Finance Receivables - Finance Receivables, net (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Finance receivables, net	$ 45,500	$ 42,172
Fair value of finance receivables	45,128	42,178
Commercial Finance Receivables [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total finance receivables	10,698	10,312
Automobile Loan [Member] | Retail Finance Receivables [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Collectively evaluated for impairment, net of fees	33,398	30,574
Individually evaluated for impairment, net of fees	2,277	2,228
Total finance receivables	35,675	32,802
Less: allowance for loan losses - collective	(508)	(561)
Less: allowance for loan losses - specific	(307)	(328)
Total finance receivables, net	34,860	31,913
Deferred income	184	228
Automobile Loan [Member] | Commercial Finance Receivables [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Collectively evaluated for impairment, net of fees	10,652	10,290
Individually evaluated for impairment, net of fees	46	22
Total finance receivables	10,698	10,312
Less: allowance for loan losses - collective	(52)	(50)
Less: allowance for loan losses - specific	(6)	(3)
Total finance receivables, net	$ 10,640	$ 10,259