Allowance for Credit Losses - Aging of Finance Receivables by Class (Details) - USD ($) $ in Millions	Sep. 30, 2018	Mar. 31, 2018
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	$ 984	$ 836
Current	69,511	69,979
Total Finance Receivables	70,495	70,815
90 Days or Greater Past Due and Accruing	56	46
30-59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	715	613
60-89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	189	158
90 Days or Greater Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	80	65
Retail Loan [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	984	836
Current	52,973	52,559
Total Finance Receivables	53,957	53,395
90 Days or Greater Past Due and Accruing	56	46
Retail Loan [Member] | 30-59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	715	613
Retail Loan [Member] | 60-89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	189	158
Retail Loan [Member] | 90 Days or Greater Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	80	65
Wholesale [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0	0
Current	9,638	10,513
Total Finance Receivables	9,638	10,513
90 Days or Greater Past Due and Accruing	0	0
Wholesale [Member] | 30-59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0	0
Wholesale [Member] | 60-89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0	0
Wholesale [Member] | 90 Days or Greater Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0	0
Real estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0	0
Current	4,552	4,630
Total Finance Receivables	4,552	4,630
90 Days or Greater Past Due and Accruing	0	0
Real estate [Member] | 30-59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0	0
Real estate [Member] | 60-89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0	0
Real estate [Member] | 90 Days or Greater Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0	0
Working capital [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0	0
Current	2,348	2,277
Total Finance Receivables	2,348	2,277
90 Days or Greater Past Due and Accruing	0	0
Working capital [Member] | 30-59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0	0
Working capital [Member] | 60-89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0	0
Working capital [Member] | 90 Days or Greater Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	$ 0	$ 0