Finance Receivables - Delinquency (Details) - Retail Finance Receivables [Member] - Automobile Loan [Member] - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
In repossession	$ 53	$ 46
In repossession (percent)	0.20%	0.20%
Total finance receivables more than 30 days delinquent or in repossession	$ 1,923	$ 1,471
Total finance receivables more than 30 days delinquent or in repossession (percent)	5.60%	5.00%
31 to 60 Days [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivable past due	$ 1,265	$ 995
Financing receivable past due (percent)	3.70%	3.40%
Greater Than 60 Days [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivable past due	$ 605	$ 430
Financing receivable past due (percent)	1.70%	1.40%
Thirty or More Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivable past due	$ 1,870	$ 1,425
Financing receivable past due (percent)	5.40%	4.80%