Allowance for Credit Losses - Allowance for Credit Losses and Finance Receivables by Portfolio Segment (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2018
Allowance for Credit Losses for Finance Receivables:
Provision for credit losses	$ 326	$ 128	$ 61	$ 75	$ 106	$ 110	$ 67	$ 89	$ 590	$ 372	$ 401
Finance Receivables:
Total Finance Receivables	74,961				71,635				74,961	71,635
Retail loan [Member]
Finance Receivables:
Total Finance Receivables	57,088				53,939				57,088	53,939
Finance Receivables, Net [Member]
Allowance for Credit Losses for Finance Receivables:
Allowance for credit losses at beginning of period				499				463	499	463
Charge-offs									(370)	(330)
Recoveries									50	50
Provision for credit losses									548	316
Allowance for credit losses at end of period	727				499				727	499	463
Ending balance: Individually evaluated for impairment	65				99				65	99
Ending balance: Collectively evaluated for impairment	662				400				662	400
Finance Receivables:
Total Finance Receivables	74,961				71,635				74,961	71,635
Ending balance: Individually evaluated for impairment	528				623				528	623
Ending balance: Collectively evaluated for impairment	74,433				71,012				74,433	71,012
Finance Receivables, Net [Member] | Retail loan [Member]
Allowance for Credit Losses for Finance Receivables:
Allowance for credit losses at beginning of period				304				312	304	312
Charge-offs									(358)	(330)
Recoveries									50	50
Provision for credit losses									490	272
Allowance for credit losses at end of period	486				304				486	304	312
Ending balance: Individually evaluated for impairment	0				0				0	0
Ending balance: Collectively evaluated for impairment	486				304				486	304
Finance Receivables:
Total Finance Receivables	57,088				53,939				57,088	53,939
Ending balance: Individually evaluated for impairment	0				0				0	0
Ending balance: Collectively evaluated for impairment	57,088				53,939				57,088	53,939
Finance Receivables, Net [Member] | Dealer products [Member]
Allowance for Credit Losses for Finance Receivables:
Allowance for credit losses at beginning of period				$ 195				$ 151	195	151
Charge-offs									(12)	0
Recoveries									0	0
Provision for credit losses									58	44
Allowance for credit losses at end of period	241				195				241	195	$ 151
Ending balance: Individually evaluated for impairment	65				99				65	99
Ending balance: Collectively evaluated for impairment	176				96				176	96
Finance Receivables:
Total Finance Receivables	17,873				17,696				17,873	17,696
Ending balance: Individually evaluated for impairment	528				623				528	623
Ending balance: Collectively evaluated for impairment	$ 17,345				$ 17,073				$ 17,345	$ 17,073