Holdings of New GM Securities - Holdings of Securities for Trust Beneficiaries (Detail) (Liquidation Basis of Accounting [Member], USD $) In Thousands, except Share data, unless otherwise specified	Dec. 31, 2014	Mar. 31, 2014
Distribution Made to Limited Partner [Line Items]
Fair Value	$ 843,834	$ 1,114,078
New GM Common Stock [Member]
Distribution Made to Limited Partner [Line Items]
Number	11,533,513
Fair Value	402,635
New GM Series A Warrants [Member]
Distribution Made to Limited Partner [Line Items]
Number	10,484,773
Fair Value	262,958
New GM Series B Warrants [Member]
Distribution Made to Limited Partner [Line Items]
Number	10,484,773
Fair Value	$ 178,241