Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Mar. 31, 2013
Assets:
Total Assets	$ 1,442,475	$ 1,668,260
Liabilities:
Liquidating distributions payable	8,466	16,555
Cash Equivalents [Member] | Money Market Funds [Member]
Assets:
Total Assets	8,435	354
Marketable Securities [Member] | Municipal Commercial Paper and Demand Notes [Member]
Assets:
Total Assets	18,110	61,006
Marketable Securities [Member] | Corporate Commercial Paper [Member]
Assets:
Total Assets	31,625	15,790
Holdings of New GM Securities [Member] | New GM Common Stock [Member]
Assets:
Total Assets	627,258	798,848
Holdings of New GM Securities [Member] | New GM Warrants [Member]
Assets:
Total Assets	757,047	792,262
Level 1 [Member]
Assets:
Total Assets	1,392,740	1,591,464
Liabilities:
Liquidating distributions payable	8,466	16,555
Level 1 [Member] | Cash Equivalents [Member] | Money Market Funds [Member]
Assets:
Total Assets	8,435	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	627,258	798,848
Level 1 [Member] | Holdings of New GM Securities [Member] | New GM Warrants [Member]
Assets:
Total Assets	757,047	792,262
Level 2 [Member]
Assets:
Total Assets	49,735	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	18,110	61,006
Level 2 [Member] | Marketable Securities [Member] | Corporate Commercial Paper [Member]
Assets:
Total Assets	31,625	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	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