Allowance for Credit Losses - Aging of Finance Receivables by Class (Details) - USD ($) $ in Millions	Jun. 30, 2017	Mar. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	$ 861	$ 781
Current	68,789	68,527
Total Finance Receivables	69,650	69,308
90 Days or Greater Past Due and Accruing	42	41
30-59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	632	589
60-89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	171	129
90 Days or Greater Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	58	63
Retail Loan [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	861	778
Current	50,514	50,631
Total Finance Receivables	51,375	51,409
90 Days or Greater Past Due and Accruing	42	41
Retail Loan [Member] | 30-59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	632	586
Retail Loan [Member] | 60-89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	171	129
Retail Loan [Member] | 90 Days or Greater Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	58	63
Wholesale [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0	0
Current	11,221	10,951
Total Finance Receivables	11,221	10,951
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	4,762	4,713
Total Finance Receivables	4,762	4,713
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	3
Current	2,292	2,232
Total Finance Receivables	2,292	2,235
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	3
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