Finance Receivables - Credit Risk Profile by Dealer grouping of Commercial Finance Receivables (Details) - Commercial Finance Receivables [Member] - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Balance at end of period	$ 10,365	$ 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	$ 1,799	$ 1,915
Percent of portfolio	17.40%	18.60%
Group II [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Balance at end of period	$ 3,708	$ 3,584
Percent of portfolio	35.80%	34.70%
Group III [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Balance at end of period	$ 3,414	$ 3,424
Percent of portfolio	32.90%	33.20%
Group IV [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Balance at end of period	$ 1,111	$ 1,048
Percent of portfolio	10.70%	10.20%
Group V [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Balance at end of period	$ 268	$ 260
Percent of portfolio	2.60%	2.50%
Group VI [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Balance at end of period	$ 65	$ 81
Percent of portfolio	0.60%	0.80%