Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Mar. 31, 2012
Assets:
Total Assets	$ 1,329,280	$ 1,655,229
Liabilities:
Liquidating Distributions payable	54,757	31,720
Cash equivalents [Member] | Money Market Funds [Member]
Assets:
Total Assets	2,414	38,396
Marketable Securities [Member] | Municipal commercial paper and demand notes [Member]
Assets:
Total Assets	65,866	25,253
Marketable Securities [Member] | Corporate commercial paper [Member]
Assets:
Total Assets	9,179	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	665,610	770,389
Holdings of New GM Securities [Member] | New GM Warrants [Member]
Assets:
Total Assets	586,211	759,873
Level 1 [Member]
Assets:
Total Assets	1,254,235	1,568,658
Liabilities:
Liquidating Distributions payable	54,757	31,720
Level 1 [Member] | Cash equivalents [Member] | Money Market Funds [Member]
Assets:
Total Assets	2,414	38,396
Level 1 [Member] | Holdings of New GM Securities [Member] | New GM Common Stock [Member]
Assets:
Total Assets	665,610	770,389
Level 1 [Member] | Holdings of New GM Securities [Member] | New GM Warrants [Member]
Assets:
Total Assets	586,211	759,873
Level 2 [Member]
Assets:
Total Assets	75,045	86,571
Level 2 [Member] | Marketable Securities [Member] | Municipal commercial paper and demand notes [Member]
Assets:
Total Assets	65,866	25,253
Level 2 [Member] | Marketable Securities [Member] | Corporate commercial paper [Member]
Assets:
Total Assets	9,179	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