Finance Receivables - Credit Quality (Details) - Automobile Loan [Member] - Retail Finance Receivables [Member] - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment [Line Items]
Finance receivables, net of fees	$ 34,320	$ 32,802
Finance receivables credit indicator, Percent	100.00%	100.00%
Prime - FICO Score 680 and greater [Member]
Financing Receivable, Recorded Investment [Line Items]
Finance receivables, net of fees	$ 18,392	$ 16,892
Finance receivables credit indicator, Percent	53.60%	51.50%
Near-prime - FICO Score 620 to 679 [Member]
Financing Receivable, Recorded Investment [Line Items]
Finance receivables, net of fees	$ 5,451	$ 5,226
Finance receivables credit indicator, Percent	15.90%	15.90%
Sub-prime - FICO Score less than 620 [Member]
Financing Receivable, Recorded Investment [Line Items]
Finance receivables, net of fees	$ 10,477	$ 10,684
Finance receivables credit indicator, Percent	30.50%	32.60%