Borrowings - Long-term Borrowings (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2020
Schedule Of Borrowings [Line Items]
Fixed rates advances from FHLB	$ 1,815	$ 1,683
Agreements to repurchase securities	101,679
Other	8,533	2,997
Long-term borrowings	6,986,186	4,382,193
Variable Rate Term Loans Due 2023 [Member] | M&T Bank Corporation [Member]
Schedule Of Borrowings [Line Items]
Variable\fixed rates term loan amount	249,756	249,824
Variable Rate Term Loans Due 2021 [Member]
Schedule Of Borrowings [Line Items]
Variable\fixed rates term loan amount	349,893	349,992
3.55% Fixed Rate Due 2023 [Member] | M&T Bank Corporation [Member]
Schedule Of Borrowings [Line Items]
Variable\fixed rates term loan amount	520,454	533,369
Variable Rate Term Loans Due 2022 [Member]
Schedule Of Borrowings [Line Items]
Variable\fixed rates term loan amount	249,758	249,858
Fixed Rate Term Loans 2.25% Due 2019 [Member]
Schedule Of Borrowings [Line Items]
Variable\fixed rates term loan amount	749,254
Fixed Rate Term Loans 2.05% Due 2020 [Member]
Schedule Of Borrowings [Line Items]
Variable\fixed rates term loan amount	749,864
Fixed Rate Term Loans 2.10% Due 2020 [Member]
Schedule Of Borrowings [Line Items]
Variable\fixed rates term loan amount	654,136
Fixed Rate Term Loans 2.625% Due 2021 [Member]
Schedule Of Borrowings [Line Items]
Variable\fixed rates term loan amount	652,714	664,400
Fixed Rate Term Loans 2.50% Due 2022 [Member]
Schedule Of Borrowings [Line Items]
Variable\fixed rates term loan amount	749,572	749,656
Variable Rate Due 2020 [Member]
Schedule Of Borrowings [Line Items]
Subordinated notes	409,361
Variable Rate Due 2021 [Member]
Schedule Of Borrowings [Line Items]
Subordinated notes	500,000	500,000
3.40% due 2027 [Member]
Schedule Of Borrowings [Line Items]
Subordinated notes	514,353	552,194
BSB Capital Trust I - 8.125%, due 2028 [Member]
Schedule Of Borrowings [Line Items]
Junior subordinated debentures associated with preferred capital securities	15,728	15,752
Provident Trust I - 8.29%, due 2028 [Member]
Schedule Of Borrowings [Line Items]
Junior subordinated debentures associated with preferred capital securities	28,235	29,099
Southern Financial Statutory Trust I - 10.60%, due 2030 [Member]
Schedule Of Borrowings [Line Items]
Junior subordinated debentures associated with preferred capital securities	6,770	6,836
First Maryland Capital I - due 2027 [Member]
Schedule Of Borrowings [Line Items]
Junior subordinated debentures associated with preferred capital securities	147,871	148,409
First Maryland Capital II - due 2027 [Member]
Schedule Of Borrowings [Line Items]
Junior subordinated debentures associated with preferred capital securities	149,943	150,606
Allfirst Asset Trust - due 2029 [Member]
Schedule Of Borrowings [Line Items]
Junior subordinated debentures associated with preferred capital securities	96,930	97,075
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	55,867	56,641
Southern Financial Capital Trust III - due 2033 [Member]
Schedule Of Borrowings [Line Items]
Junior subordinated debentures associated with preferred capital securities	$ 8,236	$ 8,338