Finance Receivables - Delinquency (Details) - Retail Finance Receivables [Member] - Automobile Loan [Member] - USD ($) $ in Millions	Sep. 30, 2017	Sep. 30, 2016
Financing Receivable, Recorded Investment, Past Due [Line Items]
In repossession	$ 55	$ 57
In repossession (percent)	0.20%	0.20%
Total finance receivables more than 30 days delinquent or in repossession	$ 1,752	$ 1,660
Total finance receivables more than 30 days delinquent or in repossession (percent)	5.40%	6.50%
31 to 60 Days [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivable past due	$ 1,176	$ 1,112
Financing receivable past due (percent)	3.60%	4.40%
Greater Than 60 Days [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivable past due	$ 521	$ 491
Financing receivable past due (percent)	1.60%	1.90%
Thirty or More Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivable past due	$ 1,697	$ 1,603
Financing receivable past due (percent)	5.20%	6.30%