Description of Trust and Reporting Policies - Additional Information (Detail) - New GM Common Stock [Member] - USD ($) $ / shares in Units, $ in Millions	Aug. 05, 2015	Jul. 07, 2015
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Number of converted New GM Series A Warrants		10,352,556
Number of shares of New GM Common Stock received in conversion of New GM Series A Warrants		7,407,155
Number of converted New GM Series B Warrants		10,352,556
Number of shares of New GM Common Stock received in conversion of New GM Series B Warrants		4,953,635
Net proceeds from the liquidation of New GM Securities	$ 741.7
Conversion rate for New GM Series A Warrants into New GM Common Stock		71.549%
Conversion rate for New GM Series B Warrants into New GM Common Stock		47.849%
Net weighted average sales price, net of expenses, for New GM Common Stock sold after June 30, 2015	$ 31.23