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	6 Months Ended
Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period	$ 990.8	$ 818.2	$ 1,025.7	$ 869.4
Total Realized and Unrealized Investment Gains (Losses) included in Earnings	(1.2)	0	(1.4)	(2.8)
Total Realized and Unrealized Investment Gains (Losses) included in Other Comprehensive Income or Loss	6.8	14.3	14.2	30.6
Purchases	44.4	5.0	61.1	5.0
Sales	(17.1)	(20.3)	(25.6)	(46.4)
Level 3 Transfers Into	163.9	154.0	233.7	157.8
Level 3 Transfers Out of	(399.1)	(445.9)	(519.2)	(488.3)
End Of Period	788.5	525.3	788.5	525.3
Public Utilities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period	324.7	218.7	173.6	264.3
Total Realized and Unrealized Investment Gains (Losses) included in Earnings	0.7	0	0.7	(1.0)
Total Realized and Unrealized Investment Gains (Losses) included in Other Comprehensive Income or Loss	3.0	3.0	4.7	6.6
Purchases	0	0	0	0
Sales	(10.2)	0	(10.5)	(6.5)
Level 3 Transfers Into	89.1	4.6	180.7	5.6
Level 3 Transfers Out of	(189.4)	(153.9)	(131.3)	(196.6)
End Of Period	217.9	72.4	217.9	72.4
Mortgage/Asset-backed Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period	32.2	2.0	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	(0.3)	0	(0.3)	0.3
Purchases	19.4	0	19.4	0
Sales	(0.1)	(0.5)	(0.1)	(3.5)
Level 3 Transfers Into	0	0	0	0
Level 3 Transfers Out of	(31.5)	0	0	0
End Of Period	19.7	1.5	19.7	1.5
All Other Corporate Bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period	633.8	557.7	829.7	580.0
Total Realized and Unrealized Investment Gains (Losses) included in Earnings	(1.9)	0	(2.1)	(1.8)
Total Realized and Unrealized Investment Gains (Losses) included in Other Comprehensive Income or Loss	3.3	10.3	8.9	22.8
Purchases	25.0	5.0	41.7	5.0
Sales	(6.8)	(19.8)	(15.0)	(36.4)
Level 3 Transfers Into	53.1	129.2	53.0	152.2
Level 3 Transfers Out of	(178.2)	(252.3)	(387.9)	(291.7)
End Of Period	528.3	430.1	528.3	430.1
Redeemable Preferred Stocks
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period	0.1	0.1	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	0.8	1.0	0.9	0.9
Purchases	0	0	0	0
Sales	0	0	0	0
Level 3 Transfers Into	21.7	20.2	0	0
Level 3 Transfers Out of	0	0	0	0
End Of Period	22.6	21.3	22.6	21.3
Equity Securites
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period	1.5	1.5	1.5	1.5
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	0.5	0.1	0.6
Purchases	2.0	6.9	2.0	6.9
Sales	0	0	0	(0.1)
Level 3 Transfers Into	0	0	0	0
Level 3 Transfers Out of	0	0	0	0
End Of Period	3.6	8.9	3.6	8.9
Embedded Derivative in Modified Coinsurance Arrangement
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period	(82.2)	(99.1)	(96.3)	(117.4)
Total Realized and Unrealized Investment Gains (Losses) included in Earnings	(4.8)	(23.6)	9.3	(5.3)
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	(87.0)	(122.7)	(87.0)	(122.7)
Foreign Governments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period		39.7
Total Realized and Unrealized Investment Gains (Losses) included in Earnings		0
Total Realized and Unrealized Investment Gains (Losses) included in Other Comprehensive Income or Loss		0
Purchases		0
Sales		0
Level 3 Transfers Into		0
Level 3 Transfers Out of		(39.7)
End Of Period		$ 0		$ 0

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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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.

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=6925170&loc=d3e19207-110258

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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 other comprehensive income (a separate component of shareholders' equity).

+ References

+ Details

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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
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=6925170&loc=d3e19207-110258

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- 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

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- 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

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)(3)
-URI http://asc.fasb.org/extlink&oid=6925170&loc=d3e19207-110258

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- 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

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)(3)
-URI http://asc.fasb.org/extlink&oid=6925170&loc=d3e19207-110258

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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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-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=6925170&loc=d3e19207-110258

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