Borrowings - Long-term Borrowings (Detail) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Schedule Of Borrowings [Line Items]
Asset-backed notes	$ 474
Other	15	$ 15
Long-term borrowings	8,201	3,965
Variable Rate Term Loans Due 2023 [Member] | M&T Bank Corporation [Member]
Schedule Of Borrowings [Line Items]
Variable\fixed rates term loan amount		250
3.55% Fixed Rate Due 2023 [Member] | M&T Bank Corporation [Member]
Schedule Of Borrowings [Line Items]
Variable\fixed rates term loan amount		494
4.55% fixed/variable due 2028 [Member] | M&T Bank Corporation [Member]
Schedule Of Borrowings [Line Items]
Variable\fixed rates term loan amount	484	477
7.41% fixed/variable due 2029 [Member] | M&T Bank Corporation [Member]
Schedule Of Borrowings [Line Items]
Variable\fixed rates term loan amount	1,028
5.05% fixed/variable due 2034 [Member] | M&T Bank Corporation [Member]
Schedule Of Borrowings [Line Items]
Variable\fixed rates term loan amount	970
Fixed Rate Term Loans 5.75% due 2024 [Member]
Schedule Of Borrowings [Line Items]
Subordinated notes	76	77
Fixed Rate Term Loans 4.65% due 2026 [Member]
Schedule Of Borrowings [Line Items]
Variable\fixed rates term loan amount	1,296
Fixed Rate Term Loans 4.70% due 2028 [Member]
Schedule Of Borrowings [Line Items]
Variable\fixed rates term loan amount	1,196
4.00% due 2024 [Member]
Schedule Of Borrowings [Line Items]
Subordinated notes	401	404
Fixed Rate Term Loans 5.40% due 2025 [Member]
Schedule Of Borrowings [Line Items]
Variable\fixed rates term loan amount	499	499
Fixed Rate Term Loans 2.90% Due 2025 [Member]
Schedule Of Borrowings [Line Items]
Variable\fixed rates term loan amount	750	750
3.40% due 2027 [Member]
Schedule Of Borrowings [Line Items]
Subordinated notes	472	463
BSB Capital Trust I - 8.125%, due 2028 [Member]
Schedule Of Borrowings [Line Items]
Junior subordinated debentures associated with preferred capital securities	16	16
Provident Trust I - 8.29%, due 2028 [Member]
Schedule Of Borrowings [Line Items]
Junior subordinated debentures associated with preferred capital securities	32	31
Southern Financial Statutory Trust I - 10.60%, due 2030 [Member]
Schedule Of Borrowings [Line Items]
Junior subordinated debentures associated with preferred capital securities	7	7
First Maryland Capital I - due 2027 [Member]
Schedule Of Borrowings [Line Items]
Junior subordinated debentures associated with preferred capital securities	150	150
First Maryland Capital II - due 2027 [Member]
Schedule Of Borrowings [Line Items]
Junior subordinated debentures associated with preferred capital securities	153	152
Allfirst Asset Trust - due 2029 [Member]
Schedule Of Borrowings [Line Items]
Junior subordinated debentures associated with preferred capital securities	98	97
BSB Capital Trust III - due 2033 [Member]
Schedule Of Borrowings [Line Items]
Junior subordinated debentures associated with preferred capital securities	15	15
Provident Statutory Trust III - Due 2033 [Member]
Schedule Of Borrowings [Line Items]
Junior subordinated debentures associated with preferred capital securities	60	59
Southern Financial Capital Trust III - due 2033 [Member]
Schedule Of Borrowings [Line Items]
Junior subordinated debentures associated with preferred capital securities	$ 9	$ 9