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	Sep. 30, 2014	Mar. 31, 2014
Distribution Made to Limited Partner [Line Items]
Fair Value	$ 997,640	$ 1,114,078
New GM Common Stock [Member]
Distribution Made to Limited Partner [Line Items]
Number	15,247,286
Fair Value	486,998
New GM Series A Warrants [Member]
Distribution Made to Limited Partner [Line Items]
Number	13,860,926
Fair Value	310,207
New GM Series B Warrants [Member]
Distribution Made to Limited Partner [Line Items]
Number	13,860,926
Fair Value	$ 200,435