Fair Value of Financial Instruments - Assets and Liabilities Carrying at Fair Value on Recurring Basis (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Jun. 30, 2013		Sep. 30, 2012		Jan. 01, 2012	Sep. 30, 2011
Fair Value Assets And Liabilities Measured On Nonrecurring Basis [Line Items]
Document Period End Date	Jun. 30, 2013
Assets
Cash and cash equivalents	$ 1,243.7		$ 1,470.7		$ 1,832.8	$ 1,137.4
Derivatives, Assets
Derivatives	227.4		200.7
Fixed maturity securities, available-for-sale:
Asset-backed loans	430.2		180.1
Derivatives, Liabilities
Equity conversion feature of preferred stock	147.3		232.0
Recurring [Member] | Fair Value [Member]
Assets
Cash and cash equivalents	1,243.7	[1],[2]	1,470.7	[1],[2]
Contingent purchase price reduction receivable	41.0	[2]	41.0	[2]
Fixed maturity securities, available-for-sale:
Asset-backed loans	430.2	[2]	180.1	[2]
Total financial assets	17,895.4	[2]	18,433.3	[2]
Liabilities
Total debt	4,574.3	[2]	2,328.8	[1],[2]
Derivatives, Liabilities
Equity conversion feature of preferred stock	147.3	[2]	232.0	[2]
Redeemable preferred stock, excluding equity conversion feature	368.9	[2]	368.9	[2]
Investment contracts, included in contractholder funds	12,354.5	[2]	12,271.9	[2]
Total financial liabilities	19,038.3	[2]	16,763.3	[2]
Recurring [Member] | Fair Value [Member] | U.S. Government securities [Member]
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	394.0	[2]	930.4	[2]
Recurring [Member] | Fair Value [Member] | Foreign exchange forward agreements [Member]
Derivatives, Assets
Derivatives	5.8	[2]	1.2	[2]
Derivatives, Liabilities
Derivative investments	4.5	[2]	10.0	[2]
Recurring [Member] | Fair Value [Member] | Commodity swap and option agreements [Member]
Derivatives, Assets
Derivatives	4.4	[2]	2.0	[2]
Derivatives, Liabilities
Derivative investments	2.5	[2]		[2]
Recurring [Member] | Fair Value [Member] | Call options [Member]
Derivatives, Assets
Derivatives	227.4	[2]	200.7	[2]
Recurring [Member] | Fair Value [Member] | Available-for-sale embedded derivatives [Member]
Fixed maturity securities, available-for-sale:
Equity securities - available-for-sale	269.8	[2]	248.1	[2]
Recurring [Member] | Fair Value [Member] | FIA embedded derivatives, included in contractholder funds [Member]
Derivatives, Liabilities
Derivative investments	1,585.9	[2]	1,550.8	[2]
Recurring [Member] | Fair Value [Member] | Future contracts [Member]
Derivatives, Liabilities
Derivative investments	0.4	[2]	0.9	[2]
Recurring [Member] | Fair Value [Member] | Asset-backed securities [Member]
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	1,437.5	[2]	1,027.9	[2]
Recurring [Member] | Fair Value [Member] | Commercial mortgage-backed securities [Member]
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	523.9	[2]	553.8	[2]
Recurring [Member] | Fair Value [Member] | Corporates [Member]
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	10,212.8	[2]	11,009.0	[2]
Recurring [Member] | Fair Value [Member] | Hybrids [Member]
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	508.3	[2]	528.2	[2]
Recurring [Member] | Fair Value [Member] | Municipals [Member]
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	1,036.9	[2]	1,224.0	[2]
Recurring [Member] | Fair Value [Member] | Agency residential mortgage-backed securities [Member]
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	111.0	[2]	155.0	[2]
Recurring [Member] | Fair Value [Member] | Non-agency residential mortgage-backed securities [Member]
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	1,353.9	[2]	660.6	[2]
Recurring [Member] | Carrying Amount [Member]
Assets
Cash and cash equivalents	1,243.7	[1],[2]	1,470.7	[1],[2]
Contingent purchase price reduction receivable	41.0	[2]	41.0	[2]
Fixed maturity securities, available-for-sale:
Asset-backed loans	430.2	[2]	180.1	[2]
Total financial assets	17,895.4	[2]	18,433.3	[2]
Liabilities
Total debt	4,554.3	[2]	2,167.0	[1],[2]
Derivatives, Liabilities
Equity conversion feature of preferred stock	147.3	[2]	232.0	[2]
Redeemable preferred stock, excluding equity conversion feature	325.4	[2]	319.2	[2]
Investment contracts, included in contractholder funds	13,756.7	[2]	13,739.6	[2]
Total financial liabilities	20,377.0	[2]	18,019.5	[2]
Recurring [Member] | Carrying Amount [Member] | U.S. Government securities [Member]
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	394.0	[2]	930.4	[2]
Recurring [Member] | Carrying Amount [Member] | Foreign exchange forward agreements [Member]
Derivatives, Assets
Derivatives	5.8	[2]	1.2	[2]
Derivatives, Liabilities
Derivative investments	4.5	[2]	10.0	[2]
Recurring [Member] | Carrying Amount [Member] | Commodity swap and option agreements [Member]
Derivatives, Assets
Derivatives	4.4	[2]	2.0	[2]
Derivatives, Liabilities
Derivative investments	2.5	[2]		[2]
Recurring [Member] | Carrying Amount [Member] | Call options [Member]
Derivatives, Assets
Derivatives	227.4	[2]	200.7	[2]
Recurring [Member] | Carrying Amount [Member] | Available-for-sale embedded derivatives [Member]
Fixed maturity securities, available-for-sale:
Equity securities - available-for-sale	269.8	[2]	248.1	[2]
Recurring [Member] | Carrying Amount [Member] | FIA embedded derivatives, included in contractholder funds [Member]
Derivatives, Liabilities
Derivative investments	1,585.9	[2]	1,550.8	[2]
Recurring [Member] | Carrying Amount [Member] | Future contracts [Member]
Derivatives, Liabilities
Derivative investments	0.4	[2]	0.9	[2]
Recurring [Member] | Carrying Amount [Member] | Asset-backed securities [Member]
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	1,437.5	[2]	1,027.9	[2]
Recurring [Member] | Carrying Amount [Member] | Commercial mortgage-backed securities [Member]
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	523.9	[2]	553.8	[2]
Recurring [Member] | Carrying Amount [Member] | Corporates [Member]
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	10,212.8	[2]	11,009.0	[2]
Recurring [Member] | Carrying Amount [Member] | Hybrids [Member]
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	508.3	[2]	528.2	[2]
Recurring [Member] | Carrying Amount [Member] | Municipals [Member]
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	1,036.9	[2]	1,224.0	[2]
Recurring [Member] | Carrying Amount [Member] | Agency residential mortgage-backed securities [Member]
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	111.0	[2]	155.0	[2]
Recurring [Member] | Carrying Amount [Member] | Non-agency residential mortgage-backed securities [Member]
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	1,353.9	[2]	660.6	[2]
Recurring [Member] | Equity securities [Member] | Fair Value [Member]
Fixed maturity securities, available-for-sale:
Equity securities - trading	67.2	[2]	146.8	[2]
Recurring [Member] | Equity securities [Member] | Carrying Amount [Member]
Fixed maturity securities, available-for-sale:
Equity securities - trading	67.2	[2]	146.8	[2]
Recurring [Member] | Fixed maturities securities [Member] | Fair Value [Member]
Fixed maturity securities, available-for-sale:
U.S. Treasuries and certificate of deposit, held-to-maturity			35.0	[2]
Recurring [Member] | Fixed maturities securities [Member] | Carrying Amount [Member]
Fixed maturity securities, available-for-sale:
U.S. Treasuries and certificate of deposit, held-to-maturity			35.0	[2]
Recurring [Member] | Policy loans [Member] | Fair Value [Member]
Fixed maturity securities, available-for-sale:
Policy loans and other invested assets	27.6	[2]	18.8	[2]
Recurring [Member] | Policy loans [Member] | Carrying Amount [Member]
Fixed maturity securities, available-for-sale:
Policy loans and other invested assets	27.6	[2]	18.8	[2]
Recurring [Member] | Level 1 [Member]
Assets
Cash and cash equivalents	1,243.7	[1],[2]	1,468.4	[1],[2]
Contingent purchase price reduction receivable		[2]		[2]
Fixed maturity securities, available-for-sale:
Asset-backed loans		[2]		[2]
Total financial assets	1,494.5	[2]	2,545.6	[2]
Liabilities
Total debt		[2]	524.0	[1],[2]
Derivatives, Liabilities
Equity conversion feature of preferred stock		[2]		[2]
Redeemable preferred stock, excluding equity conversion feature		[2]		[2]
Investment contracts, included in contractholder funds		[2]		[2]
Total financial liabilities		[2]	524.0	[2]
Recurring [Member] | Level 1 [Member] | U.S. Government securities [Member]
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	183.6	[2]	930.4	[2]
Recurring [Member] | Level 1 [Member] | Foreign exchange forward agreements [Member]
Derivatives, Assets
Derivatives		[2]		[2]
Derivatives, Liabilities
Derivative investments		[2]		[2]
Recurring [Member] | Level 1 [Member] | Commodity swap and option agreements [Member]
Derivatives, Assets
Derivatives		[2]		[2]
Derivatives, Liabilities
Derivative investments		[2]		[2]
Recurring [Member] | Level 1 [Member] | Call options [Member]
Derivatives, Assets
Derivatives		[2]		[2]
Recurring [Member] | Level 1 [Member] | Available-for-sale embedded derivatives [Member]
Fixed maturity securities, available-for-sale:
Equity securities - available-for-sale		[2]		[2]
Recurring [Member] | Level 1 [Member] | FIA embedded derivatives, included in contractholder funds [Member]
Derivatives, Liabilities
Derivative investments		[2]		[2]
Recurring [Member] | Level 1 [Member] | Future contracts [Member]
Derivatives, Liabilities
Derivative investments		[2]		[2]
Recurring [Member] | Level 1 [Member] | Asset-backed securities [Member]
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale		[2]		[2]
Recurring [Member] | Level 1 [Member] | Commercial mortgage-backed securities [Member]
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale		[2]		[2]
Recurring [Member] | Level 1 [Member] | Corporates [Member]
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale		[2]		[2]
Recurring [Member] | Level 1 [Member] | Hybrids [Member]
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale		[2]		[2]
Recurring [Member] | Level 1 [Member] | Municipals [Member]
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale		[2]		[2]
Recurring [Member] | Level 1 [Member] | Agency residential mortgage-backed securities [Member]
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale		[2]		[2]
Recurring [Member] | Level 1 [Member] | Non-agency residential mortgage-backed securities [Member]
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale		[2]		[2]
Recurring [Member] | Level 1 [Member] | Equity securities [Member]
Fixed maturity securities, available-for-sale:
Equity securities - trading	67.2	[2]	146.8	[2]
Recurring [Member] | Level 1 [Member] | Fixed maturities securities [Member]
Fixed maturity securities, available-for-sale:
U.S. Treasuries and certificate of deposit, held-to-maturity				[2]
Recurring [Member] | Level 1 [Member] | Policy loans [Member]
Fixed maturity securities, available-for-sale:
Policy loans and other invested assets		[2]		[2]
Recurring [Member] | Level 2 [Member]
Assets
Cash and cash equivalents		[1],[2]	2.3	[1],[2]
Contingent purchase price reduction receivable		[2]		[2]
Fixed maturity securities, available-for-sale:
Asset-backed loans		[2]		[2]
Total financial assets	15,440.4	[2]	15,482.8	[2]
Liabilities
Total debt	4,574.3	[2]	1,804.8	[1],[2]
Derivatives, Liabilities
Equity conversion feature of preferred stock		[2]		[2]
Redeemable preferred stock, excluding equity conversion feature		[2]		[2]
Investment contracts, included in contractholder funds		[2]		[2]
Total financial liabilities	4,581.7	[2]	1,815.7	[2]
Recurring [Member] | Level 2 [Member] | U.S. Government securities [Member]
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	210.4	[2]		[2]
Recurring [Member] | Level 2 [Member] | Foreign exchange forward agreements [Member]
Derivatives, Assets
Derivatives	5.8	[2]	1.2	[2]
Derivatives, Liabilities
Derivative investments	4.5	[2]	10.0	[2]
Recurring [Member] | Level 2 [Member] | Commodity swap and option agreements [Member]
Derivatives, Assets
Derivatives	4.4	[2]	2.0	[2]
Derivatives, Liabilities
Derivative investments	2.5	[2]		[2]
Recurring [Member] | Level 2 [Member] | Call options [Member]
Derivatives, Assets
Derivatives	227.4	[2]	200.7	[2]
Recurring [Member] | Level 2 [Member] | Available-for-sale embedded derivatives [Member]
Fixed maturity securities, available-for-sale:
Equity securities - available-for-sale	258.1	[2]	248.1	[2]
Recurring [Member] | Level 2 [Member] | FIA embedded derivatives, included in contractholder funds [Member]
Derivatives, Liabilities
Derivative investments		[2]		[2]
Recurring [Member] | Level 2 [Member] | Future contracts [Member]
Derivatives, Liabilities
Derivative investments	0.4	[2]	0.9	[2]
Recurring [Member] | Level 2 [Member] | Asset-backed securities [Member]
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	1,432.4	[2]	1,012.0	[2]
Recurring [Member] | Level 2 [Member] | Commercial mortgage-backed securities [Member]
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	518.0	[2]	548.8	[2]
Recurring [Member] | Level 2 [Member] | Corporates [Member]
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	9,773.8	[2]	10,873.7	[2]
Recurring [Member] | Level 2 [Member] | Hybrids [Member]
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	508.3	[2]	519.4	[2]
Recurring [Member] | Level 2 [Member] | Municipals [Member]
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	1,036.9	[2]	1,224.0	[2]
Recurring [Member] | Level 2 [Member] | Agency residential mortgage-backed securities [Member]
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	111.0	[2]	155.0	[2]
Recurring [Member] | Level 2 [Member] | Non-agency residential mortgage-backed securities [Member]
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	1,353.9	[2]	660.6	[2]
Recurring [Member] | Level 2 [Member] | Equity securities [Member]
Fixed maturity securities, available-for-sale:
Equity securities - trading		[2]		[2]
Recurring [Member] | Level 2 [Member] | Fixed maturities securities [Member]
Fixed maturity securities, available-for-sale:
U.S. Treasuries and certificate of deposit, held-to-maturity			35.0	[2]
Recurring [Member] | Level 2 [Member] | Policy loans [Member]
Fixed maturity securities, available-for-sale:
Policy loans and other invested assets		[2]		[2]
Recurring [Member] | Level 3 [Member]
Assets
Cash and cash equivalents		[1],[2]		[1],[2]
Contingent purchase price reduction receivable	41.0	[2]	41.0	[2]
Fixed maturity securities, available-for-sale:
Asset-backed loans	430.2	[2]	180.1	[2]
Total financial assets	960.5	[2]	404.9	[2]
Liabilities
Total debt		[2]		[1],[2]
Derivatives, Liabilities
Equity conversion feature of preferred stock	147.3	[2]	232.0	[2]
Redeemable preferred stock, excluding equity conversion feature	368.9	[2]	368.9	[2]
Investment contracts, included in contractholder funds	12,354.5	[2]	12,271.9	[2]
Total financial liabilities	14,456.6	[2]	14,423.6	[2]
Recurring [Member] | Level 3 [Member] | U.S. Government securities [Member]
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale		[2]		[2]
Recurring [Member] | Level 3 [Member] | Foreign exchange forward agreements [Member]
Derivatives, Assets
Derivatives		[2]		[2]
Derivatives, Liabilities
Derivative investments		[2]		[2]
Recurring [Member] | Level 3 [Member] | Commodity swap and option agreements [Member]
Derivatives, Assets
Derivatives		[2]		[2]
Derivatives, Liabilities
Derivative investments		[2]		[2]
Recurring [Member] | Level 3 [Member] | Call options [Member]
Derivatives, Assets
Derivatives		[2]		[2]
Recurring [Member] | Level 3 [Member] | Available-for-sale embedded derivatives [Member]
Fixed maturity securities, available-for-sale:
Equity securities - available-for-sale	11.7	[2]		[2]
Recurring [Member] | Level 3 [Member] | FIA embedded derivatives, included in contractholder funds [Member]
Derivatives, Liabilities
Derivative investments	1,585.9	[2]	1,550.8	[2]
Recurring [Member] | Level 3 [Member] | Future contracts [Member]
Derivatives, Liabilities
Derivative investments		[2]		[2]
Recurring [Member] | Level 3 [Member] | Asset-backed securities [Member]
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	5.1	[2]	15.9	[2]
Recurring [Member] | Level 3 [Member] | Commercial mortgage-backed securities [Member]
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	5.9	[2]	5.0	[2]
Recurring [Member] | Level 3 [Member] | Corporates [Member]
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	439.0	[2]	135.3	[2]
Recurring [Member] | Level 3 [Member] | Hybrids [Member]
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale	0	[2]	8.8	[2]
Recurring [Member] | Level 3 [Member] | Municipals [Member]
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale		[2]		[2]
Recurring [Member] | Level 3 [Member] | Agency residential mortgage-backed securities [Member]
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale		[2]		[2]
Recurring [Member] | Level 3 [Member] | Non-agency residential mortgage-backed securities [Member]
Fixed maturity securities, available-for-sale:
Fixed maturity securities, available-for-sale		[2]		[2]
Recurring [Member] | Level 3 [Member] | Equity securities [Member]
Fixed maturity securities, available-for-sale:
Equity securities - trading		[2]		[2]
Recurring [Member] | Level 3 [Member] | Fixed maturities securities [Member]
Fixed maturity securities, available-for-sale:
U.S. Treasuries and certificate of deposit, held-to-maturity				[2]
Recurring [Member] | Level 3 [Member] | Policy loans [Member]
Fixed maturity securities, available-for-sale:
Policy loans and other invested assets	$ 27.6	[2]	$ 18.8	[2]

[1] The fair values of cash equivalents, short-term investments and debt set forth above are generally based on quoted or observed market prices.

[2] 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.