Changes in Assets and Liabilities Measured at Fair Value on a Recurring Basis using Significant Unobservable Inputs (Level 3) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended	9 Months Ended
Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
States, Municipalities, and Political Subdivisions
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period	$ (152.0)	$ 0	$ (128.7)	$ (68.1)
Investment Gain (Loss) included in Earnings	0	0	0	0
Investment Gain (Loss) included in OCI	(0.1)	2.0	(8.4)	(0.9)
Purchases	0	0	0	0
Sales	0	0	(0.5)	(0.3)
Level 3 Transfers Into	0	108.1	32.1	43.2
Level 3 Transfers Out of	0	0	0	0
End of Period	(151.9)	(110.1)	(151.9)	(110.1)
Foreign Governments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period	(80.0)	0	(82.1)	0
Investment Gain (Loss) included in Earnings	0	0	0	0
Investment Gain (Loss) included in OCI	(1.4)	1.3	(3.5)	5.2
Purchases	0	0	0	0
Sales	0	0	0	0
Level 3 Transfers Into	0	44.8	0	40.9
Level 3 Transfers Out of	0	0	0	0
End of Period	(78.6)	(46.1)	(78.6)	(46.1)
Public Utilities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period	(354.2)	(162.8)	(574.4)	(338.9)
Investment Gain (Loss) included in Earnings	0	0	0	0
Investment Gain (Loss) included in OCI	(5.2)	8.4	(19.4)	5.4
Purchases	0	0	0	3.6
Sales	0	(1.7)	(3.6)	(3.7)
Level 3 Transfers Into	144.1	340.5	244.7	361.7
Level 3 Transfers Out of	(201.3)	(10.9)	(504.3)	(206.8)
End of Period	(291.8)	(499.1)	(291.8)	(499.1)
Mortgage/Asset-backed Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period	(14.9)	(0.6)	(0.5)	(31.7)
Investment Gain (Loss) included in Earnings	(1.3)	0	0	0
Investment Gain (Loss) included in OCI	0.9	0.4	0.1	0.9
Purchases	0	0	0	7.0
Sales	(14.0)	0	(0.1)	(0.1)
Level 3 Transfers Into	0	7.4	0	0
Level 3 Transfers Out of	0	0	0	(31.1)
End of Period	(0.5)	(8.4)	(0.5)	(8.4)
All Other Corporate Bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period	(1,042.5)	(639.6)	(1,177.8)	(665.5)
Investment Gain (Loss) included in Earnings	(1.0)	0	1.1	0
Investment Gain (Loss) included in OCI	(2.5)	19.8	(70.5)	34.0
Purchases	7.0	0	30.1	30.2
Sales	(23.0)	(5.3)	(86.9)	(49.0)
Level 3 Transfers Into	540.7	386.7	807.4	448.0
Level 3 Transfers Out of	(351.2)	(146.2)	(646.5)	(234.1)
End of Period	(1,212.5)	(894.6)	(1,212.5)	(894.6)
Redeemable Preferred Stocks
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period	(24.2)	(22.2)	(24.8)	(37.2)
Investment Gain (Loss) included in Earnings	0	0	0	(1.0)
Investment Gain (Loss) included in OCI	(0.1)	0	(0.7)	0.8
Purchases	0	0	0	0
Sales	0	0	0	(14.3)
Level 3 Transfers Into	0	0	0	0
Level 3 Transfers Out of	0	(22.2)	0	(22.7)
End of Period	(24.1)	0	(24.1)	0
Total Fixed Maturity Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period	(1,667.8)	(825.2)	(1,988.3)	(1,141.4)
Investment Gain (Loss) included in Earnings	(2.3)	0	1.1	(1.0)
Investment Gain (Loss) included in OCI	(8.4)	31.9	(102.4)	45.4
Purchases	7.0	0	30.1	40.8
Sales	(37.0)	(7.0)	(91.1)	(67.4)
Level 3 Transfers Into	684.8	887.5	1,084.2	893.8
Level 3 Transfers Out of	(552.5)	(179.3)	(1,150.8)	(494.7)
End of Period	(1,759.4)	(1,558.3)	(1,759.4)	(1,558.3)
Equity Securites
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period	(4.4)	(4.4)	(4.3)	(11.2)
Investment Gain (Loss) included in Earnings	0	0	0	0
Investment Gain (Loss) included in OCI	0	0	0.1	0
Purchases	0	0	0	0
Sales	0	0	0	(0.1)
Level 3 Transfers Into	0	0	0	0
Level 3 Transfers Out of	0	0	0	(6.7)
End of Period	(4.4)	(4.4)	(4.4)	(4.4)
Embedded Derivative in Modified Coinsurance Arrangement
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period	65.1	126.7	(83.9)	(135.7)
Investment Gain (Loss) included in Earnings	(0.3)	19.7	18.5	28.7
Investment Gain (Loss) included in OCI	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	0	0
Level 3 Transfers Into	0	0	0	0
Level 3 Transfers Out of	0	0	0	0
End of Period	$ (65.4)	$ (107.0)	$ (65.4)	$ (107.0)

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

Amount of gain (loss) recognized in earnings, arising from assets measured at fair value on a recurring basis using unobservable inputs (level 3).

+ References

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of gain (loss) recognized in other comprehensive income, arising from assets measured at fair value on a recurring basis using unobservable inputs (level 3).

+ References

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Purchases that have taken place during the period in relation to assets measured at fair value and categorized within Level 3 of the fair value hierarchy.

+ References

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Sales that have taken place during the period in relation to assets measured at fair value and categorized within Level 3 of the fair value hierarchy.

+ References

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Transfers into assets measured at fair value and categorized within Level 3 of the fair value hierarchy that have taken place during the period.

+ References

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Transfers out of assets measured at fair value and categorized within Level 3 of the fair value hierarchy that have taken place during the period.

+ References

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

This element represents an asset measured at fair value using significant unobservable inputs (Level 3) which is required for reconciliation purposes of beginning and ending balances.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=7578670&loc=d3e19207-110258

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 157

-Paragraph 32

-Subparagraph c

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

+ Details

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