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Reclassification adjustments include: (1) the unrealized holding gain or loss, net of tax, at the date of the transfer for a debt security from the held-to-maturity category transferred into the available-for-sale category. Also includes the unrealized gain or loss at the date of transfer for a debt security from the available-for-sale category transferred into the held-to-maturity category; (2) the unrealized gains or losses realized upon the sale of securities, after tax; and (3) the unrealized gains or losses realized upon the write-down of securities, after tax.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 130 -Paragraph 17, 22 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 115 -Paragraph 13 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Accounting Research Bulletin (ARB) -Number 51 -Paragraph 38 -Subparagraph c(3) Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 130 -Paragraph 24 -Subparagraph b falsefalseChange in unrealized appreciation (depreciation) of investments (net of applicable income tax benefit (expense))21false0cno_NoncreditComponentOfImpairmentLossesOnActivelyManagedFixedMaturitiescnofalsenadurationAppreciation or loss in value of the total of unsold securities (on which a noncredit impairment loss has been previously...falsefalsefalsefalsefalsefalsefalsefalsefalsefalseterselabel1falsefalsefalse00falsefalsefalsetruefalse2truefalsefalse6750000067.5falsefalsefalsetruefalse3falsefalsefalse00falsefalsefalsetruefalse4truefalsefalse6750000067.5falsefalsefalsefalsefalseMonetaryxbrli:monetaryItemTypemonetaryAppreciation or loss in value of the total of unsold securities (on which a noncredit impairment loss has been previously recognized) during the period being reported on, net of taxNo authoritative reference available.falsefalseNoncredit component of impairment losses on fixed maturities, available for sale (net of applicable income tax benefit (expense))22false0us-gaap_ComprehensiveIncomeNetOfTaxus-gaaptruecreditdurationNo definition available.falsefalsefalsefalsefalsefalsefalsefalsefalsefalsetotallabel1falsefalsefalse00falsefalsefalsetruefalse2falsefalsefalse00falsefalsefalsetruefalse3falsefalsefalse00falsefalsefalsetruefalse4truefalsefalse793400000793.4falsefalsefalsefalsefalseMonetaryxbrli:monetaryItemTypemonetaryThe change in equity [net assets] of a business enterprise during a period from transactions and other events and circumstances from non-owner sources which are attributable to the reporting entity. It includes all changes in equity during a period except those resulting from investments by owners and distributions to owners, but excludes any and all transactions which are directly or indirectly attributable to that ownership interest in subsidiary equity which is not attributable to the parent.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Accounting Research Bulletin (ARB) -Number 51 -Paragraph A5 -Appendix A Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Accounting Research Bulletin (ARB) -Number 51 -Paragraph 30 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Accounting Research Bulletin (ARB) -Number 51 -Paragraph 38 -Subparagraph c(3) Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Accounting Research Bulletin (ARB) -Number 51 -Paragraph 8, 9, 10, 11, 12, 13, 14 truefalseTotal comprehensive income (loss)23false0us-gaap_CumulativeEffectOfInitialAdoptionOfNewAccountingPrincipleus-gaaptruecreditdurationNo definition available.falsefalsefalsefalsefalsefalsefalsefalsefalsefalse1falsefalsefalse00falsefalsefalsetruefalse2truefalsefalse-6200000-6.2falsefalsefalsetruefalse3truefalsefalse-9700000-9.7falsefalsefalsetruefalse4truefalsefalse-15900000-15.9falsefalsefalsefalsefalseMonetaryxbrli:monetaryItemTypemonetaryCumulative effect of initial adoption of new accounting principle on beginning retained earnings, net of tax. This element can be used, generally, for the adjustment to retained earnings of a new accounting principle.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 154 -Paragraph 17, 18 falsefalseCumulative effect of accounting change24false0us-gaap_AdjustmentsToAdditionalPaidInCapitalConvertibleDebtWithConversionFeatureus-gaaptruecreditdurationNo definition available.falsefalsefalsefalsefalsefalsefalsefalsefalsefalselabel1truefalsefalse40000004.0falsefalsefalsetruefalse2falsefalsefalse00falsefalsefalsetruefalse3falsefalsefalse00falsefalsefalsetruefalse4truefalsefalse40000004.0falsefalsefalsefalsefalseMonetaryxbrli:monetaryItemTypemonetaryAdjustment to Additional Paid in Capital resulting from the recognition of deferred taxes for the temporary difference of the convertible debt with a beneficial conversion feature. A beneficial conversion feature is a nondetachable conversion feature that is in-the-money.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Emerging Issues Task Force (EITF) -Number 05-8 falsefalseBeneficial conversion feature related to the issuance of convertible debentures25false0cno_StockBasedCompensationcnofalsenadurationAll amounts related to share-based compensation plans. Includes the value of stock issued during the period as a result of...falsefalsefalsefalsefalsefalsefalsefalsefalsefalseterselabel1truefalsefalse1140000011.4falsefalsefalsetruefalse2falsefalsefalse00falsefalsefalsetruefalse3falsefalsefalse00falsefalsefalsetruefalse4truefalsefalse1140000011.4falsefalsefalsefalsefalseMonetaryxbrli:monetaryItemTypemonetaryAll amounts related to share-based compensation plans. Includes the value of stock issued during the period as a result of any share-based compensation plans and the amount of share-based compensation recognized during the period as expense in the income statement.No authoritative reference available.falsefalseStock option and restricted stock plans26false0us-gaap_StockholdersEquityus-gaaptruecreditinstantNo definition available.falsefalsefalsetruefalsefalsefalsefalsetruefalseperiodendlabelinstant2010-12-31T00:00:000001-01-01T00:00:001truefalsefalse44267000004426.7falsetruefalsetruefalse2truefalsefalse238300000238.3falsetruefalsetruefalse3truefalsefalse-339700000-339.7falsetruefalsetruefalse4truefalsefalse43253000004325.3falsetruefalsefalsefalseMonetaryxbrli:monetaryItemTypemonetaryTotal of all Stockholders' Equity (deficit) items, net of receivables from officers, directors owners, and affiliates of the entity which are attributable to the parent. The amount of the economic entity's stockholders' equity attributable to the parent excludes the amount of stockholders' equity which is allocable to that ownership interest in subsidiary equity which is not attributable to the parent (noncontrolling interest, minority interest). This excludes temporary equity and is sometimes called permanent equity.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Accounting Research Bulletin (ARB) -Number 51 -Paragraph A3 -Appendix A Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 4 -Section E Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 29, 30, 31 -Article 5 falsefalseBalance at Dec. 31, 2010426CONSOLIDATED STATEMENT OF SHAREHOLDERS' EQUITY (USD $)HundredThousandsUnKnownUnKnownUnKnownfalsetrue