Fair Value Measurements by Fair Value Heirarchy Input Level (Detail) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Fixed Maturity Securities
Fixed Maturity Securities	$ 45,862.0	$ 43,354.4
Other Long-term Investments
Derivative Assets	40.4	49.8
Equity Securities	1.5	1.4
Other Liabilities
Derivative Liabilities	140.0	137.8
Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	10,057.5	2,926.8
Other Long-term Investments
Derivative Assets	0.0	0.0
Equity Securities	0.0	0.0
Other Liabilities
Derivative Liabilities	0.0	0.0
Fair Value, Inputs, Level 2
Fixed Maturity Securities
Fixed Maturity Securities	33,906.2	38,546.4
Other Long-term Investments
Derivative Assets	40.4	49.8
Equity Securities	0.3	0.0
Other Liabilities
Derivative Liabilities	57.0	50.2
Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	1,898.3	1,881.2
Other Long-term Investments
Derivative Assets	0.0	0.0
Equity Securities	1.2	1.4
Other Liabilities
Derivative Liabilities	83.0	87.6
Interest Rate Swaps
Other Long-term Investments
Derivative Assets	6.3	2.4
Other Liabilities
Derivative Liabilities	6.7	12.3
Interest Rate Swaps \| Fair Value, Inputs, Level 1
Other Long-term Investments
Derivative Assets	0.0	0.0
Other Liabilities
Derivative Liabilities	0.0	0.0
Interest Rate Swaps \| Fair Value, Inputs, Level 2
Other Long-term Investments
Derivative Assets	6.3	2.4
Other Liabilities
Derivative Liabilities	6.7	12.3
Interest Rate Swaps \| Fair Value, Inputs, Level 3
Other Long-term Investments
Derivative Assets	0.0	0.0
Other Liabilities
Derivative Liabilities	0.0	0.0
Foreign Exchange Contracts
Other Long-term Investments
Derivative Assets	34.1	47.4
Other Liabilities
Derivative Liabilities	49.9	37.6
Foreign Exchange Contracts \| Fair Value, Inputs, Level 1
Other Long-term Investments
Derivative Assets	0.0	0.0
Other Liabilities
Derivative Liabilities	0.0	0.0
Foreign Exchange Contracts \| Fair Value, Inputs, Level 2
Other Long-term Investments
Derivative Assets	34.1	47.4
Other Liabilities
Derivative Liabilities	49.9	37.6
Foreign Exchange Contracts \| Fair Value, Inputs, Level 3
Other Long-term Investments
Derivative Assets	0.0	0.0
Other Liabilities
Derivative Liabilities	0.0	0.0
Credit Default Swaps
Other Liabilities
Derivative Liabilities	0.4	0.3
Credit Default Swaps \| Fair Value, Inputs, Level 1
Other Liabilities
Derivative Liabilities	0.0	0.0
Credit Default Swaps \| Fair Value, Inputs, Level 2
Other Liabilities
Derivative Liabilities	0.4	0.3
Credit Default Swaps \| Fair Value, Inputs, Level 3
Other Liabilities
Derivative Liabilities	0.0	0.0
Embedded Derivative in Modified Coinsurance Arrangement
Other Liabilities
Derivative Liabilities	83.0	87.6
Embedded Derivative in Modified Coinsurance Arrangement \| Fair Value, Inputs, Level 1
Other Liabilities
Derivative Liabilities	0.0	0.0
Embedded Derivative in Modified Coinsurance Arrangement \| Fair Value, Inputs, Level 2
Other Liabilities
Derivative Liabilities	0.0	0.0
Embedded Derivative in Modified Coinsurance Arrangement \| Fair Value, Inputs, Level 3
Other Liabilities
Derivative Liabilities	83.0	87.6
United States Government and Government Agencies and Authorities
Fixed Maturity Securities
Fixed Maturity Securities	1,538.8	1,465.2
United States Government and Government Agencies and Authorities \| Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	704.4	97.3
United States Government and Government Agencies and Authorities \| Fair Value, Inputs, Level 2
Fixed Maturity Securities
Fixed Maturity Securities	834.4	1,367.9
United States Government and Government Agencies and Authorities \| Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	0.0	0.0
States, Municipalities, and Political Subdivisions
Fixed Maturity Securities
Fixed Maturity Securities	2,290.8	2,117.1
States, Municipalities, and Political Subdivisions \| Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	79.1	0.0
States, Municipalities, and Political Subdivisions \| Fair Value, Inputs, Level 2
Fixed Maturity Securities
Fixed Maturity Securities	2,150.1	1,994.9
States, Municipalities, and Political Subdivisions \| Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	61.6	122.2
Foreign Governments
Fixed Maturity Securities
Fixed Maturity Securities	1,011.1	1,051.6
Foreign Governments \| Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	0.0	0.0
Foreign Governments \| Fair Value, Inputs, Level 2
Fixed Maturity Securities
Fixed Maturity Securities	1,011.1	998.7
Foreign Governments \| Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	0.0	52.9
Public Utilities
Fixed Maturity Securities
Fixed Maturity Securities	8,406.5	8,020.4
Public Utilities \| Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	745.1	59.1
Public Utilities \| Fair Value, Inputs, Level 2
Fixed Maturity Securities
Fixed Maturity Securities	7,207.9	7,687.2
Public Utilities \| Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	453.5	274.1
Mortgage/Asset-backed Securities
Fixed Maturity Securities
Fixed Maturity Securities	2,475.2	2,481.5
Mortgage/Asset-backed Securities \| Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	0.0	0.0
Mortgage/Asset-backed Securities \| Fair Value, Inputs, Level 2
Fixed Maturity Securities
Fixed Maturity Securities	2,475.2	2,481.5
Mortgage/Asset-backed Securities \| Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	0.0	0.0
All Other Corporate Bonds
Fixed Maturity Securities
Fixed Maturity Securities	30,092.5	28,171.4
All Other Corporate Bonds \| Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	8,528.9	2,770.4
All Other Corporate Bonds \| Fair Value, Inputs, Level 2
Fixed Maturity Securities
Fixed Maturity Securities	20,204.0	23,992.8
All Other Corporate Bonds \| Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	1,359.6	1,408.2
Redeemable Preferred Stocks
Fixed Maturity Securities
Fixed Maturity Securities	47.1	47.2
Redeemable Preferred Stocks \| Fair Value, Inputs, Level 1
Fixed Maturity Securities
Fixed Maturity Securities	0.0	0.0
Redeemable Preferred Stocks \| Fair Value, Inputs, Level 2
Fixed Maturity Securities
Fixed Maturity Securities	23.5	23.4
Redeemable Preferred Stocks \| Fair Value, Inputs, Level 3
Fixed Maturity Securities
Fixed Maturity Securities	$ 23.6	$ 23.8

Amount of debt securities categorized neither as held-to-maturity nor trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=65884222&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=66010992&loc=d3e26610-111562

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27161-111563

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 320
-Section 50
-Paragraph 2
-Subparagraph (b)-(g)
-URI http://asc.fasb.org/extlink&oid=6957658&loc=d3e62557-112803

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesDebtSecurities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 45
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41271-113958

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=73719730&loc=d3e13433-108611

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 15
-URI http://asc.fasb.org/extlink&oid=73719730&loc=d3e13495-108611

Reference 6: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=51825145&loc=SL20226008-175313

+ Details

Name:	us-gaap_DerivativeAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=51825145&loc=SL20226008-175313

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 15
-URI http://asc.fasb.org/extlink&oid=73719730&loc=d3e13495-108611

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=73719730&loc=d3e13433-108611

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 45
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41271-113958

Reference 6: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958

+ Details

Name:	us-gaap_DerivativeLiabilities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

For an unclassified balance sheet, this item represents investments in common and preferred stocks and other forms of securities that provide ownership interests in a corporation.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.1(b))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

+ Details

Name:	us-gaap_MarketableSecuritiesEquitySecurities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- References

No definition available.

+ Details

Name:	us-gaap_MarketableSecuritiesFixedMaturitiesAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- References

No definition available.

+ Details

Name:	us-gaap_OtherInvestmentsAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- References

No definition available.

+ Details

Name:	us-gaap_OtherLiabilitiesUnclassifiedAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel1Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel2Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateSwapMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_ForeignExchangeContractMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_CreditDefaultSwapMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_EmbeddedDerivativeFinancialInstrumentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_USTreasuryAndGovernmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_USStatesAndPoliticalSubdivisionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_ForeignGovernmentDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_PublicUtilityBondsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_AssetBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_CorporateDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_RedeemablePreferredStockMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: