Changes in Assets and Liabilities Measured at Fair Value on a Recurring Basis using Significant Unobservable Inputs (Level 3) (Detail) (USD $) In Millions	3 Months Ended	9 Months Ended
Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period	$ 788.5	$ 525.3	$ 1,025.7	$ 869.4
Total Realized and Unrealized Investment Gains (Losses) included in Earnings	0.1	0	(2.0)	(2.8)
Total Realized and Unrealized Investment Gains (Losses) included in Other Comprehensive Income or Loss	9.6	18.4	21.4	58.2
Purchases	30.0	20.8	91.1	20.8
Sales	(29.5)	(7.4)	(48.3)	(54.2)
Level 3 Transfers Into	339.8	235.7	395.9	321.1
Level 3 Transfers Out of	(234.6)	(93.3)	(579.9)	(513.0)
End Of Period	903.9	699.5	903.9	699.5
Public Utilities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period	217.9	72.4	173.6	264.3
Total Realized and Unrealized Investment Gains (Losses) included in Earnings	0.1	0	0.1	(1.0)
Total Realized and Unrealized Investment Gains (Losses) included in Other Comprehensive Income or Loss	9.3	6.8	17.1	9.9
Purchases	30.0	0	30.0	0
Sales	(11.9)	(1.3)	(15.7)	(8.1)
Level 3 Transfers Into	220.2	108.0	299.7	112.5
Level 3 Transfers Out of	(69.8)	(28.0)	(109.0)	(219.7)
End Of Period	395.8	157.9	395.8	157.9
US States and Political Subdivisions Debt Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period		0		0
Total Realized and Unrealized Investment Gains (Losses) included in Earnings		0		0
Total Realized and Unrealized Investment Gains (Losses) included in Other Comprehensive Income or Loss		0.5		0.5
Purchases		17.7		17.7
Sales		0		0
Level 3 Transfers Into		0		0
Level 3 Transfers Out of		0		0
End Of Period		18.2		18.2
Mortgage/Asset-backed Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period	19.7	1.5	0.7	4.7
Total Realized and Unrealized Investment Gains (Losses) included in Earnings	0	0	0	0
Total Realized and Unrealized Investment Gains (Losses) included in Other Comprehensive Income or Loss	(0.1)	0	(0.4)	0.3
Purchases	0	0	19.4	0
Sales	0	(0.8)	(0.1)	(4.3)
Level 3 Transfers Into	0	0	0	0
Level 3 Transfers Out of	0	0	0	0
End Of Period	19.6	0.7	19.6	0.7
All Other Corporate Bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period	528.3	430.1	829.7	580.0
Total Realized and Unrealized Investment Gains (Losses) included in Earnings	0	0	(2.1)	(1.8)
Total Realized and Unrealized Investment Gains (Losses) included in Other Comprehensive Income or Loss	1.3	11.1	4.7	47.5
Purchases	0	3.1	41.7	3.1
Sales	(17.6)	(5.3)	(32.5)	(41.8)
Level 3 Transfers Into	119.6	127.7	96.2	208.6
Level 3 Transfers Out of	(164.8)	(44.1)	(470.9)	(273.0)
End Of Period	466.8	522.6	466.8	522.6
Redeemable Preferred Stocks
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period	22.6	21.3	21.7	20.4
Total Realized and Unrealized Investment Gains (Losses) included in Earnings	0	0	0	0
Total Realized and Unrealized Investment Gains (Losses) included in Other Comprehensive Income or Loss	(0.9)	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	(21.2)	0	(20.3)
End Of Period	21.7	0.1	21.7	0.1
Equity Securites
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period	3.6	8.9	1.5	1.5
Total Realized and Unrealized Investment Gains (Losses) included in Earnings	(0.4)	0	(0.4)	0
Total Realized and Unrealized Investment Gains (Losses) included in Other Comprehensive Income or Loss	(2.4)	1.8	(1.7)	2.4
Purchases	0	0	2.0	6.9
Sales	0	0	0	(0.1)
Level 3 Transfers Into	9.6	0	9.0	0
Level 3 Transfers Out of	0	0	0	0
End Of Period	10.4	10.7	10.4	10.7
Embedded Derivative in Modified Coinsurance Arrangement
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period	(87.0)	(122.7)	(96.3)	(117.4)
Total Realized and Unrealized Investment Gains (Losses) included in Earnings	(33.7)	1.6	(24.4)	(3.7)
Total Realized and Unrealized Investment Gains (Losses) included in Other Comprehensive Income or Loss	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	$ (120.7)	$ (121.1)	$ (120.7)	$ (121.1)

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- Definition

This element represents total gains or losses for the period (realized and unrealized), arising from assets measured at fair value on a recurring basis using unobservable inputs (Level 3), which are included in earnings or resulted in a change in net asset value.

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 157

-Paragraph 32

-Subparagraph c(1)

-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.

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-Publisher FASB

-Name Accounting Standards Codification

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+ References

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- 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

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

-Publisher FASB

-Name Accounting Standards Codification

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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.

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

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-Publisher FASB

-Name Accounting Standards Codification

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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.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

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- 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 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.

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