Fair Value Measurements - Assets and Liabilities Not Carried at Fair Value on Recurring Basis on Consolidated Balance Sheets (Details) - USD ($) $ in Millions	Dec. 31, 2020	Mar. 31, 2020
Carrying value [Member] | Unsecured Notes and Loans Payable [Member]
Financial liabilities
Financial liabilities	$ 87,252	$ 83,172
Carrying value [Member] | Secured Notes and Loans Payable [Member]
Financial liabilities
Financial liabilities	21,720	14,568
Fair value [Member] | Unsecured Notes and Loans Payable [Member]
Financial liabilities
Financial liabilities	89,307	82,989
Fair value [Member] | Secured Notes and Loans Payable [Member]
Financial liabilities
Financial liabilities	22,054	14,608
Fair value [Member] | Level 1 [Member] | Unsecured Notes and Loans Payable [Member]
Financial liabilities
Financial liabilities	0	0
Fair value [Member] | Level 1 [Member] | Secured Notes and Loans Payable [Member]
Financial liabilities
Financial liabilities	0	0
Fair value [Member] | Level 2 [Member] | Unsecured Notes and Loans Payable [Member]
Financial liabilities
Financial liabilities	88,752	82,429
Fair value [Member] | Level 2 [Member] | Secured Notes and Loans Payable [Member]
Financial liabilities
Financial liabilities	0	0
Fair value [Member] | Level 3 [Member] | Unsecured Notes and Loans Payable [Member]
Financial liabilities
Financial liabilities	555	560
Fair value [Member] | Level 3 [Member] | Secured Notes and Loans Payable [Member]
Financial liabilities
Financial liabilities	22,054	14,608
Retail Loan [Member] | Carrying value [Member]
Financial assets
Finance receivables, net	63,859	56,360
Retail Loan [Member] | Fair value [Member]
Financial assets
Finance receivables, net	67,499	57,303
Retail Loan [Member] | Fair value [Member] | Level 1 [Member]
Financial assets
Finance receivables, net	0	0
Retail Loan [Member] | Fair value [Member] | Level 2 [Member]
Financial assets
Finance receivables, net	0	0
Retail Loan [Member] | Fair value [Member] | Level 3 [Member]
Financial assets
Finance receivables, net	67,499	57,303
Wholesale [Member] | Carrying value [Member]
Financial assets
Finance receivables, net	7,420	9,672
Wholesale [Member] | Fair value [Member]
Financial assets
Finance receivables, net	7,444	9,637
Wholesale [Member] | Fair value [Member] | Level 1 [Member]
Financial assets
Finance receivables, net	0	0
Wholesale [Member] | Fair value [Member] | Level 2 [Member]
Financial assets
Finance receivables, net	0	0
Wholesale [Member] | Fair value [Member] | Level 3 [Member]
Financial assets
Finance receivables, net	7,444	9,637
Real estate [Member] | Carrying value [Member]
Financial assets
Finance receivables, net	5,131	4,544
Real estate [Member] | Fair value [Member]
Financial assets
Finance receivables, net	5,218	4,140
Real estate [Member] | Fair value [Member] | Level 1 [Member]
Financial assets
Finance receivables, net	0	0
Real estate [Member] | Fair value [Member] | Level 2 [Member]
Financial assets
Finance receivables, net	0	0
Real estate [Member] | Fair value [Member] | Level 3 [Member]
Financial assets
Finance receivables, net	5,218	4,140
Working capital [Member] | Carrying value [Member]
Financial assets
Finance receivables, net	2,427	3,308
Working capital [Member] | Fair value [Member]
Financial assets
Finance receivables, net	2,356	2,811
Working capital [Member] | Fair value [Member] | Level 1 [Member]
Financial assets
Finance receivables, net	0	0
Working capital [Member] | Fair value [Member] | Level 2 [Member]
Financial assets
Finance receivables, net	0	0
Working capital [Member] | Fair value [Member] | Level 3 [Member]
Financial assets
Finance receivables, net	$ 2,356	$ 2,811