Finance Receivables - Credit Risk Profile by Dealer grouping of Commercial Finance Receivables (Details) - Commercial Finance Receivables [Member] - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Balance at end of period	$ 10,698	$ 10,312
Percent of portfolio	100.00%	100.00%
Group I [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Balance at end of period	$ 1,971	$ 1,915
Percent of portfolio	18.40%	18.60%
Group II [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Balance at end of period	$ 4,031	$ 3,584
Percent of portfolio	37.70%	34.70%
Group III [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Balance at end of period	$ 3,192	$ 3,424
Percent of portfolio	29.80%	33.20%
Group IV [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Balance at end of period	$ 1,001	$ 1,048
Percent of portfolio	9.40%	10.20%
Group V [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Balance at end of period	$ 426	$ 260
Percent of portfolio	4.00%	2.50%
Group VI [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Balance at end of period	$ 77	$ 81
Percent of portfolio	0.70%	0.80%