Borrowings - Long-term Borrowings (Detail) - USD ($) $ in Thousands	Dec. 31, 2022	Dec. 31, 2021
Schedule Of Borrowings [Line Items]
Fixed rates advances from FHLB	$ 5,183	$ 1,578
Other	9,535	9,570
Long-term borrowings	3,964,537	3,485,369
Variable Rate Term Loans Due 2023 [Member]
Schedule Of Borrowings [Line Items]
Variable\fixed rates term loan amount	249,961	249,893
3.55% Fixed Rate Due 2023 [Member] | M&T Bank Corporation [Member]
Schedule Of Borrowings [Line Items]
Variable\fixed rates term loan amount	493,960	516,173
4.55% fixed/variable due 2028 [Member] | M&T Bank Corporation [Member]
Schedule Of Borrowings [Line Items]
Variable\fixed rates term loan amount	477,044
Variable Rate Term Loans Due 2022 [Member]
Schedule Of Borrowings [Line Items]
Variable\fixed rates term loan amount		249,961
Fixed Rate Term Loans 2.50% Due 2022 [Member]
Schedule Of Borrowings [Line Items]
Variable\fixed rates term loan amount		653,903
Fixed Rate Term Loans 5.75% due 2024 [Member]
Schedule Of Borrowings [Line Items]
Subordinated notes	77,337
4.00% due 2024 [Member]
Schedule Of Borrowings [Line Items]
Subordinated notes	403,569
Fixed Rate Term Loans 5.40% due 2025 [Member]
Schedule Of Borrowings [Line Items]
Variable\fixed rates term loan amount	499,317
Fixed Rate Term Loans 2.90% Due 2025 [Member]
Schedule Of Borrowings [Line Items]
Variable\fixed rates term loan amount	749,824	749,740
3.40% due 2027 [Member]
Schedule Of Borrowings [Line Items]
Subordinated notes	462,727	522,867
BSB Capital Trust I - 8.125%, due 2028 [Member]
Schedule Of Borrowings [Line Items]
Junior subordinated debentures associated with preferred capital securities	15,798	15,775
Provident Trust I - 8.29%, due 2028 [Member]
Schedule Of Borrowings [Line Items]
Junior subordinated debentures associated with preferred capital securities	31,267	30,103
Southern Financial Statutory Trust I - 10.60%, due 2030 [Member]
Schedule Of Borrowings [Line Items]
Junior subordinated debentures associated with preferred capital securities	6,999	6,912
First Maryland Capital I - due 2027 [Member]
Schedule Of Borrowings [Line Items]
Junior subordinated debentures associated with preferred capital securities	149,479	148,945
First Maryland Capital II - due 2027 [Member]
Schedule Of Borrowings [Line Items]
Junior subordinated debentures associated with preferred capital securities	151,932	151,270
Allfirst Asset Trust - due 2029 [Member]
Schedule Of Borrowings [Line Items]
Junior subordinated debentures associated with preferred capital securities	97,365	97,220
BSB Capital Trust III - due 2033 [Member]
Schedule Of Borrowings [Line Items]
Junior subordinated debentures associated with preferred capital securities	15,464	15,464
Provident Statutory Trust III - Due 2033 [Member]
Schedule Of Borrowings [Line Items]
Junior subordinated debentures associated with preferred capital securities	59,132	57,547
Southern Financial Capital Trust III - due 2033 [Member]
Schedule Of Borrowings [Line Items]
Junior subordinated debentures associated with preferred capital securities	$ 8,644	$ 8,448