Variable Interest Entities and Asset Securitizations - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023	Aug. 31, 2024	Mar. 31, 2024	Dec. 31, 2023
Loss on securitization of assets			$ 0
Long-term borrowings	$ 11,583,000,000		11,583,000,000				$ 8,201,000,000
Loans	133,716,000,000		133,716,000,000				131,939,000,000
Other assets for its "investment" in the common securities recognized by the company of various trusts	22,000,000		22,000,000				22,000,000
Total assets of partnerships in which the company invested	211,785,000,000		211,785,000,000				208,264,000,000
Secured Debt
Long-term debt term (in years)						2 years
Loans and leases average rate						5.29%
Secured Debt | Automobile
Financing receivable, accrued interest, before allowance for credit loss	418,000,000		418,000,000			$ 526,000,000
Long-term debt term (in years)						2 years
Loans and leases average rate						5.29%
Secured Debt | Equipment Finance Loans And Leases
Financing receivable, accrued interest, before allowance for credit loss	742,000,000		742,000,000		$ 790,000,000
Long-term debt term (in years)					2 years
Loans and leases average rate					4.87%
Secured Debt | Third Party Investors | Automobile
Financing receivable, accrued interest, before allowance for credit loss						$ 511,000,000
Long-term borrowings	406,000,000		406,000,000
Secured Debt | Third Party Investors | Equipment Finance Loans And Leases
Financing receivable, accrued interest, before allowance for credit loss					$ 650,000,000
Long-term borrowings	614,000,000		614,000,000
Secured Debt | M&T Bank | Automobile
Financing receivable, accrued interest, before allowance for credit loss						$ 15,000,000
Secured Debt | M&T Bank | Equipment Finance Loans And Leases
Financing receivable, accrued interest, before allowance for credit loss					$ 140,000,000
Variable Interest Entity Primary Beneficiary
Total assets of partnerships in which the company invested	10,000,000,000.0		10,000,000,000.0				$ 9,800,000,000
Federal tax credits and other federal tax benefits recognized	10,000,000	$ 6,000,000	25,000,000	$ 22,000,000
Investments amortized to income tax expense	47,000,000	46,000,000	135,000,000	128,000,000
Maximum exposure to loss of investments in real estate partnerships	2,100,000,000		2,100,000,000
Variable Interest Entity Primary Beneficiary | Renewable Energy
Federal tax credits and other federal tax benefits recognized	8,000,000	10,000,000	8,000,000	10,000,000
Investments amortized to income tax expense	$ 28,000,000	$ 22,000,000	$ 28,000,000	$ 22,000,000