Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Mar. 31, 2014
ASSETS
Total Assets	$ 1,229,801	$ 1,167,413
LIABILITIES
Liquidating distributions payable	48,978	42,111
Cash Equivalents [Member] | Money Market Funds [Member]
ASSETS
Total Assets	10,780	8,953
Marketable Securities [Member] | Municipal Commercial Paper and Demand Notes [Member]
ASSETS
Total Assets	13,631	18,005
Marketable Securities [Member] | Corporate Commercial Paper [Member]
ASSETS
Total Assets	25,675	26,377
Holdings of New GM Securities [Member] | New GM Common Stock [Member]
ASSETS
Total Assets	553,542	526,533
Holdings of New GM Securities [Member] | New GM Warrants [Member]
ASSETS
Total Assets	626,173	587,545
Level 1 [Member]
ASSETS
Total Assets	1,190,495	1,123,031
LIABILITIES
Liquidating distributions payable	48,978	42,111
Level 1 [Member] | Cash Equivalents [Member] | Money Market Funds [Member]
ASSETS
Total Assets	10,780	8,953
Level 1 [Member] | Holdings of New GM Securities [Member] | New GM Common Stock [Member]
ASSETS
Total Assets	553,542	526,533
Level 1 [Member] | Holdings of New GM Securities [Member] | New GM Warrants [Member]
ASSETS
Total Assets	626,173	587,545
Level 2 [Member]
ASSETS
Total Assets	39,306	44,382
Level 2 [Member] | Marketable Securities [Member] | Municipal Commercial Paper and Demand Notes [Member]
ASSETS
Total Assets	13,631	18,005
Level 2 [Member] | Marketable Securities [Member] | Corporate Commercial Paper [Member]
ASSETS
Total Assets	$ 25,675	$ 26,377