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, 2013		Sep. 30, 2012		Sep. 30, 2011		Sep. 30, 2010
Fair Value Assets And Liabilities Measured On Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers, Net	$ (53.0)	[1]	$ (731.8)	[2]	$ (17.7)	[3]
Assets
Cash and cash equivalents	1,899.7		1,470.7		1,137.4		256.9
Derivatives, Assets
Derivatives	221.8		200.7
Fixed maturity securities, available-for-sale:
Asset Backed Loans And Other Invested Assets	560.4		180.1
Assets, Fair Value Disclosure	523.5		206.0
Derivatives, Liabilities
Equity conversion feature of preferred stock (Note 6 and Note 18)	330.8		232.0
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	(10.5)	[1]	(737.9)	[2]	(14.6)	[3]
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		[1]		[2]
Corporates
Fair Value Assets And Liabilities Measured On Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers, Net	(33.8)	[1]	18.9	[2]	(0.1)	[3]
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)	[1]	3.7	[2]	(3.0)	[3]
Fixed maturity securities, available-for-sale:
Assets, Fair Value Disclosure	10.7		0
Municipals
Fair Value Assets And Liabilities Measured On Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers, Net			(10.3)	[2]
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			(3.3)	[2]
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)	[2]
Recurring | Fair Value
Assets
Cash and cash equivalents	1,899.7	[4],[5]	1,470.7	[4],[5]
Contingent purchase price reduction receivable	41.0	[4]	41.0	[4]
Fixed maturity securities, available-for-sale:
Asset Backed Loans And Other Invested Assets	560.4	[4]	180.1	[4]
Assets, Fair Value Disclosure, Recurring	17,820.9		18,199.4
Liabilities
Total debt	4,773.2	[4],[6]	2,328.8	[4],[5]
Derivatives, Liabilities
Equity conversion feature of preferred stock (Note 6 and Note 18)	330.8	[4]	232.0	[4]
Redeemable preferred stock, excluding equity conversion feature	377.1	[4]	368.9	[4]
Investment contracts, included in contractholder funds	12,378.6	[4]	12,271.9	[4]
Liabilities, Fair Value Disclosure, Recurring	1,888.6		1,793.7
Recurring | Fair Value | U.S. Government
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	1,001.8	[4]	930.4	[4]
Recurring | Fair Value | Asset-backed securities
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	1,523.1	[4]	1,027.9	[4]
Recurring | Fair Value | Commercial mortgage-backed securities
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	454.4	[4]	553.8	[4]
Recurring | Fair Value | Corporates
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	9,418.3	[4]	11,009.0	[4]
Recurring | Fair Value | Hybrids
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	428.8	[4]	528.2	[4]
Recurring | Fair Value | Municipals
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	1,007.0	[4]	1,224.0	[4]
Recurring | Fair Value | Agency residential mortgage-backed securities
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	98.6	[4]	155.0	[4]
Recurring | Fair Value | Non-agency residential mortgage-backed securities
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	1,368.0	[4]	660.6	[4]
Recurring | Fair Value | Foreign exchange forward agreements
Derivatives, Assets
Derivatives	1.8	[4]	1.2	[4]
Derivatives, Liabilities
Derivative investments	10.0	[4]	10.0	[4]
Recurring | Fair Value | Commodity swap and option agreements
Derivatives, Assets
Derivatives	4.1	[4]	2.0	[4]
Derivatives, Liabilities
Derivative investments	2.4	[4]		[4]
Recurring | Fair Value | Call options and futures contracts
Derivatives, Assets
Derivatives	221.8	[4]	200.7	[4]
Recurring | Fair Value | Available-for-sale
Fixed maturity securities, available-for-sale:
Equity securities - available-for-sale	271.0	[4]	248.1	[4]
Recurring | Fair Value | FIA embedded derivatives, included in contractholder funds
Derivatives, Liabilities
Derivative investments	1,544.4	[4]	1,550.8	[4]
Recurring | Fair Value | Future contracts
Derivatives, Liabilities
Derivative investments	1.0	[4]	0.9	[4]
Recurring | Carrying Amount
Fixed maturity securities, available-for-sale:
Asset Backed Loans And Other Invested Assets	560.4	[4]	180.1	[4]
Liabilities
Total debt	4,896.1	[4],[6]	2,167.0	[4],[5]
Derivatives, Liabilities
Redeemable preferred stock, excluding equity conversion feature	329.4	[4]	319.2	[4]
Investment contracts, included in contractholder funds	13,703.8	[4]	13,739.6	[4]
Recurring | Equity securities | Fair Value
Fixed maturity securities, available-for-sale:
Equity securities - trading	81.5	[4]	146.8	[4]
Recurring | Fixed Maturities Securities | Fair Value
Fixed maturity securities, available-for-sale:
U.S. Treasuries and certificate of deposit, held-to-maturity			35.0	[4]
Recurring | Fixed Maturities Securities | Carrying Amount
Fixed maturity securities, available-for-sale:
U.S. Treasuries and certificate of deposit, held-to-maturity			35.0	[4]
Recurring | Policy loans | Fair Value
Fixed maturity securities, available-for-sale:
Policy loans and other invested assets	31.2	[4]	18.8	[4]
Recurring | Policy loans | Carrying Amount
Fixed maturity securities, available-for-sale:
Policy loans and other invested assets	31.2	[4]	18.8	[4]
Recurring | Level 1
Assets
Cash and cash equivalents	1,899.7	[4],[5]	1,468.4	[4],[5]
Contingent purchase price reduction receivable		[4]		[4]
Fixed maturity securities, available-for-sale:
Asset Backed Loans And Other Invested Assets		[4]		[4]
Assets, Fair Value Disclosure, Recurring	2,761.4	[4]	2,545.6
Liabilities
Total debt		[4],[6]	524.0	[4],[5]
Derivatives, Liabilities
Equity conversion feature of preferred stock (Note 6 and Note 18)		[4]		[4]
Redeemable preferred stock, excluding equity conversion feature		[4]		[4]
Investment contracts, included in contractholder funds		[4]		[4]
Liabilities, Fair Value Disclosure, Recurring	0		0
Recurring | Level 1 | U.S. Government
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	790.9	[4]	930.4	[4]
Recurring | Level 1 | Asset-backed securities
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale		[4]		[4]
Recurring | Level 1 | Commercial mortgage-backed securities
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale		[4]		[4]
Recurring | Level 1 | Corporates
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale		[4]		[4]
Recurring | Level 1 | Hybrids
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale		[4]		[4]
Recurring | Level 1 | Municipals
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale		[4]		[4]
Recurring | Level 1 | Agency residential mortgage-backed securities
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale		[4]		[4]
Recurring | Level 1 | Non-agency residential mortgage-backed securities
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale		[4]		[4]
Recurring | Level 1 | Foreign exchange forward agreements
Derivatives, Assets
Derivatives		[4]		[4]
Derivatives, Liabilities
Derivative investments		[4]		[4]
Recurring | Level 1 | Commodity swap and option agreements
Derivatives, Assets
Derivatives		[4]		[4]
Derivatives, Liabilities
Derivative investments		[4]		[4]
Recurring | Level 1 | Call options and futures contracts
Derivatives, Assets
Derivatives		[4]		[4]
Recurring | Level 1 | Available-for-sale
Fixed maturity securities, available-for-sale:
Equity securities - available-for-sale		[4]		[4]
Recurring | Level 1 | FIA embedded derivatives, included in contractholder funds
Derivatives, Liabilities
Derivative investments		[4]		[4]
Recurring | Level 1 | Future contracts
Derivatives, Liabilities
Derivative investments		[4]		[4]
Recurring | Level 1 | Equity securities
Fixed maturity securities, available-for-sale:
Equity securities - trading	70.8	[4]	146.8	[4]
Recurring | Level 1 | Fixed Maturities Securities
Fixed maturity securities, available-for-sale:
U.S. Treasuries and certificate of deposit, held-to-maturity				[4]
Recurring | Level 1 | Policy loans
Fixed maturity securities, available-for-sale:
Policy loans and other invested assets		[4]		[4]
Recurring | Level 2
Assets
Cash and cash equivalents		[4],[5]	2.3	[4],[5]
Contingent purchase price reduction receivable		[4]		[4]
Fixed maturity securities, available-for-sale:
Asset Backed Loans And Other Invested Assets		[4]		[4]
Assets, Fair Value Disclosure, Recurring	14,536.0	[4]	15,447.8
Liabilities
Total debt	4,773.2	[4],[6]	1,804.8	[4],[5]
Derivatives, Liabilities
Equity conversion feature of preferred stock (Note 6 and Note 18)		[4]		[4]
Redeemable preferred stock, excluding equity conversion feature		[4]		[4]
Investment contracts, included in contractholder funds		[4]		[4]
Liabilities, Fair Value Disclosure, Recurring	13.4		10.9
Recurring | Level 2 | U.S. Government
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	210.9	[4]		[4]
Recurring | Level 2 | Asset-backed securities
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	1,518.1	[4]	1,012.0	[4]
Recurring | Level 2 | Commercial mortgage-backed securities
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	448.7	[4]	548.8	[4]
Recurring | Level 2 | Corporates
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	8,957.2	[4]	10,873.7	[4]
Recurring | Level 2 | Hybrids
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	428.8	[4]	519.4	[4]
Recurring | Level 2 | Municipals
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	1,007.0	[4]	1,224.0	[4]
Recurring | Level 2 | Agency residential mortgage-backed securities
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	98.6	[4]	155.0	[4]
Recurring | Level 2 | Non-agency residential mortgage-backed securities
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	1,368.0	[4]	660.6	[4]
Recurring | Level 2 | Foreign exchange forward agreements
Derivatives, Assets
Derivatives	1.8	[4]	1.2	[4]
Derivatives, Liabilities
Derivative investments	10.0	[4]	10.0	[4]
Recurring | Level 2 | Commodity swap and option agreements
Derivatives, Assets
Derivatives	4.1	[4]	2.0	[4]
Derivatives, Liabilities
Derivative investments	2.4	[4]		[4]
Recurring | Level 2 | Call options and futures contracts
Derivatives, Assets
Derivatives	221.8	[4]	200.7	[4]
Recurring | Level 2 | Available-for-sale
Fixed maturity securities, available-for-sale:
Equity securities - available-for-sale	271.0	[4]	248.1	[4]
Recurring | Level 2 | FIA embedded derivatives, included in contractholder funds
Derivatives, Liabilities
Derivative investments		[4]		[4]
Recurring | Level 2 | Future contracts
Derivatives, Liabilities
Derivative investments	1.0	[4]	0.9	[4]
Recurring | Level 2 | Equity securities
Fixed maturity securities, available-for-sale:
Equity securities - trading		[4]		[4]
Recurring | Level 2 | Fixed Maturities Securities
Fixed maturity securities, available-for-sale:
U.S. Treasuries and certificate of deposit, held-to-maturity			35.0	[4]
Recurring | Level 2 | Policy loans
Fixed maturity securities, available-for-sale:
Policy loans and other invested assets		[4]		[4]
Recurring | Level 3
Assets
Cash and cash equivalents		[4],[5]		[4],[5]
Contingent purchase price reduction receivable	41.0	[4]	41.0	[4]
Fixed maturity securities, available-for-sale:
Asset Backed Loans And Other Invested Assets	560.4	[4]	180.1	[4]
Assets, Fair Value Disclosure, Recurring	523.5		206.0
Liabilities
Total debt		[4],[6]		[4],[5]
Derivatives, Liabilities
Equity conversion feature of preferred stock (Note 6 and Note 18)	330.8	[4]	232.0	[4]
Redeemable preferred stock, excluding equity conversion feature	377.1	[4]	368.9	[4]
Investment contracts, included in contractholder funds	12,378.6	[4]	12,271.9	[4]
Liabilities, Fair Value Disclosure, Recurring	1,875.2		1,782.8
Recurring | Level 3 | U.S. Government
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale		[4]		[4]
Recurring | Level 3 | Asset-backed securities
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	5.0	[4]	15.9	[4]
Recurring | Level 3 | Commercial mortgage-backed securities
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	5.7	[4]	5.0	[4]
Recurring | Level 3 | Corporates
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	461.1	[4]	135.3	[4]
Recurring | Level 3 | Hybrids
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	0	[4]	8.8	[4]
Recurring | Level 3 | Municipals
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale		[4]		[4]
Recurring | Level 3 | Agency residential mortgage-backed securities
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale		[4]		[4]
Recurring | Level 3 | Non-agency residential mortgage-backed securities
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale		[4]		[4]
Recurring | Level 3 | Foreign exchange forward agreements
Derivatives, Assets
Derivatives		[4]		[4]
Derivatives, Liabilities
Derivative investments		[4]		[4]
Recurring | Level 3 | Commodity swap and option agreements
Derivatives, Assets
Derivatives		[4]		[4]
Derivatives, Liabilities
Derivative investments		[4]		[4]
Recurring | Level 3 | Call options and futures contracts
Derivatives, Assets
Derivatives		[4]		[4]
Recurring | Level 3 | Available-for-sale
Fixed maturity securities, available-for-sale:
Equity securities - available-for-sale		[4]		[4]
Recurring | Level 3 | FIA embedded derivatives, included in contractholder funds
Derivatives, Liabilities
Derivative investments	1,544.4	[4]	1,550.8	[4]
Recurring | Level 3 | Future contracts
Derivatives, Liabilities
Derivative investments		[4]		[4]
Recurring | Level 3 | Equity securities
Fixed maturity securities, available-for-sale:
Equity securities - trading	10.7	[4]		[4]
Recurring | Level 3 | Fixed Maturities Securities
Fixed maturity securities, available-for-sale:
U.S. Treasuries and certificate of deposit, held-to-maturity				[4]
Recurring | Level 3 | Policy loans
Fixed maturity securities, available-for-sale:
Policy loans and other invested assets	$ 31.2	[4]	$ 18.8	[4]

[1] The net transfers in and out of Level 3 during the year ended September 30, 2013 were exclusively to or from Level 2.

[2] (a)The net transfers in and out of Level 3 during the year ended September 30, 2012 was exclusively to or from Level 2.

[3] (a)The net transfers in and out of Level 3 during the year ended September 30, 2011 was exclusively to or from Level 2.

[4] The carrying amounts of trade receivables, accounts payable, accrued investment income and portions of other insurance liabilities approximate fair value due to their short duration and, accordingly, they are not presented in the tables above.

[5] The fair values of cash equivalents and equity investments set forth above are generally based on quoted or observed market prices.

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