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Investments Investment Income (Detail) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2011 | Sep. 30, 2011 | Jun. 30, 2011 | Mar. 31, 2011 | Dec. 31, 2010 | Sep. 30, 2010 | Jun. 30, 2010 | Mar. 31, 2010 | Dec. 31, 2011 | Dec. 31, 2010 | Dec. 31, 2009
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |  |  |  |  |  |  |  |  |
Investment Income, Interest and Dividend |  |  |  |  |  |  |  |  | $ 2,578.3 | $ 2,545.8 | $ 2,393.8
Less Investment Expenses |  |  |  |  |  |  |  |  | 26.9 | 28.2 | 29.2
Less Investment Income on PFA Assets |  |  |  |  |  |  |  |  | 17.4 | 18.8 | 18.0
Less Amortization of Tax Credit Partnerships |  |  |  |  |  |  |  |  | 14.4 | 3.3 | 0
Net Investment Income | 634.6 | 629.2 | 637.1 | 618.7 | 634.3 | 618.4 | 629.8 | 613.0 | 2,519.6 | 2,495.5 | 2,346.6
Fixed Income Securities [Member] |  |  |  |  |  |  |  |  |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |  |  |  |  |  |  |  |  |
Investment Income, Interest and Dividend |  |  |  |  |  |  |  |  | 2,425.2 | 2,401.9 | 2,268.5
Derivative [Member] |  |  |  |  |  |  |  |  |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |  |  |  |  |  |  |  |  |
Investment Income, Interest and Dividend |  |  |  |  |  |  |  |  | 22.9 | 17.2 | 13.5
Mortgage Receivable [Member] |  |  |  |  |  |  |  |  |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |  |  |  |  |  |  |  |  |
Investment Income, Interest and Dividend |  |  |  |  |  |  |  |  | 100.1 | 91.1 | 81.0
Policy Loans [Member] |  |  |  |  |  |  |  |  |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |  |  |  |  |  |  |  |  |
Investment Income, Interest and Dividend |  |  |  |  |  |  |  |  | 14.1 | 13.9 | 12.4
Other Long-term Investments [Member] |  |  |  |  |  |  |  |  |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |  |  |  |  |  |  |  |  |
Investment Income, Interest and Dividend |  |  |  |  |  |  |  |  | 13.1 | 18.5 | 11.5
Short-term Investments [Member] |  |  |  |  |  |  |  |  |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |  |  |  |  |  |  |  |  |
Investment Income, Interest and Dividend |  |  |  |  |  |  |  |  | $ 2.9 | $ 3.2 | $ 6.9

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- Definition A credit or adjustment for government or taxing authority authorized decrease in taxes owed as a result of meeting certain tax policy conditions. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 95 -Paragraph 28 -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 230 -SubTopic 10 -Section 45 -Paragraph 28 -URI http://asc.fasb.org/extlink&oid=6943989&loc=d3e3602-108585 + Details
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- Definition Income derived from investments in debt and equity securities and on cash and cash equivalents. Interest income represents earnings which reflect the time value of money or transactions in which the payments are for the use or forbearance of money. Dividend income represents a distribution of earnings to shareholders by investee companies. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 115 -Paragraph 14 -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 SEC -Name Regulation S-X (SX) -Number 210 -Article 5 -Section 03 -Paragraph 7 -Subparagraph a, b Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.7(a),(b)) -URI http://asc.fasb.org/extlink&oid=6880815&loc=d3e20235-122688 + Details
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- Definition The income earned from investments in securities and property, equipment and other capital assets. It includes rent from property and equipment, dividends from shares in corporations, and interest from bonds, loans, mortgages, derivatives, commercial paper, bank accounts, certificates of deposits, treasuries, and other financial securities. It does not include realized gains and losses on investments. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 16 -Article 12 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph 2 -Article 7 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.2) -URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913 + Details
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