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	3 Months Ended		12 Months Ended
	Dec. 31, 2014		Sep. 30, 2014		Jun. 16, 2014
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Document Period End Date	Dec. 31, 2014
Assets, Fair Value Disclosure	$ 1,160.8		1,066.0
Other invested assets	247.0		165.0
Liabilities
Liabilities, Fair Value Disclosure	2,636.6		2,059.4
Municipal [Member] | Broker Quoted [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Valuation Technique	Broker-quoted
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] | Broker Quoted [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Assets, Fair Value Disclosure	905.5		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	2.1		2.0
Municipal Bonds [Member] | Broker Quoted [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Valuation Technique	Broker-quoted
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
Fair Value, Measurements, Recurring [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Assets, Fair Value Disclosure, Recurring	19,307.1	[1]	18,501.7	[1]
Contingent Purchase Price Reduction Receivable Fair Value Disclosure	41.5	[1]	41.5	[1]
Fair Value, Measurements, Recurring [Member] | Asset-backed Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Available-for-sale Securities	1,839.1	[1]	1,792.9	[1]
Fair Value, Measurements, Recurring [Member] | Commercial Mortgage Backed Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Available-for-sale Securities	668.4	[1]	636.9	[1]
Fair Value, Measurements, Recurring [Member] | Corporate Debt Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Available-for-sale Securities	9,721.4	[1]	9,795.8	[1]
Fair Value, Measurements, Recurring [Member] | Hybrids [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Available-for-sale Securities	1,301.5	[1]	1,316.1	[1]
Fair Value, Measurements, Recurring [Member] | Municipals [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Available-for-sale Securities	1,361.9	[1]	1,259.8	[1]
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	625.9	[1]	663.6	[1]
Nomura Fund Linked Note [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Embedded Derivative, Fair Value of Embedded Derivative Liability					11.3
Other invested assets	11.8
Fixed Index Annuity [Member] | Discounted cash flow [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Valuation Technique	Discounted cash flow
Fixed Index Annuity [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Derivative Liability	2,140.2	[1]	1,908.1	[1]
Minimum [Member] | Asset-backed Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Offered quotes	100.00%		100.00%
Minimum [Member] | Commercial Mortgage Backed Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Offered quotes	100.00%		105.00%
Minimum [Member] | Corporate Debt Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Quoted prices	65.00%		62.00%
Offered quotes			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	88.00%		88.00%
Minimum [Member] | Fixed Index Annuity [Member]
Liabilities
Market value of option	0.00%		0.00%
SWAP rates			2.00%
Surrender rates	0.50%		0.50%
Maximum [Member] | Asset-backed Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Offered quotes	109.00%		109.00%
Maximum [Member] | Commercial Mortgage Backed Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Offered quotes	122.00%		121.00%
Maximum [Member] | Corporate Debt Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Quoted prices			120.00%
Offered quotes	122.00%		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	96.00%		96.00%
Expected term	1 month 8 days		4 months 5 days
Discount rate	1.00%		1.00%
Credit insurance risk premium	12.00%		12.00%
Maximum [Member] | Fixed Index Annuity [Member]
Liabilities
Market value of option	43.00%		50.00%
SWAP rates			2.00%
Surrender rates	75.00%		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] | Commercial Mortgage Backed Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Offered quotes	115.00%		118.00%
Weighted Average [Member] | Corporate Debt Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Quoted prices	102.00%		100.00%
Offered quotes	148.00%		142.00%
Weighted Average [Member] | Municipal Debt Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Offered quotes	112.00%
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	92.00%		92.00%
Weighted Average [Member] | Fixed Index Annuity [Member]
Liabilities
Market value of option	3.00%		3.00%
SWAP rates	2.00%		3.00%
Mortality multiplier	80.00%		80.00%
Surrender rates	7.00%		7.00%
Fair Value Measurements Non Performance Spread	0.25%		0.25%
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	0.50%		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	550.2	[1]	279.3	[1]
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	57.1	[1]	59.2	[1]
Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Assets, Fair Value Disclosure, Recurring	17,596.1	[1]	17,156.4	[1]
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Asset-backed Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Available-for-sale Securities	1,804.5	[1]	1,755.9	[1]
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Commercial Mortgage Backed Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Available-for-sale Securities	548.0	[1]	553.8	[1]
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Corporate Debt Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Available-for-sale Securities	8,813.8	[1]	8,945.8	[1]
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Hybrids [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Available-for-sale Securities	1,301.5	[1]	1,316.1	[1]
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Municipals [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Available-for-sale Securities	1,323.1	[1]	1,222.6	[1]
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	562.7	[1]	598.4	[1]
Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Assets, Fair Value Disclosure, Recurring	1,160.8	[1]	1,066.0	[1]
Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Asset-backed Securities [Member] | Broker Quoted [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Available-for-sale Securities	34.6	[1]	37.0	[1]
Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Commercial Mortgage Backed Securities [Member] | Broker Quoted [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Available-for-sale Securities	120.4	[1]	83.1	[1]
Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Corporate Debt Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Available-for-sale Securities	907.6	[1]	850.0	[1]
Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Hybrids [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Available-for-sale Securities	0	[1]	0	[1]
Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Municipal Bonds [Member] | Market Pricing [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Available-for-sale Securities	38.8	[1]
Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Municipals [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Available-for-sale Securities			37.2	[1]
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	41.5	[1]	41.5	[1]
Level 3 [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			6.0	[1]
Level 3 [Member] | Available For Sale Embedded Derivatives [Member] | Fair Value, Measurements, Recurring [Member] | Equity Securities [Member] | Market Pricing [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Equity securities - available-for-sale	6.1	[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	2,140.2	[1]	1,908.1	[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	496.4	[1]	151.3	[1]
Other Invested Assets [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Other invested assets	13.8		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
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	2.0	[1]	2.1	[1]
Other Invested Assets [Member] | Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Other Invested Assets [Member] | Option Pricing [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Other invested assets			11.2
Other Invested Assets [Member] | Level 3 [Member] | Nomura Fund Linked Note [Member] | Fair Value, Measurements, Recurring [Member] | Other Invested Assets [Member] | Option Pricing [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Other invested assets	$ 11.8

[1] December 31, 2014 September 30, 2014 Level 1 Level 2 Level 3 Fair Value Level 1 Level 2 Level 3 Fair ValueAssets Contingent purchase price reduction receivable$— $— $41.5 $41.5 $— $— $41.5 $41.5Derivatives: Interest rate contracts— 0.6 — 0.6 — 0.6 — 0.6Commodity swap and option agreements— 19.9 — 19.9 — 3.2 — 3.2Foreign exchange forward agreements— 17.3 — 17.3 — 12.8 — 12.8Call options and futures contracts— 306.2 — 306.2 — 296.3 — 296.3Fixed maturity securities, available-for-sale: Asset-backed securities— 1,804.5 34.6 1,839.1 — 1,755.9 37.0 1,792.9Commercial mortgage-backed securities— 548.0 120.4 668.4 — 553.8 83.1 636.9Corporates— 8,813.8 907.6 9,721.4 — 8,945.8 850.0 9,795.8Hybrids— 1,301.5 — 1,301.5 — 1,316.1 — 1,316.1Municipals— 1,323.1 38.8 1,361.9 — 1,222.6 37.2 1,259.8Residential mortgage-backed securities— 2,201.9 — 2,201.9 — 2,114.0 — 2,114.0U.S. Government401.6 182.0 — 583.6 115.6 180.4 — 296.0Equity securities: Available-for-sale57.1 562.7 6.1 625.9 59.2 598.4 6.0 663.6Trading91.5 — — 91.5 104.5 — — 104.5Other invested assets— 2.0 11.8 13.8 — 2.1 11.2 13.3Funds withheld receivable (a)— 512.6 — 512.6 — 154.4 — 154.4Total financial assets$550.2 $17,596.1 $1,160.8 $19,307.1 $279.3 $17,156.4 $1,066.0 $18,501.7 Liabilities Derivatives: FIA embedded derivatives, included in contractholder funds$— $— $2,140.2 $2,140.2 $— $— $1,908.1 $1,908.1Front Street future policyholder benefit liability— — 496.4 496.4 — — 151.3 151.3Foreign exchange forward agreements and contracts— 2.6 — 2.6 — 0.1 — 0.1Futures contracts— 1.9 — 1.9 — 0.4 — 0.4Commodity contracts— 0.9 — 0.9 — 0.5 — 0.5Interest rate contracts— 1.9 — 1.9 — 1.8 — 1.8Total financial liabilities$— $7.3 $2,636.6 $2,643.9 $— $2.8 $2,059.4 $2,062.2