Fair Value of Financial Instruments - Schedule of Unobservable Inputs Used for Level Three Fair Value Measurements of Financial Instruments on Recurring Basis (Detail) - USD ($)
$ in Millions

Fair Value of Financial Instruments - Schedule of Unobservable Inputs Used for Level Three Fair Value Measurements of Financial Instruments on Recurring Basis (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Jun. 30, 2015	Sep. 30, 2014	Jun. 16, 2014
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Document Period End Date	Jun. 30, 2015
Assets, Fair Value Disclosure	$ 1,148.1	$ 1,066.0
Other invested assets	442.4	165.0
Liabilities
Liabilities, Fair Value Disclosure	2,768.3	2,059.4
Fair Value, Measurements, Recurring [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Assets, Fair Value Disclosure, Recurring	19,238.2	18,501.7
Contingent Purchase Price Reduction Receivable Fair Value Disclosure	0.0	41.5
Asset-backed Securities [Member] \| Fair Value, Measurements, Recurring [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Available-for-sale Securities	$ 1,887.5	1,792.9
Municipal [Member] \| Broker Quoted [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Valuation Technique	Broker-quoted
Commercial Mortgage Backed Securities [Member] \| Fair Value, Measurements, Recurring [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Available-for-sale Securities	$ 821.6	636.9
Commercial Mortgage Backed Securities [Member] \| Broker Quoted [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Valuation Technique	Broker-quoted
Corporate Debt Securities [Member] \| Fair Value, Measurements, Recurring [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Available-for-sale Securities	$ 9,551.1	9,795.8
Corporate Debt Securities [Member] \| Broker Quoted [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Assets, Fair Value Disclosure	$ 883.8	848.0
Corporate Debt Securities [Member] \| Matrix Pricing [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Valuation Technique	Matrix Pricing
Assets, Fair Value Disclosure	$ 33.1	2.0
Hybrids [Member] \| Fair Value, Measurements, Recurring [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Available-for-sale Securities	$ 1,241.1	1,316.1
Municipal Bonds [Member] \| Broker Quoted [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Valuation Technique	Broker-quoted
Municipals [Member] \| Fair Value, Measurements, Recurring [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Available-for-sale Securities	$ 1,450.6	1,259.8
Equities [Member] \| Broker Quoted [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Valuation Technique	Broker-quoted
Other Invested Assets [Member] \| Black Scholes Model [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Valuation Technique	Black Scholes model
Other Invested Assets [Member] \| Discounted Cash Flow [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Valuation Technique	Discounted cash flow
Contingent Purchase Price Reduction Receivable [Member] \| Discounted Cash Flow [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Valuation Technique	Discounted cash flow
Available For Sale Embedded Derivatives [Member] \| Fair Value, Measurements, Recurring [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Equity securities - available-for-sale	$ 579.6	663.6
Nomura Fund Linked Note [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Embedded Derivative, Fair Value of Embedded Derivative Liability	11.7		$ 11.3
Fixed Index Annuity [Member] \| Fair Value, Measurements, Recurring [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Derivative Liability	$ 2,173.7	$ 1,908.1
Front Street Future Policyholder Benefits Reserve [Member] \| Discounted Cash Flow [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Valuation Technique	Discounted cash flow
Minimum [Member] \| Asset-backed Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Offered quotes	0.00%	100.00%
Minimum [Member] \| Asset-backed Securities [Member] \| Matrix Pricing [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Quoted prices	0.00%	0.00%
Minimum [Member] \| Commercial Mortgage Backed Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Quoted prices	106.00%	0.00%
Offered quotes	100.00%	105.00%
Minimum [Member] \| Corporate Debt Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Quoted prices	67.00%
Offered quotes	75.00%
Minimum [Member] \| Corporate Debt Securities [Member] \| Broker Quoted [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Offered quotes		62.00%
Minimum [Member] \| Corporate Debt Securities [Member] \| Matrix Pricing [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Quoted prices		0.00%
Minimum [Member] \| Hybrids [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Quoted prices
Minimum [Member] \| Contingent Purchase Price Reduction Receivable [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Probability of collection	0.00%	88.00%
Minimum [Member] \| Fixed Index Annuity [Member]
Liabilities
Market value of option	0.00%	0.00%
SWAP rates
Surrender rates	0.50%
Minimum [Member] \| Fixed Index Annuity [Member] \| Discounted Cash Flow [Member]
Liabilities
SWAP rates		2.00%
Surrender rates		0.50%
Maximum [Member] \| Asset-backed Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Offered quotes	0.00%	109.00%
Maximum [Member] \| Asset-backed Securities [Member] \| Matrix Pricing [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Quoted prices	0.00%	0.00%
Maximum [Member] \| Commercial Mortgage Backed Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Quoted prices	123.00%	0.00%
Offered quotes	121.00%	121.00%
Maximum [Member] \| Corporate Debt Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Quoted prices	150.00%
Offered quotes	123.00%
Maximum [Member] \| Corporate Debt Securities [Member] \| Broker Quoted [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Offered quotes		120.00%
Maximum [Member] \| Corporate Debt Securities [Member] \| Matrix Pricing [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Quoted prices		0.00%
Maximum [Member] \| Hybrids [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Quoted prices
Maximum [Member] \| Contingent Purchase Price Reduction Receivable [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Probability of collection	0.00%	96.00%
Expected term	5 days
Discount rate	1.00%	1.00%
Credit insurance risk premium	7.00%	12.00%
Maximum [Member] \| Contingent Purchase Price Reduction Receivable [Member] \| Discounted Cash Flow [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Expected term		4 months 5 days
Maximum [Member] \| Fixed Index Annuity [Member]
Liabilities
Market value of option	42.00%	50.00%
SWAP rates
Surrender rates	75.00%
Maximum [Member] \| Fixed Index Annuity [Member] \| Discounted Cash Flow [Member]
Liabilities
SWAP rates		3.00%
Surrender rates		75.00%
Weighted Average [Member] \| Asset-backed Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Offered quotes	100.00%	101.00%
Weighted Average [Member] \| Asset-backed Securities [Member] \| Matrix Pricing [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Quoted prices	110.00%	0.00%
Weighted Average [Member] \| Commercial Mortgage Backed Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Quoted prices	115.00%	0.00%
Offered quotes	114.00%	118.00%
Weighted Average [Member] \| Corporate Debt Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Quoted prices	103.00%
Offered quotes	103.00%
Weighted Average [Member] \| Corporate Debt Securities [Member] \| Broker Quoted [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Offered quotes		100.00%
Weighted Average [Member] \| Corporate Debt Securities [Member] \| Matrix Pricing [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Quoted prices		142.00%
Weighted Average [Member] \| Municipal Debt Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Offered quotes	115.00%
Weighted Average [Member] \| Hybrids [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Quoted prices		107.00%
Weighted Average [Member] \| Equities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Offered quotes	100.00%	100.00%
Weighted Average [Member] \| Nomura Fund Linked Note [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Offered quotes	100.00%	100.00%
Weighted Average [Member] \| Contingent Purchase Price Reduction Receivable [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Probability of collection	94.00%	92.00%
Weighted Average [Member] \| Fixed Index Annuity [Member]
Liabilities
Market value of option	3.00%	3.00%
SWAP rates	2.00%
Mortality multiplier	80.00%	80.00%
Surrender rates	7.00%
Fair Value Measurements Non Performance Spread	0.25%	0.25%
Weighted Average [Member] \| Fixed Index Annuity [Member] \| Discounted Cash Flow [Member]
Liabilities
SWAP rates		2.00%
Surrender rates		7.00%
Front Street Future Policyholder Benefits Reserve [Member] \| Minimum [Member]
Liabilities
Fair Value Measurements Non Performance Spread	0.50%	0.50%
Front Street Future Policyholder Benefits Reserve [Member] \| Maximum [Member]
Liabilities
Fair Value Measurements Non Performance Spread	1.50%	1.50%
Front Street Future Policyholder Benefits Reserve [Member] \| Weighted Average [Member]
Liabilities
Risk margin for uncertainty	1.00%	0.50%
Level 1 [Member] \| Fair Value, Measurements, Recurring [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Assets, Fair Value Disclosure, Recurring	$ 527.8	$ 279.3
Level 1 [Member] \| Available For Sale Embedded Derivatives [Member] \| Fair Value, Measurements, Recurring [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Equity securities - available-for-sale	26.7	59.2
Level 2 [Member] \| Fair Value, Measurements, Recurring [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Assets, Fair Value Disclosure, Recurring	17,562.3	17,156.4
Level 2 [Member] \| Asset-backed Securities [Member] \| Fair Value, Measurements, Recurring [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Available-for-sale Securities	1,854.4	1,755.9
Level 2 [Member] \| Commercial Mortgage Backed Securities [Member] \| Fair Value, Measurements, Recurring [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Available-for-sale Securities	681.5	553.8
Level 2 [Member] \| Corporate Debt Securities [Member] \| Fair Value, Measurements, Recurring [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Available-for-sale Securities	8,634.2	8,945.8
Level 2 [Member] \| Hybrids [Member] \| Fair Value, Measurements, Recurring [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Available-for-sale Securities	1,241.1	1,316.1
Level 2 [Member] \| Municipals [Member] \| Fair Value, Measurements, Recurring [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Available-for-sale Securities	1,413.1	1,222.6
Level 2 [Member] \| Available For Sale Embedded Derivatives [Member] \| Fair Value, Measurements, Recurring [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Equity securities - available-for-sale	546.9	598.4
Level 3 [Member] \| Fair Value, Measurements, Recurring [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Assets, Fair Value Disclosure, Recurring	1,148.1	1,066.0
Level 3 [Member] \| Asset-backed Securities [Member] \| Fair Value, Measurements, Recurring [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Available-for-sale Securities	33.1
Level 3 [Member] \| Asset-backed Securities [Member] \| Broker Quoted [Member] \| Fair Value, Measurements, Recurring [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Available-for-sale Securities	33.1	37.0
Level 3 [Member] \| Commercial Mortgage Backed Securities [Member] \| Fair Value, Measurements, Recurring [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Available-for-sale Securities	140.1
Level 3 [Member] \| Commercial Mortgage Backed Securities [Member] \| Broker Quoted [Member] \| Fair Value, Measurements, Recurring [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Available-for-sale Securities	140.1	83.1
Level 3 [Member] \| Corporate Debt Securities [Member] \| Fair Value, Measurements, Recurring [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Available-for-sale Securities	916.9	850.0
Level 3 [Member] \| Hybrids [Member] \| Fair Value, Measurements, Recurring [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Available-for-sale Securities	0.0	0.0
Level 3 [Member] \| Municipal Bonds [Member] \| Market Pricing [Member] \| Fair Value, Measurements, Recurring [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Available-for-sale Securities	37.5
Level 3 [Member] \| Municipals [Member] \| Fair Value, Measurements, Recurring [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Available-for-sale Securities		37.2
Level 3 [Member] \| Contingent Purchase Price Reduction Receivable [Member] \| Fair Value, Measurements, Recurring [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Contingent Purchase Price Reduction Receivable Fair Value Disclosure	0.0	41.5
Level 3 [Member] \| Available For Sale Embedded Derivatives [Member] \| Equity Securities [Member] \| Broker Quoted [Member] \| Fair Value, Measurements, Recurring [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Equity securities - available-for-sale	6.0	6.0
Level 3 [Member] \| Fixed Index Annuity [Member] \| Fair Value, Measurements, Recurring [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Derivative Liability	2,173.7	1,908.1
Level 3 [Member] \| Front Street Future Policyholder Benefits Reserve [Member] \| Fair Value, Measurements, Recurring [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Derivative Liability	594.6	151.3
Other Invested Assets [Member] \| Fair Value, Measurements, Recurring [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Other invested assets	14.5	13.3
Other Invested Assets [Member] \| Level 1 [Member] \| Fair Value, Measurements, Recurring [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Other invested assets	0.0	0.0
Other Invested Assets [Member] \| Level 2 [Member] \| Fair Value, Measurements, Recurring [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Other invested assets		2.1
Other Invested Assets [Member] \| Level 3 [Member] \| Other Invested Assets [Member] \| Fair Value, Measurements, Recurring [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Other invested assets	14.5	11.2
Other Invested Assets [Member] \| Level 3 [Member] \| Other Invested Assets [Member] \| Discounted Cash Flow [Member] \| Fair Value, Measurements, Recurring [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Other invested assets	2.8	$ 0.0
Other Invested Assets [Member] \| Level 3 [Member] \| Nomura Fund Linked Note [Member] \| Other Invested Assets [Member] \| Option Pricing [Member] \| Fair Value, Measurements, Recurring [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Other invested assets	$ 11.7