Finance Receivables - Delinquency (Details) - Retail Finance Receivables [Member] - Automobile Loan [Member] - USD ($) $ in Millions	Mar. 31, 2019	Mar. 31, 2018
Financing Receivable, Recorded Investment, Past Due [Line Items]
In repossession	$ 47	$ 53
In repossession (percent)	0.10%	0.20%
Total finance receivables more than 30 days delinquent or in repossession	$ 1,507	$ 1,923
Total finance receivables more than 30 days delinquent or in repossession (percent)	3.60%	5.60%
31 to 60 Days [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivable past due	$ 1,048	$ 1,265
Financing receivable past due (percent)	2.50%	3.70%
Greater Than 60 Days [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivable past due	$ 412	$ 605
Financing receivable past due (percent)	1.00%	1.70%
Thirty or More Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivable past due	$ 1,460	$ 1,870
Financing receivable past due (percent)	3.50%	5.40%