Finance Receivables - Credit Quality (Details) - Retail Finance Receivables [Member] - Automobile Loan [Member] - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment [Line Items]
Amount	$ 32,802	$ 26,400
Percent	100.00%	100.00%
Prime - FICO score 680 and greater [Member]
Financing Receivable, Recorded Investment [Line Items]
Amount	$ 16,892	$ 11,417
Percent	51.50%	43.20%
Near-prime FICO score 620 to 679 [Member]
Financing Receivable, Recorded Investment [Line Items]
Amount	$ 5,226	$ 4,222
Percent	15.90%	16.00%
Sub-prime FICO score Less than 620 [Member]
Financing Receivable, Recorded Investment [Line Items]
Amount	$ 10,684	$ 10,761
Percent	32.60%	40.80%