Derivative Financial Instruments - Fair Value of Outstanding Derivative Contracts in Condensed Consolidated Balance Sheets (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Sep. 30, 2015
Derivatives, Fair Value [Line Items]
Funds Withheld Receivable	$ 1,664.0	$ 1,710.1
FIA embedded derivatives [Member] | Contractholder funds [Member]
Derivatives, Fair Value [Line Items]
Total liability derivatives	139.9	142.3
Derivatives designated as hedging instruments [Member]
Derivatives, Fair Value [Line Items]
Total asset derivatives	7.5	5.6
Total liability derivatives	7.1	9.6
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	1.2	1.4
Derivatives designated as hedging instruments [Member] | Interest Rate Contract [Member] | Other liabilities [Member]
Derivatives, Fair Value [Line Items]
Total liability derivatives	0.7	1.2
Derivatives designated as hedging instruments [Member] | Foreign Exchange Forward [Member] | Other assets [Member]
Derivatives, Fair Value [Line Items]
Total asset derivatives	0.8	0.4
Derivatives designated as hedging instruments [Member] | Foreign Exchange Contract [Member] | Receivables, net [Member]
Derivatives, Fair Value [Line Items]
Total asset derivatives	6.7	5.2
Derivatives designated as hedging instruments [Member] | Foreign Exchange Contract [Member] | Accounts payable and other current liabilities [Member]
Derivatives, Fair Value [Line Items]
Total liability derivatives	0.0	1.5
Derivatives designated as hedging instruments [Member] | Foreign Exchange Contract [Member] | Other liabilities [Member]
Derivatives, Fair Value [Line Items]
Total liability derivatives	0.1	0.0
Derivatives designated as hedging instruments [Member] | Commodity contracts [Member] | Accounts payable and other current liabilities [Member]
Derivatives, Fair Value [Line Items]
Total liability derivatives	4.3	4.7
Derivatives designated as hedging instruments [Member] | Commodity contracts [Member] | Other liabilities [Member]
Derivatives, Fair Value [Line Items]
Total liability derivatives	0.8	0.8
Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Total asset derivatives	19.2	20.3
Total liability derivatives	147.6	152.1
Not Designated as Hedging Instrument [Member] | Foreign Exchange Forward [Member] | Accounts payable and other current liabilities [Member]
Derivatives, Fair Value [Line Items]
Total liability derivatives	0.6	0.1
Not Designated as Hedging Instrument [Member] | Call options [Member] | Other assets [Member]
Derivatives, Fair Value [Line Items]
Total asset derivatives	2.2	1.0
Not Designated as Hedging Instrument [Member] | Call options [Member] | Funds withheld for reinsurance liability [Member]
Derivatives, Fair Value [Line Items]
Total asset derivatives	8.1	5.4
Not Designated as Hedging Instrument [Member] | Foreign Exchange Contract [Member] | Receivables, net [Member]
Derivatives, Fair Value [Line Items]
Total asset derivatives	0.1	0.4
Not Designated as Hedging Instrument [Member] | Commodity contracts [Member] | Receivables, net [Member]
Derivatives, Fair Value [Line Items]
Total asset derivatives	1.3	7.9
Not Designated as Hedging Instrument [Member] | Commodity contracts [Member] | Accounts payable and other current liabilities [Member]
Derivatives, Fair Value [Line Items]
Total liability derivatives	0.0	0.1
Fair Value, Measurements, Recurring [Member]
Derivatives, Fair Value [Line Items]
Funds Withheld Receivable	1,664.0	1,710.1
Assets, Fair Value Disclosure, Recurring	1,683.2	1,780.1
Liabilities, Fair Value Disclosure, Recurring	776.6	781.3
Fair Value, Measurements, Recurring [Member] | Interest Rate Contract [Member]
Derivatives, Fair Value [Line Items]
Derivatives	10.3	6.4
Fair Value, Measurements, Recurring [Member] | Commodity Option [Member]
Derivatives, Fair Value [Line Items]
Derivatives	1.3	7.9
Derivative Liability	5.1	5.6
Fair Value, Measurements, Recurring [Member] | Foreign Exchange Forward [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability	0.7	1.6
Fair Value, Measurements, Recurring [Member] | Interest Rate Swap [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability	1.9	2.6
Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Derivatives, Fair Value [Line Items]
Funds Withheld Receivable	53.9	45.8
Assets, Fair Value Disclosure, Recurring	53.9	78.6
Liabilities, Fair Value Disclosure, Recurring	0.0	0.0
Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Interest Rate Contract [Member]
Derivatives, Fair Value [Line Items]
Derivatives	0.0	0.0
Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Fixed Index Annuity [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability	139.9	142.3
Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Front Street Future Policyholder Benefits Reserve [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability		0.0
Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Derivatives, Fair Value [Line Items]
Funds Withheld Receivable	1,535.3	1,579.9
Assets, Fair Value Disclosure, Recurring	1,554.5	1,600.2
Liabilities, Fair Value Disclosure, Recurring	7.7	9.8
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Interest Rate Contract [Member]
Derivatives, Fair Value [Line Items]
Derivatives	10.3	6.4
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Commodity Option [Member]
Derivatives, Fair Value [Line Items]
Derivatives	1.3	7.9
Derivative Liability	5.1	5.6
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Foreign Exchange Forward [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability	0.7	1.6
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Interest Rate Swap [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability	1.9	2.6
Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Derivatives, Fair Value [Line Items]
Funds Withheld Receivable	74.8	84.4
Assets, Fair Value Disclosure, Recurring	74.8	101.3
Liabilities, Fair Value Disclosure, Recurring	768.9	771.5
Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Interest Rate Contract [Member]
Derivatives, Fair Value [Line Items]
Derivatives	0.0	0.0
Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Fixed Index Annuity [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability	139.9	142.3
Corporate Debt Securities [Member] | Fair Value, Measurements, Recurring [Member]
Derivatives, Fair Value [Line Items]
Available-for-sale Securities	0.0	14.1
Corporate Debt Securities [Member] | Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Derivatives, Fair Value [Line Items]
Available-for-sale Securities	0.0	0.0
Corporate Debt Securities [Member] | Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Derivatives, Fair Value [Line Items]
Available-for-sale Securities	0.0	14.1
Equity [Member] | Fair Value, Measurements, Recurring [Member]
Derivatives, Fair Value [Line Items]
Trading Securities	0.0	32.8
Equity [Member] | Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Derivatives, Fair Value [Line Items]
Trading Securities	0.0	32.8
Other Invested Assets [Member] | Fair Value, Measurements, Recurring [Member]
Derivatives, Fair Value [Line Items]
Other invested assets	0.0	2.8
Other Invested Assets [Member] | Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Derivatives, Fair Value [Line Items]
Other invested assets	0.0	0.0
Other Invested Assets [Member] | Other Invested Assets [Member] | Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Derivatives, Fair Value [Line Items]
Other invested assets	0.0
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		2.8
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	$ 629.0	$ 629.2