, Recurring | Estimate of Fair Value Measurement [Member] | Front Street Future Policyholder Benefits Reserve [Member]
Derivatives, Liabilities
Derivative Liability	151.3
Fair Value, Measurements, Recurring | Foreign exchange contracts
Derivatives, Assets
Derivatives	12.8
Fair Value, Measurements, Recurring | Equity securities
Fixed maturity securities, available-for-sale:
Trading Securities	104.5	81.5
Fair Value, Measurements, Recurring | Other invested assets
Fixed maturity securities, available-for-sale:
Other invested assets	13.3
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets
Contingent purchase price reduction receivable
Fixed maturity securities, available-for-sale:
Funds Withheld Receivable	0
Assets, Fair Value Disclosure, Recurring	279.3	861.7
Derivatives, Liabilities
Equity Conversion Option Of Preferred Stock
Liabilities, Fair Value Disclosure, Recurring	0	0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Asset-backed securities
Fixed maturity securities, available-for-sale:
Available-for-sale Securities
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Commercial mortgage-backed securities
Fixed maturity securities, available-for-sale:
Available-for-sale Securities
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Corporates
Fixed maturity securities, available-for-sale:
Available-for-sale Securities
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Hybrids
Fixed maturity securities, available-for-sale:
Available-for-sale Securities
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Municipals
Fixed maturity securities, available-for-sale:
Available-for-sale Securities
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Agency residential mortgage-backed securities
Fixed maturity securities, available-for-sale:
Available-for-sale Securities
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Non-agency residential mortgage-backed securities
Fixed maturity securities, available-for-sale:
Available-for-sale Securities
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Foreign exchange forward agreements
Derivatives, Assets
Derivatives
Derivatives, Liabilities
Derivative Liability
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Commodity swap and option agreements
Derivatives, Assets
Derivatives
Derivatives, Liabilities
Derivative Liability
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Call options
Derivatives, Assets
Derivatives
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Available-for-sale
Fixed maturity securities, available-for-sale:
Equity securities - available-for-sale	59.2
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | FIA embedded derivatives, included in contractholder funds
Derivatives, Liabilities
Derivative Liability
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Front Street Future Policyholder Benefits Reserve [Member]
Derivatives, Liabilities
Derivative Liability
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Future contracts
Derivatives, Liabilities
Derivative Liability
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Interest Rate Swap [Member]
Derivatives, Liabilities
Derivative Liability
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Foreign exchange contracts
Derivatives, Assets
Derivatives
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Equity securities
Fixed maturity securities, available-for-sale:
Trading Securities	104.5	70.8
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Other invested assets
Fixed maturity securities, available-for-sale:
Other invested assets	0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 [Member]
Assets
Contingent purchase price reduction receivable
Fixed maturity securities, available-for-sale:
Funds Withheld Receivable	154.4
Assets, Fair Value Disclosure, Recurring	17,156.4	14,536.0
Derivatives, Liabilities
Equity Conversion Option Of Preferred Stock
Liabilities, Fair Value Disclosure, Recurring	2.8	13.4
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 [Member] | Asset-backed securities
Fixed maturity securities, available-for-sale:
Available-for-sale Securities	1,755.9	1,518.1
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 [Member] | Commercial mortgage-backed securities
Fixed maturity securities, available-for-sale:
Available-for-sale Securities	553.8	448.7
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 [Member] | Corporates
Fixed maturity securities, available-for-sale:
Available-for-sale Securities	8,945.8	8,957.2
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 [Member] | Hybrids
Fixed maturity securities, available-for-sale:
Available-for-sale Securities	1,316.1	428.8
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 [Member] | Municipals
Fixed maturity securities, available-for-sale:
Available-for-sale Securities	1,222.6	1,007.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 [Member] | Agency residential mortgage-backed securities
Fixed maturity securities, available-for-sale:
Available-for-sale Securities	107.3	98.6
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 [Member] | Non-agency residential mortgage-backed securities
Fixed maturity securities, available-for-sale:
Available-for-sale Securities	2,006.7	1,368.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 [Member] | Foreign exchange forward agreements
Derivatives, Assets
Derivatives	0.6	1.8
Derivatives, Liabilities
Derivative Liability	0.1	10.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 [Member] | Commodity swap and option agreements
Derivatives, Assets
Derivatives	3.2	4.1
Derivatives, Liabilities
Derivative Liability	0.4	2.4
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 [Member] | Call options
Derivatives, Assets
Derivatives	296.3	221.8
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 [Member] | Available-for-sale
Fixed maturity securities, available-for-sale:
Equity securities - available-for-sale	598.4	271.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 [Member] | FIA embedded derivatives, included in contractholder funds
Derivatives, Liabilities
Derivative Liability
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 [Member] | Front Street Future Policyholder Benefits Reserve [Member]
Derivatives, Liabilities
Derivative Liability
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 [Member] | Future contracts
Derivatives, Liabilities
Derivative Liability	0.5	1.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 [Member] | Interest Rate Swap [Member]
Derivatives, Liabilities
Derivative Liability	1.8
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 [Member] | Foreign exchange contracts
Derivatives, Assets
Derivatives	12.8
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 [Member] | Equity securities
Fixed maturity securities, available-for-sale:
Trading Securities
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 [Member] | Other invested assets
Fixed maturity securities, available-for-sale:
Other invested assets	2.1
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 [Member]
Assets
Contingent purchase price reduction receivable	41.5	41.0
Fixed maturity securities, available-for-sale:
Funds Withheld Receivable	0
Assets, Fair Value Disclosure, Recurring	1,066.0	523.5
Derivatives, Liabilities
Equity Conversion Option Of Preferred Stock		330.8
Liabilities, Fair Value Disclosure, Recurring	2,059.4	1,875.2
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 [Member] | Asset-backed securities
Fixed maturity securities, available-for-sale:
Available-for-sale Securities	37.0	5.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 [Member] | Commercial mortgage-backed securities
Fixed maturity securities, available-for-sale:
Available-for-sale Securities	83.1	5.7
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 [Member] | Corporates
Fixed maturity securities, available-for-sale:
Available-for-sale Securities	850.0	461.1
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 [Member] | Hybrids
Fixed maturity securities, available-for-sale:
Available-for-sale Securities	0	0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 [Member] | Municipals
Fixed maturity securities, available-for-sale:
Available-for-sale Securities	37.2
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 [Member] | Agency residential mortgage-backed securities
Fixed maturity securities, available-for-sale:
Available-for-sale Securities
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 [Member] | Non-agency residential mortgage-backed securities
Fixed maturity securities, available-for-sale:
Available-for-sale Securities
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 [Member] | Foreign exchange forward agreements
Derivatives, Assets
Derivatives
Derivatives, Liabilities
Derivative Liability
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 [Member] | Commodity swap and option agreements
Derivatives, Assets
Derivatives
Derivatives, Liabilities
Derivative Liability
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 [Member] | Call options
Derivatives, Assets
Derivatives
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 [Member] | Available-for-sale
Fixed maturity securities, available-for-sale:
Equity securities - available-for-sale	6.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 [Member] | FIA embedded derivatives, included in contractholder funds
Derivatives, Liabilities
Derivative Liability	1,908.1	1,544.4
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 [Member] | Front Street Future Policyholder Benefits Reserve [Member]
Derivatives, Liabilities
Derivative Liability	151.3
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 [Member] | Future contracts
Derivatives, Liabilities
Derivative Liability
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 [Member] | Interest Rate Swap [Member]
Derivatives, Liabilities
Derivative Liability
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 [Member] | Foreign exchange contracts
Derivatives, Assets
Derivatives
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 [Member] | Equity securities
Fixed maturity securities, available-for-sale:
Trading Securities		10.7
Option Pricing [Member] | Other invested assets
Fixed maturity securities, available-for-sale:
Assets, Fair Value Disclosure		0
Option Pricing [Member] | 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	11.2
U.S. Government | Fair Value, Measurements, Recurring
Fixed maturity securities, available-for-sale:
Available-for-sale Securities	296.0	1,001.8
U.S. Government | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fixed maturity securities, available-for-sale:
Available-for-sale Securities	115.6	790.9
U.S. Government | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 [Member]
Fixed maturity securities, available-for-sale:
Available-for-sale Securities	180.4	210.9
U.S. Government | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 [Member]
Fixed maturity securities, available-for-sale:
Available-for-sale Securities

[1] (a)The net transfers in and out of Level 3 during Fiscal 2012 was exclusively to or from Level 2.