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, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Financing Receivable Allowance For Credit Losses [Abstract]
Provision for credit losses			$ 65	$ 248
Finance Receivables, Net [Member]
Financing Receivable Allowance For Credit Losses [Abstract]
Allowance for credit losses at beginning of period	$ 1,196	$ 1,143	1,215	727
Charge-offs	(55)	(46)	(88)	(115)
Recoveries	15	12	32	22
Provision for credit losses	68	65	65	248
Allowance for credit losses at end of period	1,224	1,174	1,224	1,174
Finance Receivables, Net [Member] | ASU 2016-13 [Member]
Financing Receivable Allowance For Credit Losses [Abstract]
Adoption of ASU 2016-13	0			292
Finance Receivables, Net [Member] | Retail loan [Member]
Financing Receivable Allowance For Credit Losses [Abstract]
Allowance for credit losses at beginning of period	1,052	933	1,075	486
Charge-offs	(55)	(46)	(88)	(115)
Recoveries	15	12	32	21
Provision for credit losses	123	121	116	347
Allowance for credit losses at end of period	1,135	1,020	1,135	1,020
Finance Receivables, Net [Member] | Retail loan [Member] | ASU 2016-13 [Member]
Financing Receivable Allowance For Credit Losses [Abstract]
Adoption of ASU 2016-13	0			281
Finance Receivables, Net [Member] | Dealer products [Member]
Financing Receivable Allowance For Credit Losses [Abstract]
Allowance for credit losses at beginning of period	144	210	140	241
Charge-offs	0	0	0	0
Recoveries	0	0	0	1
Provision for credit losses	(55)	(56)	(51)	(99)
Allowance for credit losses at end of period	89	$ 154	$ 89	154
Finance Receivables, Net [Member] | Dealer products [Member] | ASU 2016-13 [Member]
Financing Receivable Allowance For Credit Losses [Abstract]
Adoption of ASU 2016-13	$ 0			$ 11