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	3 Months Ended	12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Funds Withheld Receivable	$ 1,664.0	$ 1,710.1
Assets, Fair Value Disclosure	74.8	101.3
Liabilities
Liabilities, Fair Value Disclosure	768.9	771.5
Fair Value, Measurements, Recurring [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Assets, Fair Value Disclosure, Recurring	1,683.2	1,780.1
Funds Withheld Receivable	$ 1,664.0	1,710.1
Funds withheld for reinsurance liability [Member] | Market Pricing [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Valuation Technique	Market pricing
Funds withheld for reinsurance liability [Member] | NAV [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Valuation Technique	Loan value
Funds withheld for reinsurance liability [Member] | Broker Quoted [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Valuation Technique	Broker-quoted
Funds withheld for reinsurance liability [Member] | Matrix Pricing [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Valuation Technique	Matrix pricing
Funds withheld for reinsurance liability [Member] | Discounted Cash Flow [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Valuation Technique	Discounted Cash Flow
Corporate Debt Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Available-for-sale Securities	$ 0.0	$ 14.1
Corporate Debt Securities [Member] | Broker Quoted [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Valuation Technique	Broker-quoted
Other Invested Assets [Member] | Discounted Cash Flow [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Valuation Technique	Discounted Cash Flow
Fixed Index Annuity [Member] | Discounted Cash Flow [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Valuation Technique	Discounted cash flow
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] | Funds Withheld Receivables [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount rate	6.00%	6.00%
Quoted prices	100.00%	100.00%
Offered quotes	98.00%	99.00%
Minimum [Member] | Corporate Debt Securities [Member] | Broker Quoted [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Offered quotes
Minimum [Member] | Other Invested Assets [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Probability of collection
Minimum [Member] | Fixed Index Annuity [Member]
Liabilities
Market value of option	0.00%	0.00%
SWAP rates
Surrender rates	0.50%	0.50%
Maximum [Member] | Funds Withheld Receivables [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount rate	13.00%	12.00%
Quoted prices	120.00%	122.00%
Offered quotes	100.00%	103.00%
Maximum [Member] | Corporate Debt Securities [Member] | Broker Quoted [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Offered quotes
Maximum [Member] | Other Invested Assets [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Probability of collection
Maximum [Member] | Fixed Index Annuity [Member]
Liabilities
Market value of option	29.00%	32.00%
SWAP rates
Surrender rates	75.00%	75.00%
Weighted Average [Member] | Funds Withheld Receivables [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount rate	7.00%	8.00%
Quoted prices	110.00%	112.00%
Offered quotes	99.00%	101.00%
Weighted Average [Member] | Loans and Finance Receivables [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Offered quotes	100.00%	100.00%
Weighted Average [Member] | Corporate Debt Securities [Member] | Broker Quoted [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Offered quotes		83.00%
Weighted Average [Member] | Other Invested Assets [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Probability of collection		50.00%
Discount rate		10.00%
Weighted Average [Member] | Policy Loans [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Discount rate	100.00%	100.00%
Weighted Average [Member] | Fixed Index Annuity [Member]
Liabilities
Market value of option	1.00%	1.00%
SWAP rates	2.00%
Mortality multiplier	80.00%	80.00%
Surrender rates	13.00%	13.00%
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%
Front Street Future Policyholder Benefits Reserve [Member] | Minimum [Member]
Liabilities
Fair Value Measurements Non Performance Spread	0.00%	0.16%
Risk margin for uncertainty	0.50%	0.50%
Front Street Future Policyholder Benefits Reserve [Member] | Maximum [Member]
Liabilities
Fair Value Measurements Non Performance Spread	0.00%	0.46%
Risk margin for uncertainty	1.00%	1.00%
Front Street Future Policyholder Benefits Reserve [Member] | Weighted Average [Member]
Liabilities
Risk margin for uncertainty
Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Assets, Fair Value Disclosure, Recurring	$ 1,554.5	$ 1,600.2
Funds Withheld Receivable	1,535.3	1,579.9
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	0.0	0.0
Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Assets, Fair Value Disclosure, Recurring	74.8	101.3
Funds Withheld Receivable	74.8	84.4
Level 3 [Member] | Discounted Cash Flow [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Funds Withheld Receivable	15.4	19.5
Other invested assets	0.0	2.8
Level 3 [Member] | Funds Withheld Receivables [Member] | Market Pricing [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Assets, Fair Value Disclosure	5.3	9.7
Level 3 [Member] | Funds Withheld Receivables [Member] | NAV [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Assets, Fair Value Disclosure	8.8	9.0
Level 3 [Member] | Funds Withheld Receivables [Member] | Broker Quoted [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Assets, Fair Value Disclosure	11.6	6.7
Level 3 [Member] | Funds Withheld Receivables [Member] | Matrix Pricing [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Assets, Fair Value Disclosure	33.7	39.5
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	0.0	14.1
Level 3 [Member] | Corporate Debt Securities [Member] | Broker Quoted [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Assets, Fair Value Disclosure	0.0	14.1
Level 3 [Member] | Fixed Index Annuity [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Derivative Liability	139.9	142.3
Level 3 [Member] | Front Street Future Policyholder Benefits Reserve [Member] | Discounted Cash Flow [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Derivative Liability	629.0	629.2
Other Invested Assets [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Other invested assets	0.0	2.8
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	$ 0.0
Other Invested Assets [Member] | Level 3 [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		$ 2.8