Allowance for Credit Losses - Allowance for Credit Losses for Finance Receivables and Finance Receivables by Portfolio Segment (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Mar. 31, 2018
Allowance for Credit Losses for Finance Receivables:
Provision for credit losses	$ 67	$ 127	$ 156	$ 212
Finance Receivables:
Total Finance Receivables	70,495		70,495		$ 70,815
Retail Loan [Member]
Finance Receivables:
Total Finance Receivables	53,957		53,957		53,395
Finance Receivables, Net [Member]
Allowance for Credit Losses for Finance Receivables:
Allowance for credit losses at beginning of period	463	453	463	467
Charge-offs	(76)	(86)	(151)	(165)
Recoveries	12	13	26	26
Provision for credit losses	48	102	109	154
Allowance for credit losses at end of period	447	482	447	482
Ending balance: Individually evaluated for impairment	57	45	57	45
Ending balance: Collectively evaluated for impairment	390	437	390	437
Finance Receivables:
Total Finance Receivables	70,495	68,386	70,495	68,386	$ 70,815
Ending balance: Individually evaluated for impairment	446	403	446	403
Ending balance: Collectively evaluated for impairment	70,049	67,983	70,049	67,983
Finance Receivables, Net [Member] | Retail Loan [Member]
Allowance for Credit Losses for Finance Receivables:
Allowance for credit losses at beginning of period	313	322	312	344
Charge-offs	(76)	(86)	(151)	(165)
Recoveries	12	13	26	26
Provision for credit losses	58	110	120	154
Allowance for credit losses at end of period	307	359	307	359
Ending balance: Individually evaluated for impairment	0	0	0	0
Ending balance: Collectively evaluated for impairment	307	359	307	359
Finance Receivables:
Total Finance Receivables	53,957	52,155	53,957	52,155
Ending balance: Individually evaluated for impairment	0	0	0	0
Ending balance: Collectively evaluated for impairment	53,957	52,155	53,957	52,155
Finance Receivables, Net [Member] | Dealer Products [Member]
Allowance for Credit Losses for Finance Receivables:
Allowance for credit losses at beginning of period	150	131	151	123
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Provision for credit losses	(10)	(8)	(11)	0
Allowance for credit losses at end of period	140	123	140	123
Ending balance: Individually evaluated for impairment	57	45	57	45
Ending balance: Collectively evaluated for impairment	83	78	83	78
Finance Receivables:
Total Finance Receivables	16,538	16,231	16,538	16,231
Ending balance: Individually evaluated for impairment	446	403	446	403
Ending balance: Collectively evaluated for impairment	$ 16,092	$ 15,828	$ 16,092	$ 15,828