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Investments - Available-For-Sale Securities (Parenthetical) (Detail) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Schedule Of Available For Sale Securities [Line Items]    
Asset-backed and mortgage-backed securities impairment   1.6us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionInOtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParentAvailableforsaleSecurities
Asset-backed securities and collateralized mortgage obligations [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Commercial paper secured by mortgaged-backed securities   5.6us-gaap_MoneyMarketFundsAtCarryingValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mdr_AssetBackedSecuritiesAndCollateralizedMortgageObligationsMember
Maximum [Member] | Commercial paper [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Available-for-sale securities, maturities 1 year 1 year
Maximum [Member] | Asset-backed securities and collateralized mortgage obligations [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Available-for-sale securities, maturities   26 years