Derivative Financial Instruments - Fair Value of Outstanding Derivative Contracts in Condensed Consolidated Balance Sheets (Detail) - USD ($) $ in Millions	6 Months Ended
	Mar. 31, 2017	Sep. 30, 2016	Sep. 30, 2015
Derivatives, Fair Value [Line Items]
Document Period End Date	Mar. 31, 2017
Funds Withheld Receivable	$ 1,634.2	$ 1,650.4
Derivatives designated as hedging instruments [Member]
Derivatives, Fair Value [Line Items]
Total asset derivatives	8.5	8.5
Total liability derivatives	1.6	3.0
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	0.0	0.7
Derivatives designated as hedging instruments [Member] | Interest Rate Contract [Member] | Other liabilities [Member]
Derivatives, Fair Value [Line Items]
Total liability derivatives	0.0	0.4
Derivatives designated as hedging instruments [Member] | Commodity Swap And Option Agreements [Member] | Receivables, net [Member]
Derivatives, Fair Value [Line Items]
Total asset derivatives	4.1	2.9
Derivatives designated as hedging instruments [Member] | Commodity Swap And Option Agreements [Member] | Other assets [Member]
Derivatives, Fair Value [Line Items]
Total asset derivatives	0.2		$ 0.0
Derivatives designated as hedging instruments [Member] | Foreign Exchange Forward [Member] | Other assets [Member]
Derivatives, Fair Value [Line Items]
Total asset derivatives	0.1	0.1
Derivatives designated as hedging instruments [Member] | Foreign Exchange Contract [Member] | Receivables, net [Member]
Derivatives, Fair Value [Line Items]
Total asset derivatives	4.1	5.5
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	1.3	1.7
Derivatives designated as hedging instruments [Member] | Foreign Exchange Contract [Member] | Other liabilities [Member]
Derivatives, Fair Value [Line Items]
Total liability derivatives	0.2	0.1
Derivatives designated as hedging instruments [Member] | Commodity contracts [Member] | Accounts payable and other current liabilities [Member]
Derivatives, Fair Value [Line Items]
Total liability derivatives	0.1	0.1
Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Total asset derivatives	32.7	25.9
Total liability derivatives	123.2	134.4
Not Designated as Hedging Instrument [Member] | Embedded Derivative Financial Instruments [Member] | Accounts payable and other current liabilities [Member]
Derivatives, Fair Value [Line Items]
Total liability derivatives	121.0	131.2
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	0.2
Not Designated as Hedging Instrument [Member] | Call options [Member] | Other assets [Member]
Derivatives, Fair Value [Line Items]
Total asset derivatives	10.5	5.9
Not Designated as Hedging Instrument [Member] | Call options [Member] | Funds withheld for reinsurance liability [Member]
Derivatives, Fair Value [Line Items]
Total asset derivatives	13.3	11.3
Not Designated as Hedging Instrument [Member] | Foreign Exchange Contract [Member] | Receivables, net [Member]
Derivatives, Fair Value [Line Items]
Total asset derivatives	0.4	0.2
Fair Value, Measurements, Recurring [Member]
Derivatives, Fair Value [Line Items]
Funds Withheld Receivable	1,543.4	1,535.1
Assets, Fair Value Disclosure, Recurring	1,576.1	1,561.0
Liabilities, Fair Value Disclosure, Recurring	784.4	766.2
Fair Value, Measurements, Recurring [Member] | Commodity Option [Member]
Derivatives, Fair Value [Line Items]
Derivatives	4.3	2.9
Derivative Liability	0.1	0.1
Fair Value, Measurements, Recurring [Member] | Foreign Exchange Forward [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability	2.1	2.0
Fair Value, Measurements, Recurring [Member] | Call options [Member]
Derivatives, Fair Value [Line Items]
Derivatives	10.5	5.9
Fair Value, Measurements, Recurring [Member] | Fixed Index Annuity [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability	121.0	131.2
Fair Value, Measurements, Recurring [Member] | Interest Rate Swap [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability		1.1
Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Derivatives, Fair Value [Line Items]
Funds Withheld Receivable	80.5	69.9
Assets, Fair Value Disclosure, Recurring	80.5	69.9
Liabilities, Fair Value Disclosure, Recurring	0.0	0.0
Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Call options [Member]
Derivatives, Fair Value [Line Items]
Derivatives	0.0	0.0
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	1,413.8	1,387.1
Assets, Fair Value Disclosure, Recurring	1,446.5	1,413.0
Liabilities, Fair Value Disclosure, Recurring	2.2	3.2
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Commodity Option [Member]
Derivatives, Fair Value [Line Items]
Derivatives	4.3	2.9
Derivative Liability	0.1	0.1
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Foreign Exchange Forward [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability	2.1	2.0
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Call options [Member]
Derivatives, Fair Value [Line Items]
Derivatives	10.5	5.9
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Interest Rate Swap [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability		1.1
Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Derivatives, Fair Value [Line Items]
Funds Withheld Receivable	49.1	78.1
Assets, Fair Value Disclosure, Recurring	49.1	78.1
Liabilities, Fair Value Disclosure, Recurring	782.2	763.0
Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Call options [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	121.0	131.2
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	$ 661.2	$ 631.8