Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (Liquidation Basis of Accounting [Member], USD $)	Mar. 31, 2015	Mar. 31, 2014
ASSETS
Total Assets	$ 968,071,000	$ 1,167,413,000
LIABILITIES
Liquidating distributions payable	7,714,000	42,111,000
Cash Equivalents [Member] | Money Market Funds [Member]
ASSETS
Total Assets	19,150,000	8,953,000
Marketable Securities [Member] | Municipal Commercial Paper and Demand Notes [Member]
ASSETS
Total Assets	12,064,000	18,005,000
Marketable Securities [Member] | Corporate Commercial Paper [Member]
ASSETS
Total Assets	18,880,000	26,377,000
Holdings of New GM Securities [Member] | New GM Common Stock [Member]
ASSETS
Total Assets	427,151,000	526,533,000
Holdings of New GM Securities [Member] | New GM Warrants [Member]
ASSETS
Total Assets	490,826,000	587,545,000
Level 1 [Member]
ASSETS
Total Assets	937,127,000	1,123,031,000
LIABILITIES
Liquidating distributions payable	7,714,000	42,111,000
Level 1 [Member] | Cash Equivalents [Member] | Money Market Funds [Member]
ASSETS
Total Assets	19,150,000	8,953,000
Level 1 [Member] | Holdings of New GM Securities [Member] | New GM Common Stock [Member]
ASSETS
Total Assets	427,151,000	526,533,000
Level 1 [Member] | Holdings of New GM Securities [Member] | New GM Warrants [Member]
ASSETS
Total Assets	490,826,000	587,545,000
Level 2 [Member]
ASSETS
Total Assets	30,944,000	44,382,000
Level 2 [Member] | Marketable Securities [Member] | Municipal Commercial Paper and Demand Notes [Member]
ASSETS
Total Assets	12,064,000	18,005,000
Level 2 [Member] | Marketable Securities [Member] | Corporate Commercial Paper [Member]
ASSETS
Total Assets	$ 18,880,000	$ 26,377,000