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Fair Values of Financial Instruments - Additional Information (Detail) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2011 | Dec. 31, 2010 | Dec. 31, 2009
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Carrying amount of policy loans | $ 3,051.4 | $ 2,996.1 |
Embedded Derivative in Modified Coinsurance Arrangement | (39.4) | 21.1 | 243.1
Fair Value, Inputs, Level 1 [Member] | Pricing Service |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Percent of fixed maturity securities valued | 10.90% |  |
Other Than Fair Value Inputs Level 1 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Percent of fixed maturity securities valued | 89.10% |  |
Fair Value, Inputs, Level 2 [Member] | Pricing Service |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Percent of fixed maturity securities valued | 71.10% |  |
Fair Value, Inputs, Level 2 [Member] | Other Observable Market Data |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Percent of fixed maturity securities valued | 4.10% |  |
Fair Value Inputs Level 2 Or Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Percent of fixed maturity securities valued | 13.90% |  |
Policy Loans Ceded |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Carrying amount of policy loans | $ 2,838.3 | $ 2,790.5 |

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- Definition Net Increase or Decrease in the fair value of the embedded derivative or group of embedded derivatives included in earnings in the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 133 -Paragraph 44A, 44B -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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 Statement of Financial Accounting Standard (FAS) -Number 133 -Paragraph 12-16, 44A, 44B -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 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -URI http://asc.fasb.org/extlink&oid=6935481&loc=SL5624171-113959 + Details
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- Definition Gross amount of loans made to policy and contract holders using their life insurance contracts' cash value or separate accounts as collateral. + 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(e)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Audit and Accounting Guide (AAG) -Number AAG-LHI -Chapter 11 -Paragraph 104, 105, 106 -IssueDate 2006-05-01 -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 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 7 -Section 03 -Paragraph 1 -Subparagraph e Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.7(a)(7)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 + Details
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