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Holdings of New GM Securities - Securities Pending Distribution or Set Aside (Detail) (USD $)
In Thousands, except Share data, unless otherwise specified
Dec. 31, 2014
Distribution Made to Limited Partner [Line Items]  
Fair Value $ 345,438ck0000040730_SecuritiesAtFairValuePendingDistributionOrSetAside
New GM Common Stock [Member]  
Distribution Made to Limited Partner [Line Items]  
Number 4,721,463ck0000040730_NumberOfSecuritiesPendingDistributionOrSetAside
/ us-gaap_DistributionsMadeToMemberOrLimitedPartnerByDistributionTypeAxis
= ck0000040730_NewGeneralMotorsCompanyCommonStockMember
Fair Value 164,826ck0000040730_SecuritiesAtFairValuePendingDistributionOrSetAside
/ us-gaap_DistributionsMadeToMemberOrLimitedPartnerByDistributionTypeAxis
= ck0000040730_NewGeneralMotorsCompanyCommonStockMember
New GM Series A Warrants [Member]  
Distribution Made to Limited Partner [Line Items]  
Number 4,292,110ck0000040730_NumberOfSecuritiesPendingDistributionOrSetAside
/ us-gaap_DistributionsMadeToMemberOrLimitedPartnerByDistributionTypeAxis
= ck0000040730_NewGeneralMotorsCompanySeriesaWarrantsMember
Fair Value 107,646ck0000040730_SecuritiesAtFairValuePendingDistributionOrSetAside
/ us-gaap_DistributionsMadeToMemberOrLimitedPartnerByDistributionTypeAxis
= ck0000040730_NewGeneralMotorsCompanySeriesaWarrantsMember
New GM Series B Warrants [Member]  
Distribution Made to Limited Partner [Line Items]  
Number 4,292,110ck0000040730_NumberOfSecuritiesPendingDistributionOrSetAside
/ us-gaap_DistributionsMadeToMemberOrLimitedPartnerByDistributionTypeAxis
= ck0000040730_NewGeneralMotorsCompanySeriesBWarrantsMember
Fair Value $ 72,966ck0000040730_SecuritiesAtFairValuePendingDistributionOrSetAside
/ us-gaap_DistributionsMadeToMemberOrLimitedPartnerByDistributionTypeAxis
= ck0000040730_NewGeneralMotorsCompanySeriesBWarrantsMember