Allowance for Credit Losses - Allowance for Credit Losses and Finance Receivables by Portfolio Segment (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Mar. 31, 2019
Allowance for Credit Losses for Finance Receivables:
Provision for credit losses	$ 61	$ 67	$ 136	$ 156
Finance Receivables:
Total Finance Receivables	73,933		73,933		$ 71,635
Retail Loan [Member]
Finance Receivables:
Total Finance Receivables	56,602		56,602		53,939
Finance Receivables, Net [Member]
Allowance for Credit Losses for Finance Receivables:
Allowance for credit losses at beginning of period	504	463	499	463
Charge-offs	(80)	(76)	(152)	(151)
Recoveries	13	12	27	26
Provision for credit losses	68	48	131	109
Allowance for credit losses at end of period	505	447	505	447
Ending balance: Individually evaluated for impairment	90	57	90	57
Ending balance: Collectively evaluated for impairment	415	390	415	390
Finance Receivables:
Total Finance Receivables	73,933	70,495	73,933	70,495	$ 71,635
Ending balance: Individually evaluated for impairment	610	446	610	446
Ending balance: Collectively evaluated for impairment	73,323	70,049	73,323	70,049
Finance Receivables, Net [Member] | Retail Loan [Member]
Allowance for Credit Losses for Finance Receivables:
Allowance for credit losses at beginning of period	327	313	304	312
Charge-offs	(80)	(76)	(152)	(151)
Recoveries	13	12	27	26
Provision for credit losses	57	58	138	120
Allowance for credit losses at end of period	317	307	317	307
Ending balance: Individually evaluated for impairment	0	0	0	0
Ending balance: Collectively evaluated for impairment	317	307	317	307
Finance Receivables:
Total Finance Receivables	56,602	53,957	56,602	53,957
Ending balance: Individually evaluated for impairment	0	0	0	0
Ending balance: Collectively evaluated for impairment	56,602	53,957	56,602	53,957
Finance Receivables, Net [Member] | Dealer Products [Member]
Allowance for Credit Losses for Finance Receivables:
Allowance for credit losses at beginning of period	177	150	195	151
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Provision for credit losses	11	(10)	(7)	(11)
Allowance for credit losses at end of period	188	140	188	140
Ending balance: Individually evaluated for impairment	90	57	90	57
Ending balance: Collectively evaluated for impairment	98	83	98	83
Finance Receivables:
Total Finance Receivables	17,331	16,538	17,331	16,538
Ending balance: Individually evaluated for impairment	610	446	610	446
Ending balance: Collectively evaluated for impairment	$ 16,721	$ 16,092	$ 16,721	$ 16,092