Derivative Financial Instruments - Fair Value of Outstanding Derivative Contracts in Condensed Consolidated Balance Sheets (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Dec. 31, 2014		Sep. 30, 2014
Derivatives, Fair Value [Line Items]
Document Period End Date	Dec. 31, 2014
Derivatives	$ 306.2		$ 296.3
Other invested assets	247.0		165.0
Call options [Member]
Derivatives, Fair Value [Line Items]
Total asset derivatives	306.2		296.3
Nomura Fund Linked Note [Member]
Derivatives, Fair Value [Line Items]
Other invested assets	11.8
Derivatives designated as hedging instruments [Member]
Derivatives, Fair Value [Line Items]
Total asset derivatives	18.0		14.2
Total liability derivatives	2.7		1.9
Derivatives designated as hedging instruments [Member] | Interest Rate Contract [Member] | Accounts payable and other current liabilities [Member]
Derivatives, Fair Value [Line Items]
Total liability derivatives	1.9		1.8
Derivatives designated as hedging instruments [Member] | Commodity swap and option agreements [Member] | Receivables, net [Member]
Derivatives, Fair Value [Line Items]
Total asset derivatives	0.6		0.6
Derivatives designated as hedging instruments [Member] | Commodity swap and option agreements [Member] | Other assets [Member]
Derivatives, Fair Value [Line Items]
Total asset derivatives	0.4		1.3
Derivatives designated as hedging instruments [Member] | Foreign exchange forward agreements [Member] | Receivables, net [Member]
Derivatives, Fair Value [Line Items]
Total asset derivatives	0.1		0.3
Derivatives designated as hedging instruments [Member] | Foreign Exchange Contract [Member] | Other assets [Member]
Derivatives, Fair Value [Line Items]
Total asset derivatives	16.9		12.0
Derivatives designated as hedging instruments [Member] | Commodity contracts [Member] | Accounts payable and other current liabilities [Member]
Derivatives, Fair Value [Line Items]
Total liability derivatives	0.8		0.1
Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Total asset derivatives	355.8		324.1
Total liability derivatives	2,147.5		1,910.9
Not Designated as Hedging Instrument [Member] | Foreign exchange forward agreements [Member] | Accounts payable and other current liabilities [Member]
Derivatives, Fair Value [Line Items]
Total liability derivatives			0.1
Not Designated as Hedging Instrument [Member] | Foreign exchange forward agreements [Member] | Other liabilities [Member]
Derivatives, Fair Value [Line Items]
Total liability derivatives	2.6
Not Designated as Hedging Instrument [Member] | Foreign Exchange Contract [Member] | Receivables, net [Member]
Derivatives, Fair Value [Line Items]
Total asset derivatives	0.3		0.5
Not Designated as Hedging Instrument [Member] | Commodity contracts [Member] | Receivables, net [Member]
Derivatives, Fair Value [Line Items]
Total asset derivatives			1.9
Not Designated as Hedging Instrument [Member] | Commodity contracts [Member] | Derivatives For Trading And Investment [Member]
Derivatives, Fair Value [Line Items]
Total asset derivatives	19.5
Not Designated as Hedging Instrument [Member] | Commodity contracts [Member] | Other liabilities [Member]
Derivatives, Fair Value [Line Items]
Total liability derivatives	0.1		0.3
Not Designated as Hedging Instrument [Member] | Call options [Member] | Derivatives For Trading And Investment [Member]
Derivatives, Fair Value [Line Items]
Total asset derivatives	306.2		296.3
Not Designated as Hedging Instrument [Member] | FIA embedded derivatives [Member] | Contractholder funds [Member]
Derivatives, Fair Value [Line Items]
Total liability derivatives			1,908.1
Not Designated as Hedging Instrument [Member] | Futures contracts [Member] | Derivatives For Trading And Investment [Member]
Derivatives, Fair Value [Line Items]
Total asset derivatives	11.8
Not Designated as Hedging Instrument [Member] | Futures contracts [Member] | Contractholder funds [Member]
Derivatives, Fair Value [Line Items]
Total liability derivatives	2,140.2
Not Designated as Hedging Instrument [Member] | Futures contracts [Member] | Other liabilities [Member]
Derivatives, Fair Value [Line Items]
Total liability derivatives	1.9		0.5
Not Designated as Hedging Instrument [Member] | Embedded Derivative Financial Instruments [Member] | Derivatives For Trading And Investment [Member]
Derivatives, Fair Value [Line Items]
Total asset derivatives			11.2
Fair Value, Measurements, Recurring [Member]
Derivatives, Fair Value [Line Items]
Contingent Purchase Price Reduction Receivable Fair Value Disclosure	41.5	[1]	41.5	[1]
Funds Withheld Receivable	512.6	[1]	154.4	[1]
Assets, Fair Value Disclosure, Recurring	19,307.1	[1]	18,501.7	[1]
Liabilities, Fair Value Disclosure, Recurring	2,643.9	[1]	2,062.2	[1]
Fair Value, Measurements, Recurring [Member] | Interest Rate Contract [Member]
Derivatives, Fair Value [Line Items]
Derivatives	0.6	[1]	0.6	[1]
Fair Value, Measurements, Recurring [Member] | Commodity Option [Member]
Derivatives, Fair Value [Line Items]
Derivatives	19.9	[1]	3.2	[1]
Derivative Liability	0.9	[1]	0.5	[1]
Fair Value, Measurements, Recurring [Member] | Foreign exchange forward agreements [Member]
Derivatives, Fair Value [Line Items]
Derivatives			12.8	[1]
Derivative Liability	2.6	[1]	0.4	[1]
Fair Value, Measurements, Recurring [Member] | Call options [Member]
Derivatives, Fair Value [Line Items]
Derivatives	306.2	[1]	296.3	[1]
Fair Value, Measurements, Recurring [Member] | Available For Sale Embedded Derivatives [Member]
Derivatives, Fair Value [Line Items]
Equity securities - available-for-sale	625.9	[1]	663.6	[1]
Fair Value, Measurements, Recurring [Member] | Fixed Index Annuity [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability	2,140.2	[1]	1,908.1	[1]
Fair Value, Measurements, Recurring [Member] | Future Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability	1.9
Fair Value, Measurements, Recurring [Member] | Interest Rate Swap [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability	1.9	[1]	1.8	[1]
Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Derivatives, Fair Value [Line Items]
Funds Withheld Receivable	0	[1]	0	[1]
Assets, Fair Value Disclosure, Recurring	550.2	[1]	279.3	[1]
Liabilities, Fair Value Disclosure, Recurring	0	[1]	0	[1]
Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Available For Sale Embedded Derivatives [Member]
Derivatives, Fair Value [Line Items]
Equity securities - available-for-sale	57.1	[1]	59.2	[1]
Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Derivatives, Fair Value [Line Items]
Funds Withheld Receivable	512.6	[1]	154.4	[1]
Assets, Fair Value Disclosure, Recurring	17,596.1	[1]	17,156.4	[1]
Liabilities, Fair Value Disclosure, Recurring	7.3	[1]	2.8	[1]
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Interest Rate Contract [Member]
Derivatives, Fair Value [Line Items]
Derivatives	0.6	[1]	0.6	[1]
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Commodity Option [Member]
Derivatives, Fair Value [Line Items]
Derivatives	19.9	[1]	3.2	[1]
Derivative Liability	0.9	[1]	0.5	[1]
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Foreign exchange forward agreements [Member]
Derivatives, Fair Value [Line Items]
Derivatives			12.8	[1]
Derivative Liability	2.6	[1]	0.4	[1]
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Call options [Member]
Derivatives, Fair Value [Line Items]
Derivatives	306.2	[1]	296.3	[1]
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Available For Sale Embedded Derivatives [Member]
Derivatives, Fair Value [Line Items]
Equity securities - available-for-sale	562.7	[1]	598.4	[1]
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Future Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability	1.9		0.1
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Interest Rate Swap [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability	1.9	[1]	1.8	[1]
Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Derivatives, Fair Value [Line Items]
Funds Withheld Receivable	0	[1]	0	[1]
Assets, Fair Value Disclosure, Recurring	1,160.8	[1]	1,066.0	[1]
Liabilities, Fair Value Disclosure, Recurring	2,636.6	[1]	2,059.4	[1]
Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Available For Sale Embedded Derivatives [Member]
Derivatives, Fair Value [Line Items]
Equity securities - available-for-sale			6.0	[1]
Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Fixed Index Annuity [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability	2,140.2	[1]	1,908.1	[1]
Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Front Street Future Policyholder Benefits Reserve [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability	496.4	[1]	151.3	[1]
Contingent Purchase Price Reduction Receivable [Member] | Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Derivatives, Fair Value [Line Items]
Contingent Purchase Price Reduction Receivable Fair Value Disclosure	41.5	[1]	41.5	[1]
Asset-backed Securities [Member] | Fair Value, Measurements, Recurring [Member]
Derivatives, Fair Value [Line Items]
Available-for-sale Securities	1,839.1	[1]	1,792.9	[1]
Asset-backed Securities [Member] | Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Derivatives, Fair Value [Line Items]
Available-for-sale Securities	1,804.5	[1]	1,755.9	[1]
Commercial Mortgage Backed Securities [Member] | Fair Value, Measurements, Recurring [Member]
Derivatives, Fair Value [Line Items]
Available-for-sale Securities	668.4	[1]	636.9	[1]
Commercial Mortgage Backed Securities [Member] | Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Derivatives, Fair Value [Line Items]
Available-for-sale Securities	548.0	[1]	553.8	[1]
Corporate Debt Securities [Member] | Fair Value, Measurements, Recurring [Member]
Derivatives, Fair Value [Line Items]
Available-for-sale Securities	9,721.4	[1]	9,795.8	[1]
Corporate Debt Securities [Member] | Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Derivatives, Fair Value [Line Items]
Available-for-sale Securities	8,813.8	[1]	8,945.8	[1]
Corporate Debt Securities [Member] | Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Derivatives, Fair Value [Line Items]
Available-for-sale Securities	907.6	[1]	850.0	[1]
Hybrids [Member] | Fair Value, Measurements, Recurring [Member]
Derivatives, Fair Value [Line Items]
Available-for-sale Securities	1,301.5	[1]	1,316.1	[1]
Hybrids [Member] | Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Derivatives, Fair Value [Line Items]
Available-for-sale Securities	1,301.5	[1]	1,316.1	[1]
Hybrids [Member] | Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Derivatives, Fair Value [Line Items]
Available-for-sale Securities	0	[1]	0	[1]
Municipals [Member] | Fair Value, Measurements, Recurring [Member]
Derivatives, Fair Value [Line Items]
Available-for-sale Securities	1,361.9	[1]	1,259.8	[1]
Municipals [Member] | Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Derivatives, Fair Value [Line Items]
Available-for-sale Securities	1,323.1	[1]	1,222.6	[1]
Municipals [Member] | Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Derivatives, Fair Value [Line Items]
Available-for-sale Securities			37.2	[1]
Residential Mortgage Backed Securities [Member] | Fair Value, Measurements, Recurring [Member]
Derivatives, Fair Value [Line Items]
Available-for-sale Securities	2,201.9	[1]	2,114.0	[1]
Residential Mortgage Backed Securities [Member] | Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Derivatives, Fair Value [Line Items]
Available-for-sale Securities	2,201.9	[1]	2,114.0	[1]
Broker Quoted [Member] | Asset-backed Securities [Member] | Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Derivatives, Fair Value [Line Items]
Available-for-sale Securities	34.6	[1]	37.0	[1]
Broker Quoted [Member] | Commercial Mortgage Backed Securities [Member] | Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Derivatives, Fair Value [Line Items]
Available-for-sale Securities	120.4	[1]	83.1	[1]
Market Pricing [Member] | Municipal Bonds [Member] | Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Derivatives, Fair Value [Line Items]
Available-for-sale Securities	38.8	[1]
Market Pricing [Member] | Equity Securities [Member] | Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Available For Sale Embedded Derivatives [Member]
Derivatives, Fair Value [Line Items]
Equity securities - available-for-sale	6.1	[1]
U.S. Government securities [Member] | Fair Value, Measurements, Recurring [Member]
Derivatives, Fair Value [Line Items]
Available-for-sale Securities	583.6	[1]	296.0	[1]
U.S. Government securities [Member] | Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Derivatives, Fair Value [Line Items]
Available-for-sale Securities	401.6	[1]	115.6	[1]
U.S. Government securities [Member] | Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Derivatives, Fair Value [Line Items]
Available-for-sale Securities	182.0	[1]	180.4	[1]
Equity [Member] | Fair Value, Measurements, Recurring [Member]
Derivatives, Fair Value [Line Items]
Trading Securities	91.5	[1]	104.5	[1]
Equity [Member] | Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Derivatives, Fair Value [Line Items]
Trading Securities	91.5	[1]	104.5	[1]
Other Invested Assets [Member]
Derivatives, Fair Value [Line Items]
Trading Securities			165.0
Other Invested Assets [Member] | Fair Value, Measurements, Recurring [Member]
Derivatives, Fair Value [Line Items]
Other invested assets	13.8		13.3
Other Invested Assets [Member] | Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Derivatives, Fair Value [Line Items]
Other invested assets	0		0
Other Invested Assets [Member] | Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Derivatives, Fair Value [Line Items]
Other invested assets	2.0	[1]	2.1	[1]
Other Invested Assets [Member] | Option Pricing [Member] | Other Invested Assets [Member] | Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Derivatives, Fair Value [Line Items]
Other invested assets			11.2
Other Invested Assets [Member] | Option Pricing [Member] | Other Invested Assets [Member] | Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Nomura Fund Linked Note [Member]
Derivatives, Fair Value [Line Items]
Other invested assets	11.8
Estimate of Fair Value Measurement [Member] | Fair Value, Measurements, Recurring [Member] | Front Street Future Policyholder Benefits Reserve [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability	496.4	[1]	151.3	[1]
Estimate of Fair Value Measurement [Member] | Fair Value, Measurements, Recurring [Member] | Future Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability			$ 0.1

[1] December 31, 2014 September 30, 2014 Level 1 Level 2 Level 3 Fair Value Level 1 Level 2 Level 3 Fair ValueAssets Contingent purchase price reduction receivable$— $— $41.5 $41.5 $— $— $41.5 $41.5Derivatives: Interest rate contracts— 0.6 — 0.6 — 0.6 — 0.6Commodity swap and option agreements— 19.9 — 19.9 — 3.2 — 3.2Foreign exchange forward agreements— 17.3 — 17.3 — 12.8 — 12.8Call options and futures contracts— 306.2 — 306.2 — 296.3 — 296.3Fixed maturity securities, available-for-sale: Asset-backed securities— 1,804.5 34.6 1,839.1 — 1,755.9 37.0 1,792.9Commercial mortgage-backed securities— 548.0 120.4 668.4 — 553.8 83.1 636.9Corporates— 8,813.8 907.6 9,721.4 — 8,945.8 850.0 9,795.8Hybrids— 1,301.5 — 1,301.5 — 1,316.1 — 1,316.1Municipals— 1,323.1 38.8 1,361.9 — 1,222.6 37.2 1,259.8Residential mortgage-backed securities— 2,201.9 — 2,201.9 — 2,114.0 — 2,114.0U.S. Government401.6 182.0 — 583.6 115.6 180.4 — 296.0Equity securities: Available-for-sale57.1 562.7 6.1 625.9 59.2 598.4 6.0 663.6Trading91.5 — — 91.5 104.5 — — 104.5Other invested assets— 2.0 11.8 13.8 — 2.1 11.2 13.3Funds withheld receivable (a)— 512.6 — 512.6 — 154.4 — 154.4Total financial assets$550.2 $17,596.1 $1,160.8 $19,307.1 $279.3 $17,156.4 $1,066.0 $18,501.7 Liabilities Derivatives: FIA embedded derivatives, included in contractholder funds$— $— $2,140.2 $2,140.2 $— $— $1,908.1 $1,908.1Front Street future policyholder benefit liability— — 496.4 496.4 — — 151.3 151.3Foreign exchange forward agreements and contracts— 2.6 — 2.6 — 0.1 — 0.1Futures contracts— 1.9 — 1.9 — 0.4 — 0.4Commodity contracts— 0.9 — 0.9 — 0.5 — 0.5Interest rate contracts— 1.9 — 1.9 — 1.8 — 1.8Total financial liabilities$— $7.3 $2,636.6 $2,643.9 $— $2.8 $2,059.4 $2,062.2