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Income Taxes - Comprehensive Income Tax Benefit and Expense (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended 12 Months Ended
Dec. 31, 2014
Sep. 30, 2014
Jun. 30, 2014
Mar. 31, 2014
Dec. 31, 2013
Sep. 30, 2013
Jun. 30, 2013
Mar. 31, 2013
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Income Tax Benefit (Expense) [Abstract]                      
Income Tax Expense $ (29.8)us-gaap_IncomeTaxExpenseBenefit $ 0.8us-gaap_IncomeTaxExpenseBenefit $ (1.9)us-gaap_IncomeTaxExpenseBenefit $ (16.7)us-gaap_IncomeTaxExpenseBenefit $ (26.6)us-gaap_IncomeTaxExpenseBenefit $ (33.4)us-gaap_IncomeTaxExpenseBenefit $ (13.9)us-gaap_IncomeTaxExpenseBenefit $ (26.0)us-gaap_IncomeTaxExpenseBenefit $ (47.6)us-gaap_IncomeTaxExpenseBenefit $ (99.9)us-gaap_IncomeTaxExpenseBenefit $ (30.6)us-gaap_IncomeTaxExpenseBenefit
Income Tax Expense - Discontinued Operations                 (1.1)us-gaap_DiscontinuedOperationTaxEffectOfDiscontinuedOperation (1.7)us-gaap_DiscontinuedOperationTaxEffectOfDiscontinuedOperation (7.3)us-gaap_DiscontinuedOperationTaxEffectOfDiscontinuedOperation
Unrealized Appreciation on Securities                 (82.8)us-gaap_OtherComprehensiveIncomeLossAvailableForSaleSecuritiesTax 129.1us-gaap_OtherComprehensiveIncomeLossAvailableForSaleSecuritiesTax (34.4)us-gaap_OtherComprehensiveIncomeLossAvailableForSaleSecuritiesTax
Foreign Currency Translation Adjustments on Investments                 0.4us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTranslationAdjustmentTax (0.1)us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTranslationAdjustmentTax (0.6)us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTranslationAdjustmentTax
Tax Effects from Postretirement Benefit Plans                 34.7us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansTax (30.4)us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansTax 4.6us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansTax
Tax Effects from Long-Term Equity-based Compensation included in Pain-in Capital                 (1.0)us-gaap_IncomeTaxEffectsAllocatedDirectlyToEquityEquityTransactions 0.2us-gaap_IncomeTaxEffectsAllocatedDirectlyToEquityEquityTransactions (0.1)us-gaap_IncomeTaxEffectsAllocatedDirectlyToEquityEquityTransactions
Comprehensive Income Tax Expense                 $ 97.4kmpr_ComprehensiveIncomeLossTax $ 2.8kmpr_ComprehensiveIncomeLossTax $ 68.4kmpr_ComprehensiveIncomeLossTax