Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - Liquidation Basis of Accounting [Member] - USD ($) $ in Thousands	Sep. 30, 2015	Mar. 31, 2015
LIABILITIES
Liquidating distributions payable	$ 131,885	$ 7,714
Fair Value, Measurements, Recurring [Member]
ASSETS
Total Assets	799,587	968,071
LIABILITIES
Liquidating distributions payable	131,885	7,714
Fair Value, Measurements, Recurring [Member] | Cash Equivalents [Member] | Money Market Funds [Member]
ASSETS
Total Assets	37,580	19,150
Fair Value, Measurements, Recurring [Member] | Marketable Securities [Member] | U.S. Treasury Bills [Member]
ASSETS
Total Assets	739,722
Fair Value, Measurements, Recurring [Member] | Marketable Securities [Member] | Corporate Commercial Paper [Member]
ASSETS
Total Assets	15,225	18,880
Fair Value, Measurements, Recurring [Member] | Marketable Securities [Member] | U.S. Government Agency Bonds [Member]
ASSETS
Total Assets	7,060
Fair Value, Measurements, Recurring [Member] | Marketable Securities [Member] | Municipal Commercial Paper and Demand Notes [Member]
ASSETS
Total Assets		12,064
Fair Value, Measurements, Recurring [Member] | Holdings of New GM Securities [Member] | New GM Common Stock [Member]
ASSETS
Total Assets		427,151
Fair Value, Measurements, Recurring [Member] | Holdings of New GM Securities [Member] | New GM Warrants [Member]
ASSETS
Total Assets		490,826
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
ASSETS
Total Assets	37,580	937,127
LIABILITIES
Liquidating distributions payable	131,885	7,714
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Cash Equivalents [Member] | Money Market Funds [Member]
ASSETS
Total Assets	37,580	19,150
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Holdings of New GM Securities [Member] | New GM Common Stock [Member]
ASSETS
Total Assets		427,151
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Holdings of New GM Securities [Member] | New GM Warrants [Member]
ASSETS
Total Assets		490,826
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
ASSETS
Total Assets	762,007	30,944
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Marketable Securities [Member] | U.S. Treasury Bills [Member]
ASSETS
Total Assets	739,722
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Marketable Securities [Member] | Corporate Commercial Paper [Member]
ASSETS
Total Assets	15,225	18,880
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Marketable Securities [Member] | U.S. Government Agency Bonds [Member]
ASSETS
Total Assets	$ 7,060
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Marketable Securities [Member] | Municipal Commercial Paper and Demand Notes [Member]
ASSETS
Total Assets		$ 12,064