Finance Receivables - Delinquency (Details) - Retail Finance Receivables [Member] - Automobile Loan [Member] - USD ($) $ in Millions	Sep. 30, 2018	Sep. 30, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
In repossession	$ 53	$ 55
In repossession (percent)	0.20%	0.20%
Total finance receivables more than 30 days delinquent or in repossession	$ 1,853	$ 1,752
Total finance receivables more than 30 days delinquent or in repossession (percent)	4.90%	5.40%
31 to 60 Days [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivable past due	$ 1,302	$ 1,176
Financing receivable past due (percent)	3.40%	3.60%
Greater Than 60 Days [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivable past due	$ 498	$ 521
Financing receivable past due (percent)	1.30%	1.60%
Thirty or More Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivable past due	$ 1,800	$ 1,697
Financing receivable past due (percent)	4.70%	5.20%