Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Mar. 31, 2012
Assets:
Total Assets	$ 1,750,471	$ 1,655,229
Liabilities:
Liquidating Distributions Payable	67,704	31,720
Cash equivalents [Member] | Money Market Funds [Member]
Assets:
Total Assets	422	38,396
Marketable Securities [Member] | Municipal commercial paper and demand notes [Member]
Assets:
Total Assets	59,316	25,253
Marketable Securities [Member] | Corporate commercial paper [Member]
Assets:
Total Assets	5,398	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	839,007	770,389
Holdings of New GM Securities [Member] | New GM Warrants [Member]
Assets:
Total Assets	846,328	759,873
Level 1 [Member]
Assets:
Total Assets	1,685,757	1,568,658
Liabilities:
Liquidating Distributions Payable	67,704	31,720
Level 1 [Member] | Cash equivalents [Member] | Money Market Funds [Member]
Assets:
Total Assets	422	38,396
Level 1 [Member] | Holdings of New GM Securities [Member] | New GM Common Stock [Member]
Assets:
Total Assets	839,007	770,389
Level 1 [Member] | Holdings of New GM Securities [Member] | New GM Warrants [Member]
Assets:
Total Assets	846,328	759,873
Level 2 [Member]
Assets:
Total Assets	64,714	86,571
Level 2 [Member] | Marketable Securities [Member] | Municipal commercial paper and demand notes [Member]
Assets:
Total Assets	59,316	25,253
Level 2 [Member] | Marketable Securities [Member] | Corporate commercial paper [Member]
Assets:
Total Assets	5,398	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