Fair Value Measurements (Narratives) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2011
Fair Value Measurements [Abstract]
Derivative asset counterparty credit valuation adjustment	$ 1	$ 3
Derivative liability non-performance credit valuation adjustment	1	1
Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gain (loss) from changes in fair value of net finance receivables	12	22	24
Nonrecurring [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net finance receivables	$ 208	$ 166