Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Mar. 31, 2012
Assets:
Total Assets	$ 1,668,260	$ 1,655,229
Liabilities:
Liquidating distributions payable	16,555	31,720
Cash equivalents [Member] | Money Market Funds [Member]
Assets:
Total Assets	354	38,396
Marketable Securities [Member] | Municipal commercial paper and demand notes [Member]
Assets:
Total Assets	61,006	25,253
Marketable Securities [Member] | Corporate commercial paper [Member]
Assets:
Total Assets	15,790	59,317
Marketable Securities [Member] | U.S. government agency bonds [Member]
Assets:
Total Assets		2,001
Holdings of New GM Securities [Member] | New GM Common Stock [Member]
Assets:
Total Assets	798,848	770,389
Holdings of New GM Securities [Member] | New GM Warrants [Member]
Assets:
Total Assets	792,262	759,873
Level 1 [Member]
Assets:
Total Assets	1,591,464	1,568,658
Liabilities:
Liquidating distributions payable	16,555	31,720
Level 1 [Member] | Cash equivalents [Member] | Money Market Funds [Member]
Assets:
Total Assets	354	38,396
Level 1 [Member] | Holdings of New GM Securities [Member] | New GM Common Stock [Member]
Assets:
Total Assets	798,848	770,389
Level 1 [Member] | Holdings of New GM Securities [Member] | New GM Warrants [Member]
Assets:
Total Assets	792,262	759,873
Level 2 [Member]
Assets:
Total Assets	76,796	86,571
Level 2 [Member] | Marketable Securities [Member] | Municipal commercial paper and demand notes [Member]
Assets:
Total Assets	61,006	25,253
Level 2 [Member] | Marketable Securities [Member] | Corporate commercial paper [Member]
Assets:
Total Assets	15,790	59,317
Level 2 [Member] | Marketable Securities [Member] | U.S. government agency bonds [Member]
Assets:
Total Assets		2,001
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] | Marketable Securities [Member] | U.S. government agency bonds [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