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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 $ (112)met_OtherThanTemporaryImpairmentsOnDebtSecuritiesTransferredToOtherComprehensiveIncomeLoss $ (218)met_OtherThanTemporaryImpairmentsOnDebtSecuritiesTransferredToOtherComprehensiveIncomeLoss
Noncredit OTTI losses recognized 10met_NoncreditLossesOnDebtSecuritiesIncludedInAccumulatedOtherComprehensiveIncomeLoss 17met_NoncreditLossesOnDebtSecuritiesIncludedInAccumulatedOtherComprehensiveIncomeLoss
Securities sold with previous noncredit OTTI loss 12us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesBeforeTax 53us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesBeforeTax
Subsequent changes in estimated fair value 6met_ChangeInEstimatedFairValueOfDebtSecuritiesWithNoncreditOttiLossesPreviouslyIncludedInAccumulatedOtherComprehensiveIncomeLoss 36met_ChangeInEstimatedFairValueOfDebtSecuritiesWithNoncreditOttiLossesPreviouslyIncludedInAccumulatedOtherComprehensiveIncomeLoss
Balance, end of period $ (84)met_OtherThanTemporaryImpairmentsOnDebtSecuritiesTransferredToOtherComprehensiveIncomeLoss $ (112)met_OtherThanTemporaryImpairmentsOnDebtSecuritiesTransferredToOtherComprehensiveIncomeLoss