Related Party Transactions (Narratives) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2011
Related Party Transaction [Line Items]
Credit Support Fees	$ 72	$ 33	$ 34
Notes Payable	48	61
Loans payable to affiliate	0	2,201
Notes Receivable	931	1,052
Accounts Receivable	22	17
Accounts Payable	192	58
Subvention Revenues	940	949	965
Stock-based Compensation Expense	2	2
Operating Leases to Affiliates	7	4
Employee benefits expense	30	27	27
Investments in affiliates commercial paper	2	0
TFSC Credit Support Agreement [Member]
Related Party Transaction [Line Items]
Credit Support Fees	72	33	34
TMCC-TFSB Loan Agreement [Member]
Related Party Transaction [Line Items]
Maximum Lending Amount	150
TFSB Master Participation Agreement [Member]
Related Party Transaction [Line Items]
Annual Maximum Participation	60
Loan Purchased	$ 55	$ 54
TMIS Service Type One [Member]
Related Party Transaction [Line Items]
Risk Ceded to Reinsures	99.00%	99.00%