Related Party Transactions - Related Party Transactions Included in Consolidated Statements of Income (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2019
Net financing revenues:
Depreciation on operating leases	$ (1,448)	$ (1,450)	$ (1,349)	$ (1,685)	$ (1,900)	$ (1,712)	$ (1,583)	$ (1,625)	$ (5,932)	$ (6,820)	$ (6,909)
Voluntary protection contract revenues and insurance earned premiums:
Voluntary protection contract revenues and insurance earned premiums	242	241	238	235	241	231	232	229	956	933	904
Expenses:
Operating and administrative expenses	399	354	389	345	460	406	358	337	1,487	1,561	1,385
Voluntary protection contract expenses and insurance losses	$ 104	$ 98	$ 92	$ 75	$ 111	$ 116	$ 115	$ 113	369	455	446
TMCC-affiliated companies [Member]
Net financing revenues:
Manufacturer's subvention and other revenues									1,997	2,065	1,930
Depreciation on operating leases									(113)	(59)	(24)
Interest expense:
Credit support fees, interest and other expenses									117	96	96
Voluntary protection contract revenues and insurance earned premiums:
Voluntary protection contract revenues and insurance earned premiums									173	181	181
Investment and other income, net:
Interest and other income									18	26	16
Expenses:
Operating and administrative expenses									91	102	95
Voluntary protection contract expenses and insurance losses									$ 0	$ 0	$ (3)