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Investments (Changes in Fixed Maturity Securities with Noncredit OTTI Losses) (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended 12 Months Ended
Mar. 31, 2015
Dec. 31, 2014
Changes in fixed maturity securities with noncredit OTTI losses in accumulated other comprehensive income (loss)    
Balance, beginning of period $ (34)mlus_OtherThanTemporaryImpairmentsOnDebtSecuritiesTransferredToOtherComprehensiveIncomeLoss $ (45)mlus_OtherThanTemporaryImpairmentsOnDebtSecuritiesTransferredToOtherComprehensiveIncomeLoss
Noncredit OTTI losses and subsequent changes recognized 1mlus_NoncreditLossesOnDebtSecuritiesIncludedInAccumulatedOtherComprehensiveIncomeLoss 6mlus_NoncreditLossesOnDebtSecuritiesIncludedInAccumulatedOtherComprehensiveIncomeLoss
Securities sold with previous noncredit OTTI loss 2us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesBeforeTax 9us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesBeforeTax
Subsequent changes in estimated fair value 0mlus_ChangeInEstimatedFairValueOfDebtSecuritiesWithNoncreditOttiLossesPreviouslyIncludedInAccumulatedOtherComprehensiveIncomeLoss (4)mlus_ChangeInEstimatedFairValueOfDebtSecuritiesWithNoncreditOttiLossesPreviouslyIncludedInAccumulatedOtherComprehensiveIncomeLoss
Balance, end of period $ (31)mlus_OtherThanTemporaryImpairmentsOnDebtSecuritiesTransferredToOtherComprehensiveIncomeLoss $ (34)mlus_OtherThanTemporaryImpairmentsOnDebtSecuritiesTransferredToOtherComprehensiveIncomeLoss