Description of Trust and Reporting Policies - Additional Information (Detail) - New GM Common Stock [Member] - Subsequent Event [Member] - USD ($) $ / shares in Units, $ in Millions		1 Months Ended
	Jul. 07, 2015	Aug. 05, 2015
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Net proceeds from the liquidation of New GM Securities		$ 741.7
Net weighted average sales price, net of expenses, for New GM Common Stock sold after June 30, 2015		$ 31.23
New GM Series A Warrants [Member]
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
Conversion rate for New GM Series A Warrants into New GM Common Stock	71.549%
New GM Series B Warrants [Member]
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
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
Conversion rate for New GM Series B Warrants into New GM Common Stock	47.849%