Borrowings - Long-term Borrowings (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Schedule Of Borrowings [Line Items]
Fixed rates advances from FHLB	$ 576,446	$ 576,876
Agreements to repurchase securities	409,154	421,771
Other	35,174	9,635
Long-term borrowings	8,444,914	8,141,430
Variable Rate Term Loans Due 2023 [Member] | M&T Bank Corporation [Member]
Schedule Of Borrowings [Line Items]
Variable\fixed rates term loan amount	249,688
Variable Rate Term Loans Due 2021 [Member]
Schedule Of Borrowings [Line Items]
Variable\fixed rates term loan amount	349,794
3.55% Fixed Rate Due 2023 [Member] | M&T Bank Corporation [Member]
Schedule Of Borrowings [Line Items]
Variable\fixed rates term loan amount	506,021
Variable Rate Term Loans Due 2022 [Member]
Schedule Of Borrowings [Line Items]
Variable\fixed rates term loan amount	249,658	249,558
Fixed Rate Term Loans 1.45% Due 2018 [Member]
Schedule Of Borrowings [Line Items]
Variable\fixed rates term loan amount		499,907
Fixed Rate Term Loans 2.25% Due 2019 [Member]
Schedule Of Borrowings [Line Items]
Variable\fixed rates term loan amount	645,801	644,977
Fixed Rate Term Loans 2.30% Due 2019 [Member]
Schedule Of Borrowings [Line Items]
Variable\fixed rates term loan amount		746,919
Fixed Rate Term Loans 2.05% Due 2020 [Member]
Schedule Of Borrowings [Line Items]
Variable\fixed rates term loan amount	737,793	739,961
Fixed Rate Term Loans 2.10% Due 2020 [Member]
Schedule Of Borrowings [Line Items]
Variable\fixed rates term loan amount	741,965	743,788
Fixed Rate Term Loans 2.625% Due 2021 [Member]
Schedule Of Borrowings [Line Items]
Variable\fixed rates term loan amount	646,301
Fixed Rate Term Loans 2.50% Due 2022 [Member]
Schedule Of Borrowings [Line Items]
Variable\fixed rates term loan amount	634,525	638,872
Fixed Rate Term Loans 2.90% Due 2025 [Member]
Schedule Of Borrowings [Line Items]
Variable\fixed rates term loan amount	749,488	749,404
8.5% due 2018 [Member]
Schedule Of Borrowings [Line Items]
Subordinated notes		200,000
Variable Rate Due 2020 [Member]
Schedule Of Borrowings [Line Items]
Subordinated notes	409,361	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	481,692	491,176
BSB Capital Trust I - 8.125%, due 2028 [Member]
Schedule Of Borrowings [Line Items]
Junior subordinated debentures associated with preferred capital securities	15,705	15,682
Provident Trust I - 8.29%, due 2028 [Member]
Schedule Of Borrowings [Line Items]
Junior subordinated debentures associated with preferred capital securities	27,489	26,847
Southern Financial Statutory Trust I - 10.60%, due 2030 [Member]
Schedule Of Borrowings [Line Items]
Junior subordinated debentures associated with preferred capital securities	6,713	6,664
First Maryland Capital I - due 2027 [Member]
Schedule Of Borrowings [Line Items]
Junior subordinated debentures associated with preferred capital securities	147,333	146,794
First Maryland Capital II - due 2027 [Member]
Schedule Of Borrowings [Line Items]
Junior subordinated debentures associated with preferred capital securities	149,280	148,617
Allfirst Asset Trust - due 2029 [Member]
Schedule Of Borrowings [Line Items]
Junior subordinated debentures associated with preferred capital securities	96,785	96,640
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	55,143	54,466
Southern Financial Capital Trust III - due 2033 [Member]
Schedule Of Borrowings [Line Items]
Junior subordinated debentures associated with preferred capital securities	$ 8,141	$ 8,051