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, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2017
Allowance for Credit Losses for Finance Receivables:
Provision for credit losses	$ 106	$ 110	$ 67	$ 89	$ 81	$ 108	$ 127	$ 85	$ 372	$ 401	$ 582
Finance Receivables:
Total Finance Receivables	71,635				70,815				71,635	70,815
Retail Loan [Member]
Finance Receivables:
Total Finance Receivables	53,939				53,395				53,939	53,395
Finance Receivables, Net [Member]
Allowance for Credit Losses for Finance Receivables:
Allowance for credit losses at beginning of period				463				467	463	467
Charge-offs									(330)	(355)
Recoveries									50	50
Provision for credit losses									316	301
Allowance for credit losses at end of period	499				463				499	463	467
Ending balance: Individually evaluated for impairment	99				70				99	70
Ending balance: Collectively evaluated for impairment	400				393				400	393
Finance Receivables:
Total Finance Receivables	71,635				70,815				71,635	70,815
Ending balance: Individually evaluated for impairment	623				495				623	495
Ending balance: Collectively evaluated for impairment	71,012				70,320				71,012	70,320
Finance Receivables, Net [Member] | Retail Loan [Member]
Allowance for Credit Losses for Finance Receivables:
Allowance for credit losses at beginning of period				312				344	312	344
Charge-offs									(330)	(355)
Recoveries									50	50
Provision for credit losses									272	273
Allowance for credit losses at end of period	304				312				304	312	344
Ending balance: Individually evaluated for impairment	0				0				0	0
Ending balance: Collectively evaluated for impairment	304				312				304	312
Finance Receivables:
Total Finance Receivables	53,939				53,395				53,939	53,395
Ending balance: Individually evaluated for impairment	0				0				0	0
Ending balance: Collectively evaluated for impairment	53,939				53,395				53,939	53,395
Finance Receivables, Net [Member] | Dealer Products [Member]
Allowance for Credit Losses for Finance Receivables:
Allowance for credit losses at beginning of period				$ 151				$ 123	151	123
Charge-offs									0	0
Recoveries									0	0
Provision for credit losses									44	28
Allowance for credit losses at end of period	195				151				195	151	$ 123
Ending balance: Individually evaluated for impairment	99				70				99	70
Ending balance: Collectively evaluated for impairment	96				81				96	81
Finance Receivables:
Total Finance Receivables	17,696				17,420				17,696	17,420
Ending balance: Individually evaluated for impairment	623				495				623	495
Ending balance: Collectively evaluated for impairment	$ 17,073				$ 16,925				$ 17,073	$ 16,925