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, unless otherwise specified	6 Months Ended	12 Months Ended
	Mar. 31, 2013	Sep. 30, 2012
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Assets, Fair Value Disclosure	419.0	206.0
Liabilities
Liabilities, Fair Value Disclosure	1,842.3	1,782.8
Asset-backed securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Assets, Fair Value Disclosure	5.3	15.9
Commercial mortgage-backed securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Assets, Fair Value Disclosure	6.2	5.0
Corporates [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Assets, Fair Value Disclosure	2.6	32.0
Hybrids [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Assets, Fair Value Disclosure	10.0	0
Broker-quoted [Member] | Asset-backed securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Valuation Technique	Broker-quoted
Broker-quoted [Member] | Commercial mortgage-backed securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Valuation Technique	Market pricing
Broker-quoted [Member] | Corporates [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Valuation Technique	Broker-quoted
Broker-quoted [Member] | Hybrids [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Valuation Technique	Broker-quoted
Market pricing [Member] | Corporates [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Valuation Technique	Broker-quoted
Assets, Fair Value Disclosure	353.9	103.3
Market pricing [Member] | Hybrids [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Valuation Technique	Broker-quoted
Assets, Fair Value Disclosure		8.8
Contingent purchase price reduction receivable [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Assets, Fair Value Disclosure	41.0	41.0
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
FIA embedded derivatives, included in contractholder funds [Member]
Liabilities
Liabilities, Fair Value Disclosure	1,639.6	1,550.8
FIA embedded derivatives, included in contractholder funds [Member] | Discounted cash flow [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Valuation Technique	Discounted cash flow
Equity conversion feature of preferred stock [Member]
Liabilities
Liabilities, Fair Value Disclosure	202.7	232.0
Equity conversion feature of preferred stock [Member] | Monte Carlo simulation / Option model [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Valuation Technique	Monte Carlo simulation / Option model
Minimum [Member] | Asset-backed securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Offered quotes		100.00%
Minimum [Member] | Corporates [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Quoted prices	0.00%	0.00%
Offered quotes	88.00%	88.00%
Minimum [Member] | Hybrids [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Quoted prices		0.00%
Minimum [Member] | Contingent purchase price reduction receivable [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Probability of collection	88.00%	88.00%
Minimum [Member] | FIA embedded derivatives, included in contractholder funds [Member]
Liabilities
Market value of option	0.00%	0.00%
SWAP rates	0.95%	0.76%
Surrender rates	0.50%	2.00%
Non-performance spread	0.25%	0.25%
Minimum [Member] | Equity conversion feature of preferred stock [Member]
Liabilities
Annualized volatility of equity	43.00%	41.00%
Discount yield	11.00%	12.00%
Calibration adjustment	12.00%	10.00%
Maximum [Member] | Asset-backed securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Offered quotes	109.00%	110.00%
Maximum [Member] | Commercial mortgage-backed securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Offered quotes		101.00%
Maximum [Member] | Corporates [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Quoted prices	141.00%	141.00%
Offered quotes	116.00%	158.00%
Maximum [Member] | Hybrids [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Quoted prices		103.00%
Maximum [Member] | Contingent purchase price reduction receivable [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Probability of collection	96.00%	96.00%
Expected term	9 months	9 months
Discount rate	0.94%	0.72%
Credit insurance risk premium	11.00%	12.00%
Maximum [Member] | FIA embedded derivatives, included in contractholder funds [Member]
Liabilities
Market value of option	35.00%	31.00%
SWAP rates	2.00%	2.00%
Mortality multiplier	80.00%	70.00%
Surrender rates	75.00%	50.00%
Non-performance spread	0.25%	0.25%
Maximum [Member] | Equity conversion feature of preferred stock [Member]
Liabilities
Discount yield		13.00%
Non-cash accretion rate	0.00%	0.00%
Calibration adjustment	14.00%	13.00%
Weighted average [Member] | Asset-backed securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Offered quotes		103.00%
Weighted average [Member] | Commercial mortgage-backed securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Offered quotes	104.00%
Weighted average [Member] | Corporates [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Quoted prices	91.00%	69.00%
Offered quotes	123.00%	98.00%
Weighted average [Member] | Hybrids [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Quoted prices		25.00%
Offered quotes	100.00%
Weighted average [Member] | Contingent purchase price reduction receivable [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Probability of collection	92.00%	92.00%
Weighted average [Member] | FIA embedded derivatives, included in contractholder funds [Member]
Liabilities
Market value of option	4.00%	4.00%
SWAP rates	1.50%	1.00%
Surrender rates	7.00%	7.00%
Non-performance spread	0.25%	0.25%
Weighted average [Member] | Equity conversion feature of preferred stock [Member]
Liabilities
Discount yield		12.00%
Calibration adjustment	13.00%	11.00%