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Marketable Securities (Narratives) (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
CreditRatingAgency
Dec. 31, 2014
Marketable Securities    
Gross unrealized losses (pre-tax) on marketable securities $ 0us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax $ 1us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
Gross unrealized gains (pre-tax) on marketable securities 1us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax 1us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
Percentage of asset-backed securities rated AAA/A-1+, Aaa/P-1, or AAA/F1+ 92.20%mmm_AvailableForSaleAssetBackedSecuritiesPercentageOfSecuritiesRatedAtHighestGradeDisclosure  
Estimated fair value of current plus long term asset backed securities $ 286mmm_EstimatedFairValueOfCurrentPlusLongTermAssetBackedSecurities  
Number of rating agencies for which asset backed securities must be rated 2mmm_NumberOfCreditAgencyRatingsNeededAssetBackedSecurities  
Number of rating agencies for asset backed securities that must be either Moody's or Standard and Poor's 1mmm_NumberOfSpecificCreditRatingAgenciesNeededAssetBackedSecurities