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		12 Months Ended
		Sep. 30, 2016		Sep. 30, 2015		Sep. 30, 2014	Sep. 30, 2013
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Value		$ 763.0		$ 771.5		$ 302.1	$ 471.3
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings		48.4		15.8		30.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Other Comprehensive Income (Loss)		0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Purchases		0.0		444.2		150.6
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Sales		0.0		0.0		0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements		56.9		(9.4)		349.8
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Transfers, Net		0.0	[1]	0.0	[2]	0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings		(1.4)		(10.5)		3.3
Funds Withheld Receivable		1,650.4		1,710.1
Assets, Fair Value Disclosure		78.1		91.6
Liabilities
Liabilities, Fair Value Disclosure		763.0		771.5
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)						0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases		36.2		30.4		41.3
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales		48.0		14.1		0.7
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements		5.5		50.0		13.5
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers, Net		5.2	[1]	19.1	[2]	0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value		78.1		91.6		116.7	86.3
Funds Withheld Receivables [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings				(0.5)		1.5
Liabilities
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases				30.4		23.5
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales				14.1		0.7
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value				74.7		58.9	34.6
Funds withheld for reinsurance liability [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings		(3.6)
Liabilities
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)		0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases		36.2
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales		34.4
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements		0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers, Net	[1]	5.2
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value		78.1		74.7
Corporate Debt Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings		(0.5)		(2.2)		0.0
Liabilities
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)						0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases		0.0				16.3
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales		13.6				0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements		0.0				0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers, Net						0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value				14.1		16.3	0.0
Other invested assets
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings		2.7		(16.3)
Liabilities
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)		0.0		0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases		0.0		0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales		0.0		0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements		5.5		0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers, Net		0.0	[1]	19.1	[2]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value		0.0		2.8		0.0
Fair Value, Measurements, Recurring
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Funds Withheld Receivable		1,535.1		1,638.1
Fair Value, Measurements, Recurring | Corporate Debt Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Available-for-sale Securities		$ 0.0		14.1
Contingent Purchase Price Reduction Receivable [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings				8.5		0.5
Liabilities
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)						0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases						0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales						0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements				$ 50.0		0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers, Net						0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value						41.5	$ 41.0
Minimum [Member] | FIA embedded derivatives, included in contractholder funds
Liabilities
Market value of option		0.00%		0.00%
Maximum | FIA embedded derivatives, included in contractholder funds
Liabilities
Market value of option		27.00%		32.00%
Weighted average | Other invested assets
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Probability of collection		0.00%		50.00%
Fair Value Inputs, Discount Rate		0.00%		10.00%
Weighted average | FIA embedded derivatives, included in contractholder funds
Liabilities
Market value of option		2.00%		1.00%
SWAP rates		1.00%		2.00%
Mortality multiplier		80.00%		80.00%
Non-performance spread		0.25%		0.25%
Broker Quoted [Member] | Funds withheld for reinsurance liability [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Valuation Technique		Broker-quoted
Broker Quoted [Member] | Corporate Debt Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Valuation Technique		Broker-quoted
Broker Quoted [Member] | Minimum [Member] | Corporate Debt Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Offered quotes		0.00%		0.00%
Broker Quoted [Member] | Minimum [Member] | Funds withheld for reinsurance liability [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs, Discount Rate		97.00%		99.00%
Broker Quoted [Member] | Maximum | Corporate Debt Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Offered quotes		0.00%		0.00%
Broker Quoted [Member] | Maximum | Funds withheld for reinsurance liability [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs, Discount Rate		100.00%		103.00%
Broker Quoted [Member] | Weighted average | Corporate Debt Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Offered quotes		0.00%		83.00%
Broker Quoted [Member] | Weighted average | Funds withheld for reinsurance liability [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs, Discount Rate		100.00%		101.00%
Discounted Cash Flow [Member] | Funds withheld for reinsurance liability [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Valuation Technique		Loan Recovery Value
Discounted Cash Flow [Member] | Other invested assets
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Valuation Technique		Discounted Cash Flow
Discounted Cash Flow [Member] | FIA embedded derivatives, included in contractholder funds
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Valuation Technique		Discounted cash flow
Discounted Cash Flow [Member] | Minimum [Member] | FIA embedded derivatives, included in contractholder funds
Liabilities
Surrender rates		0.50%		0.50%
Discounted Cash Flow [Member] | Maximum | FIA embedded derivatives, included in contractholder funds
Liabilities
Surrender rates		75.00%		75.00%
Discounted Cash Flow [Member] | Weighted average | FIA embedded derivatives, included in contractholder funds
Liabilities
Surrender rates		10.00%		13.00%
Matrix Pricing [Member] | Funds withheld for reinsurance liability [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Valuation Technique		Matrix pricing
Matrix Pricing [Member] | Minimum [Member] | Funds withheld for reinsurance liability [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs, Discount Rate		98.00%		100.00%
Matrix Pricing [Member] | Maximum | Funds withheld for reinsurance liability [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs, Discount Rate		122.00%		122.00%
Matrix Pricing [Member] | Weighted average | Funds withheld for reinsurance liability [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs, Discount Rate		109.00%		112.00%
Cost Approach Valuation Technique [Member] | Funds withheld for reinsurance liability [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Valuation Technique		Loan Recovery Value
Cost Approach Valuation Technique [Member] | Minimum [Member] | Loan Participations and Assignments [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs, Discount Rate		52.00%
Cost Approach Valuation Technique [Member] | Minimum [Member] | Funds withheld for reinsurance liability [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs, Discount Rate		56.00%		6.00%
Cost Approach Valuation Technique [Member] | Maximum | Loan Participations and Assignments [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs, Discount Rate		100.00%
Cost Approach Valuation Technique [Member] | Maximum | Funds withheld for reinsurance liability [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs, Discount Rate		100.00%		12.00%
Cost Approach Valuation Technique [Member] | Weighted average | Loan Participations and Assignments [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs, Discount Rate		71.00%		100.00%
Cost Approach Valuation Technique [Member] | Weighted average | Funds withheld for reinsurance liability [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs, Discount Rate		82.00%		8.00%
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Funds Withheld Receivable		$ 78.1		$ 74.7
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring | Corporate Debt Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Available-for-sale Securities		0.0		14.1
Fair Value, Inputs, Level 3 [Member] | FIA embedded derivatives, included in contractholder funds | Fair Value, Measurements, Recurring
Liabilities
Derivative Liability		131.2		142.3
Fair Value, Inputs, Level 3 [Member] | Broker Quoted [Member] | Funds Withheld Receivables [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Assets, Fair Value Disclosure		35.7		6.7
Fair Value, Inputs, Level 3 [Member] | Broker Quoted [Member] | Corporate Debt Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Assets, Fair Value Disclosure		0.0		14.1
Fair Value, Inputs, Level 3 [Member] | Discounted Cash Flow [Member] | Fair Value, Measurements, Recurring
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Funds Withheld Receivable		5.4		19.2
Other invested assets		$ 0.0		2.8
Fair Value, Inputs, Level 3 [Member] | Discounted Cash Flow [Member] | Front Street Future Policyholder Benefits Reserve [Member] | Fair Value, Measurements, Recurring
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Valuation Technique		Discounted cash flow
Liabilities
Derivative Liability		$ 631.8		629.2
Fair Value, Inputs, Level 3 [Member] | Matrix Pricing [Member] | Funds Withheld Receivables [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Assets, Fair Value Disclosure		35.2		39.1
Fair Value, Inputs, Level 3 [Member] | Cost Approach Valuation Technique [Member] | Funds Withheld Receivables [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Assets, Fair Value Disclosure		1.8		9.7
Other invested assets | Fair Value, Measurements, Recurring
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Other invested assets		0.0		2.8
Other invested assets | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring | Other invested assets
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Other invested assets		0.0
Other invested assets | Fair Value, Inputs, Level 3 [Member] | Option Pricing [Member] | Fair Value, Measurements, Recurring | Other invested assets
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Other invested assets				2.8
Front Street Future Policyholder Benefits Reserve [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Value		631.8		629.2		151.3
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings		59.5		24.3		7.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Purchases				444.2		150.6
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements		$ (56.9)		$ (9.4)		$ 6.3
Front Street Future Policyholder Benefits Reserve [Member] | Minimum [Member]
Liabilities
Non-performance spread				0.16%
Risk margin for uncertainty				0.50%
Front Street Future Policyholder Benefits Reserve [Member] | Maximum
Liabilities
Non-performance spread				0.46%
Risk margin for uncertainty				1.00%
Front Street Future Policyholder Benefits Reserve [Member] | Weighted average
Liabilities
Non-performance spread		0.32%
Risk margin for uncertainty		0.50%

[1] (a) During Fiscal 2016, the net transfer to Level 3 was exclusively from Level 2. The transfers into Level 3 were related to changes in the primary pricing source and changes in the observability of external information used in determining the fair value.

[2] (a) During Fiscal 2015, the net transfer to Level 3 was related to a loan receivable previously classified as a related party loan.