Borrowings - Long-term Borrowings (Parenthetical) (Detail)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Variable Rate Term Loans Due 2023 [Member] | M&T Bank Corporation [Member]
Schedule Of Borrowings [Line Items]
Debt maturity date	2023	2023
Variable Rate Term Loans Due 2021 [Member]
Schedule Of Borrowings [Line Items]
Debt maturity date	2021	2021
3.55% Fixed Rate Due 2023 [Member] | M&T Bank Corporation [Member]
Schedule Of Borrowings [Line Items]
Debt maturity date	2023	2023
Interest rate of debt instrument	3.55%	3.55%
Variable Rate Term Loans Due 2022 [Member]
Schedule Of Borrowings [Line Items]
Debt maturity date	2022	2022
Fixed Rate Term Loans 2.25% Due 2019 [Member]
Schedule Of Borrowings [Line Items]
Debt maturity date	2019	2019
Interest rate of debt instrument	2.25%	2.25%
Fixed Rate Term Loans 2.10% Due 2020 [Member]
Schedule Of Borrowings [Line Items]
Debt maturity date	2020	2020
Interest rate of debt instrument	2.10%	2.10%
Fixed Rate Term Loans 2.05% Due 2020 [Member]
Schedule Of Borrowings [Line Items]
Debt maturity date	2020	2020
Interest rate of debt instrument	2.05%	2.05%
Fixed Rate Term Loans 2.625% Due 2021 [Member]
Schedule Of Borrowings [Line Items]
Debt maturity date	2021	2021
Interest rate of debt instrument	2.625%	2.625%
Fixed Rate Term Loans 2.90% Due 2025 [Member]
Schedule Of Borrowings [Line Items]
Debt maturity date	2025	2025
Interest rate of debt instrument	2.90%	2.90%
Fixed Rate Term Loans 2.50% Due 2022 [Member]
Schedule Of Borrowings [Line Items]
Debt maturity date	2022	2022
Interest rate of debt instrument	2.50%	2.50%
Variable Rate Due 2020 [Member]
Schedule Of Borrowings [Line Items]
Debt maturity date	2020	2020
Variable Rate Due 2021 [Member]
Schedule Of Borrowings [Line Items]
Debt maturity date	2021	2021
3.40% due 2027 [Member]
Schedule Of Borrowings [Line Items]
Debt maturity date	2027	2027
Interest rate of debt instrument	3.40%	3.40%
BSB Capital Trust I - 8.125%, due 2028 [Member]
Schedule Of Borrowings [Line Items]
Debt maturity date	2028	2028
Interest rate of debt instrument	8.125%	8.125%
Provident Trust I - 8.29%, due 2028 [Member]
Schedule Of Borrowings [Line Items]
Debt maturity date	2028	2028
Interest rate of debt instrument	8.29%	8.29%
Southern Financial Statutory Trust I - 10.60%, due 2030 [Member]
Schedule Of Borrowings [Line Items]
Debt maturity date	2030	2030
Interest rate of debt instrument	10.60%	10.60%
First Maryland Capital I - due 2027 [Member]
Schedule Of Borrowings [Line Items]
Debt maturity date	2027	2027
First Maryland Capital II - due 2027 [Member]
Schedule Of Borrowings [Line Items]
Debt maturity date	2027	2027
Allfirst Asset Trust - due 2029 [Member]
Schedule Of Borrowings [Line Items]
Debt maturity date	2029	2029
BSB Capital Trust III - due 2033 [Member]
Schedule Of Borrowings [Line Items]
Debt maturity date	2033	2033
Provident Statutory Trust III - Due 2033 [Member]
Schedule Of Borrowings [Line Items]
Debt maturity date	2033	2033
Southern Financial Capital Trust III - due 2033 [Member]
Schedule Of Borrowings [Line Items]
Debt maturity date	2033	2033