Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013
Assets:
Total Assets	$ 1,167,413	$ 1,668,260
Liabilities:
Liquidating distributions payable	42,111	16,555
Cash Equivalents [Member] | Money Market Funds [Member]
Assets:
Total Assets	8,953	354
Marketable Securities [Member] | Municipal Commercial Paper and Demand Notes [Member]
Assets:
Total Assets	18,005	61,006
Marketable Securities [Member] | Corporate Commercial Paper [Member]
Assets:
Total Assets	26,377	15,790
Holdings of New GM Securities [Member] | New GM Common Stock [Member]
Assets:
Total Assets	526,533	798,848
Holdings of New GM Securities [Member] | New GM Warrants [Member]
Assets:
Total Assets	587,545	792,262
Level 1 [Member]
Assets:
Total Assets	1,123,031	1,591,464
Liabilities:
Liquidating distributions payable	42,111	16,555
Level 1 [Member] | Cash Equivalents [Member] | Money Market Funds [Member]
Assets:
Total Assets	8,953	354
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	526,533	798,848
Level 1 [Member] | Holdings of New GM Securities [Member] | New GM Warrants [Member]
Assets:
Total Assets	587,545	792,262
Level 2 [Member]
Assets:
Total Assets	44,382	76,796
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	18,005	61,006
Level 2 [Member] | Marketable Securities [Member] | Corporate Commercial Paper [Member]
Assets:
Total Assets	26,377	15,790
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
Liabilities:
Liquidating distributions payable
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