Allowance for Credit Losses - Allowance for Credit Losses for Finance Receivables and Finance Receivables by Portfolio Segment (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Mar. 31, 2017
Allowance for Credit Losses for Finance Receivables:
Provisions	$ 108	$ 183	$ 320	$ 396
Finance Receivables:
Total Finance Receivables	69,984		69,984		$ 69,308
Retail Loan [Member]
Finance Receivables:
Total Finance Receivables	52,892		52,892		51,409
Finance Receivables, Net [Member]
Allowance for Credit Losses for Finance Receivables:
Allowance for credit losses at beginning of period	482	389	467	421
Charge-offs	(92)	(113)	(257)	(294)
Recoveries	9	12	35	38
Provisions	92	133	246	256
Allowance for credit losses at end of period	491	421	491	421
Ending balance: Individually evaluated for impairment	52	26	52	26
Ending balance: Collectively evaluated for impairment	439	395	439	395
Finance Receivables:
Total Finance Receivables	69,984	68,789	69,984	68,789	$ 69,308
Ending balance: Individually evaluated for impairment	458	438	458	438
Ending balance: Collectively evaluated for impairment	69,526	68,351	69,526	68,351
Finance Receivables, Net [Member] | Retail Loan [Member]
Allowance for Credit Losses for Finance Receivables:
Allowance for credit losses at beginning of period	359	295	344	289
Charge-offs	(92)	(113)	(257)	(294)
Recoveries	9	12	35	38
Provisions	81	120	235	281
Allowance for credit losses at end of period	357	314	357	314
Ending balance: Individually evaluated for impairment	0	0	0	0
Ending balance: Collectively evaluated for impairment	357	314	357	314
Finance Receivables:
Total Finance Receivables	52,892	51,570	52,892	51,570
Ending balance: Individually evaluated for impairment	0	0	0	0
Ending balance: Collectively evaluated for impairment	52,892	51,570	52,892	51,570
Finance Receivables, Net [Member] | Dealer Products [Member]
Allowance for Credit Losses for Finance Receivables:
Allowance for credit losses at beginning of period	123	94	123	132
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Provisions	11	13	11	(25)
Allowance for credit losses at end of period	134	107	134	107
Ending balance: Individually evaluated for impairment	52	26	52	26
Ending balance: Collectively evaluated for impairment	82	81	82	81
Finance Receivables:
Total Finance Receivables	17,092	17,219	17,092	17,219
Ending balance: Individually evaluated for impairment	458	438	458	438
Ending balance: Collectively evaluated for impairment	$ 16,634	$ 16,781	$ 16,634	$ 16,781