Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Mar. 31, 2013
Assets:
Total Assets	$ 2,069,148	$ 1,668,260
Liabilities:
Liquidating distributions payable	25,046	16,555
Cash equivalents [Member] | Money market funds [Member]
Assets:
Total Assets	858	354
Marketable Securities [Member] | Municipal commercial paper and demand notes [Member]
Assets:
Total Assets	62,645	61,006
Marketable Securities [Member] | Corporate commercial paper [Member]
Assets:
Total Assets	7,044	15,790
Holdings of New GM Securities [Member] | New GM Common Stock [Member]
Assets:
Total Assets	951,163	798,848
Holdings of New GM Securities [Member] | New GM Warrants [Member]
Assets:
Total Assets	1,047,438	792,262
Level 1 [Member]
Assets:
Total Assets	1,999,459	1,591,464
Liabilities:
Liquidating distributions payable	25,046	16,555
Level 1 [Member] | Cash equivalents [Member] | Money market funds [Member]
Assets:
Total Assets	858	354
Level 1 [Member] | Holdings of New GM Securities [Member] | New GM Common Stock [Member]
Assets:
Total Assets	951,163	798,848
Level 1 [Member] | Holdings of New GM Securities [Member] | New GM Warrants [Member]
Assets:
Total Assets	1,047,438	792,262
Level 2 [Member]
Assets:
Total Assets	69,689	76,796
Level 2 [Member] | Marketable Securities [Member] | Municipal commercial paper and demand notes [Member]
Assets:
Total Assets	62,645	61,006
Level 2 [Member] | Marketable Securities [Member] | Corporate commercial paper [Member]
Assets:
Total Assets	7,044	15,790
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