Finance Receivables - Delinquency (Details) - Retail Finance Receivables [Member] - Automobile Loan [Member] - USD ($) $ in Millions	Sep. 30, 2019	Sep. 30, 2018
Financing Receivable, Past Due [Line Items]
Financing receivable past due	$ 1,766	$ 1,800
Financing receivable past due (percent)	4.20%	4.70%
In repossession	$ 48	$ 53
In repossession (percent)	0.10%	0.20%
Total finance receivables more than 30 days delinquent or in repossession	$ 1,814	$ 1,853
Total finance receivables more than 30 days delinquent or in repossession (percent)	4.30%	4.90%
31 to 60 Days [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable past due	$ 1,252	$ 1,302
Financing receivable past due (percent)	3.00%	3.40%
Greater Than 60 Days [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable past due	$ 514	$ 498
Financing receivable past due (percent)	1.20%	1.30%