Allowance for Credit Losses - Allowance for Credit Losses and Finance Receivables by Portfolio Segment (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Mar. 31, 2019
Allowance for Credit Losses for Finance Receivables:
Provision for credit losses	$ 128	$ 110	$ 264	$ 266
Finance Receivables:
Total Finance Receivables	74,324		74,324		$ 71,635
Retail Loan [Member]
Finance Receivables:
Total Finance Receivables	57,324		57,324		53,939
Finance Receivables, Net [Member]
Allowance for Credit Losses for Finance Receivables:
Allowance for credit losses at beginning of period	505	447	499	463
Charge-offs	(117)	(87)	(269)	(238)
Recoveries	12	12	39	38
Provision for credit losses	123	103	254	212
Allowance for credit losses at end of period	523	475	523	475
Ending balance: Individually evaluated for impairment	105	88	105	88
Ending balance: Collectively evaluated for impairment	418	387	418	387
Finance Receivables:
Total Finance Receivables	74,324	70,736	74,324	70,736	$ 71,635
Ending balance: Individually evaluated for impairment	617	596	617	596
Ending balance: Collectively evaluated for impairment	73,707	70,140	73,707	70,140
Finance Receivables, Net [Member] | Retail Loan [Member]
Allowance for Credit Losses for Finance Receivables:
Allowance for credit losses at beginning of period	317	307	304	312
Charge-offs	(117)	(87)	(269)	(238)
Recoveries	12	12	39	38
Provision for credit losses	116	69	254	189
Allowance for credit losses at end of period	328	301	328	301
Ending balance: Individually evaluated for impairment	0	0	0	0
Ending balance: Collectively evaluated for impairment	328	301	328	301
Finance Receivables:
Total Finance Receivables	57,324	53,992	57,324	53,992
Ending balance: Individually evaluated for impairment	0	0	0	0
Ending balance: Collectively evaluated for impairment	57,324	53,992	57,324	53,992
Finance Receivables, Net [Member] | Dealer Products [Member]
Allowance for Credit Losses for Finance Receivables:
Allowance for credit losses at beginning of period	188	140	195	151
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Provision for credit losses	7	34	0	23
Allowance for credit losses at end of period	195	174	195	174
Ending balance: Individually evaluated for impairment	105	88	105	88
Ending balance: Collectively evaluated for impairment	90	86	90	86
Finance Receivables:
Total Finance Receivables	17,000	16,744	17,000	16,744
Ending balance: Individually evaluated for impairment	617	596	617	596
Ending balance: Collectively evaluated for impairment	$ 16,383	$ 16,148	$ 16,383	$ 16,148