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Transfers of Assets between Level 1 and Level 2 (Detail) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2011 | Dec. 31, 2010
Fixed Maturity Securities |  |
Level 1 from Level 2 | $ 2,006.1 | $ 2,556.1
Level 2 from Level 1 | 3,262.5 | 2,389.4
United States Government and Government Agencies and Authorities |  |
Fixed Maturity Securities |  |
Level 1 from Level 2 | 169.8 | 95.9
Level 2 from Level 1 | 0 | 0
States, Municipalities, and Political Subdivisions |  |
Fixed Maturity Securities |  |
Level 1 from Level 2 | 47.7 | 36.2
Level 2 from Level 1 | 274.9 | 33.7
Foreign Governments |  |
Fixed Maturity Securities |  |
Level 1 from Level 2 | 0 | 0.7
Level 2 from Level 1 | 0.7 | 0
Public Utilities |  |
Fixed Maturity Securities |  |
Level 1 from Level 2 | 362.1 | 483.2
Level 2 from Level 1 | 534.2 | 673.6
All Other Corporate Bonds |  |
Fixed Maturity Securities |  |
Level 1 from Level 2 | 1,426.5 | 1,940.1
Level 2 from Level 1 | 2,452.7 | 1,676.6
Redeemable Preferred Stocks [Member] |  |
Fixed Maturity Securities |  |
Level 1 from Level 2 | 0 | 0
Level 2 from Level 1 | $ 0 | $ 5.5

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- Definition The amount of the transfers of assets or liabilities measured on a recurring basis out of Level 1 of the fair value hierarchy into Level 2. + 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 (bb) -URI http://asc.fasb.org/extlink&oid=6925170&loc=d3e19207-110258 + Details
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- Definition The amount of the transfers of assets or liabilities measured on a recurring basis out of Level 2 of the fair value hierarchy into Level 1. + 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 (bb) -URI http://asc.fasb.org/extlink&oid=6925170&loc=d3e19207-110258 + Details
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