Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (Liquidation Basis of Accounting [Member], USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Mar. 31, 2014
ASSETS
Total Assets	$ 886,001	$ 1,167,413
LIABILITIES
Liquidating distributions payable	6,033	42,111
Cash Equivalents [Member] | Money Market Funds [Member]
ASSETS
Total Assets	10,079	8,953
Marketable Securities [Member] | Municipal Commercial Paper and Demand Notes [Member]
ASSETS
Total Assets	10,921	18,005
Marketable Securities [Member] | Corporate Commercial Paper [Member]
ASSETS
Total Assets	21,167	26,377
Holdings of New GM Securities [Member] | New GM Common Stock [Member]
ASSETS
Total Assets	402,635	526,533
Holdings of New GM Securities [Member] | New GM Warrants [Member]
ASSETS
Total Assets	441,199	587,545
Level 1 [Member]
ASSETS
Total Assets	853,913	1,123,031
LIABILITIES
Liquidating distributions payable	6,033	42,111
Level 1 [Member] | Cash Equivalents [Member] | Money Market Funds [Member]
ASSETS
Total Assets	10,079	8,953
Level 1 [Member] | Holdings of New GM Securities [Member] | New GM Common Stock [Member]
ASSETS
Total Assets	402,635	526,533
Level 1 [Member] | Holdings of New GM Securities [Member] | New GM Warrants [Member]
ASSETS
Total Assets	441,199	587,545
Level 2 [Member]
ASSETS
Total Assets	32,088	44,382
Level 2 [Member] | Marketable Securities [Member] | Municipal Commercial Paper and Demand Notes [Member]
ASSETS
Total Assets	10,921	18,005
Level 2 [Member] | Marketable Securities [Member] | Corporate Commercial Paper [Member]
ASSETS
Total Assets	$ 21,167	$ 26,377