Fair Value Measurements - Assets and Liabilities Not Carried at Fair Value on Recurring Basis on Consolidated Balance Sheets (Details) - USD ($) $ in Millions	Dec. 31, 2019	Mar. 31, 2019
Carrying value [Member] | Unsecured Notes and Loans Payable [Member]
Financial liabilities
Financial liabilities	$ 84,190	$ 80,521
Carrying value [Member] | Secured Notes and Loans Payable [Member]
Financial liabilities
Financial liabilities	13,344	12,401
Fair value [Member] | Unsecured Notes and Loans Payable [Member]
Financial liabilities
Financial liabilities	85,432	81,369
Fair value [Member] | Secured Notes and Loans Payable [Member]
Financial liabilities
Financial liabilities	13,447	12,428
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	84,072	79,056
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	1,360	2,313
Fair value [Member] | Level 3 [Member] | Secured Notes and Loans Payable [Member]
Financial liabilities
Financial liabilities	13,447	12,428
Retail Loan [Member] | Carrying value [Member]
Financial assets
Finance receivables, net	56,641	53,013
Retail Loan [Member] | Fair value [Member]
Financial assets
Finance receivables, net	57,383	53,247
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	57,383	53,247
Wholesale [Member] | Carrying value [Member]
Financial assets
Finance receivables, net	9,398	10,293
Wholesale [Member] | Fair value [Member]
Financial assets
Finance receivables, net	9,485	10,369
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	9,485	10,369
Real estate [Member] | Carrying value [Member]
Financial assets
Finance receivables, net	4,554	4,550
Real estate [Member] | Fair value [Member]
Financial assets
Finance receivables, net	4,626	4,534
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	4,626	4,534
Working capital [Member] | Carrying value [Member]
Financial assets
Finance receivables, net	2,741	2,510
Working capital [Member] | Fair value [Member]
Financial assets
Finance receivables, net	2,784	2,554
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	$ 2,784	$ 2,554