Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - Liquidation Basis of Accounting [Member] - USD ($) $ in Thousands	Jun. 30, 2015	Mar. 31, 2015
ASSETS
Total Assets	$ 853,722	$ 968,071
LIABILITIES
Liquidating distributions payable	14,645	7,714
Cash Equivalents [Member] | Money Market Funds [Member]
ASSETS
Total Assets	15,086	19,150
Marketable Securities [Member] | Municipal Commercial Paper and Demand Notes [Member]
ASSETS
Total Assets	10,500	12,064
Marketable Securities [Member] | Corporate Commercial Paper [Member]
ASSETS
Total Assets	37,369	18,880
Holdings of New GM Securities [Member] | New GM Common Stock [Member]
ASSETS
Total Assets	379,563	427,151
Holdings of New GM Securities [Member] | New GM Warrants [Member]
ASSETS
Total Assets	411,204	490,826
Level 1 [Member]
ASSETS
Total Assets	805,853	937,127
LIABILITIES
Liquidating distributions payable	14,645	7,714
Level 1 [Member] | Cash Equivalents [Member] | Money Market Funds [Member]
ASSETS
Total Assets	15,086	19,150
Level 1 [Member] | Holdings of New GM Securities [Member] | New GM Common Stock [Member]
ASSETS
Total Assets	379,563	427,151
Level 1 [Member] | Holdings of New GM Securities [Member] | New GM Warrants [Member]
ASSETS
Total Assets	411,204	490,826
Level 2 [Member]
ASSETS
Total Assets	47,869	30,944
Level 2 [Member] | Marketable Securities [Member] | Municipal Commercial Paper and Demand Notes [Member]
ASSETS
Total Assets	10,500	12,064
Level 2 [Member] | Marketable Securities [Member] | Corporate Commercial Paper [Member]
ASSETS
Total Assets	$ 37,369	$ 18,880