Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $)	Sep. 30, 2013	Mar. 31, 2013
Assets:
Total Assets	$ 2,253,085,000	$ 1,668,260,000
Liabilities:
Liquidating distributions payable	100,447,000	16,555,000
Cash Equivalents [Member] | Money Market Funds [Member]
Assets:
Total Assets	32,913,000	354,000
Marketable Securities [Member] | Municipal Commercial Paper and Demand Notes [Member]
Assets:
Total Assets	18,349,000	61,006,000
Marketable Securities [Member] | Corporate Commercial Paper [Member]
Assets:
Total Assets	12,086,000	15,790,000
Holdings of New GM Securities [Member] | New GM Common Stock [Member]
Assets:
Total Assets	1,024,290,000	798,848,000
Holdings of New GM Securities [Member] | New GM Warrants [Member]
Assets:
Total Assets	1,165,447,000	792,262,000
Level 1 [Member]
Assets:
Total Assets	2,222,650,000	1,591,464,000
Liabilities:
Liquidating distributions payable	100,447,000	16,555,000
Level 1 [Member] | Cash Equivalents [Member] | Money Market Funds [Member]
Assets:
Total Assets	32,913,000	354,000
Level 1 [Member] | Marketable Securities [Member] | Municipal Commercial Paper and Demand Notes [Member]
Assets:
Total Assets
Level 1 [Member] | Marketable Securities [Member] | Corporate Commercial Paper [Member]
Assets:
Total Assets
Level 1 [Member] | Holdings of New GM Securities [Member] | New GM Common Stock [Member]
Assets:
Total Assets	1,024,290,000	798,848,000
Level 1 [Member] | Holdings of New GM Securities [Member] | New GM Warrants [Member]
Assets:
Total Assets	1,165,447,000	792,262,000
Level 2 [Member]
Assets:
Total Assets	30,435,000	76,796,000
Liabilities:
Liquidating distributions payable
Level 2 [Member] | Cash Equivalents [Member] | Money Market Funds [Member]
Assets:
Total Assets
Level 2 [Member] | Marketable Securities [Member] | Municipal Commercial Paper and Demand Notes [Member]
Assets:
Total Assets	18,349,000	61,006,000
Level 2 [Member] | Marketable Securities [Member] | Corporate Commercial Paper [Member]
Assets:
Total Assets	12,086,000	15,790,000
Level 2 [Member] | Holdings of New GM Securities [Member] | New GM Common Stock [Member]
Assets:
Total Assets
Level 2 [Member] | Holdings of New GM Securities [Member] | New GM Warrants [Member]
Assets:
Total Assets
Level 3 [Member]
Assets:
Total Assets	0	0
Liabilities:
Liquidating distributions payable	0	0
Level 3 [Member] | Cash Equivalents [Member] | Money Market Funds [Member]
Assets:
Total Assets
Level 3 [Member] | Marketable Securities [Member] | Municipal Commercial Paper and Demand Notes [Member]
Assets:
Total Assets
Level 3 [Member] | Marketable Securities [Member] | Corporate Commercial Paper [Member]
Assets:
Total Assets
Level 3 [Member] | Holdings of New GM Securities [Member] | New GM Common Stock [Member]
Assets:
Total Assets
Level 3 [Member] | Holdings of New GM Securities [Member] | New GM Warrants [Member]
Assets:
Total Assets