Borrowings - Long-term Borrowings (Parenthetical) (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Schedule Of Borrowings [Line Items]
Debt Maturity, Start Date	Jan. 01, 2027
Debt Maturity, End Date	Dec. 31, 2033
Variable Rate Due 2016 [Member]
Schedule Of Borrowings [Line Items]
Debt maturity date	2017	2017
Senior Notes Due 2017 [Member]
Schedule Of Borrowings [Line Items]
Debt maturity date	2022	2022
Fixed Rate Term Loans 1.25% Due 2017 [Member]
Schedule Of Borrowings [Line Items]
Debt maturity date	2017	2017
Interest rate of debt instrument	1.25%	1.25%
Fixed Rate Term Loans 1.40% Due 2017 [Member]
Schedule Of Borrowings [Line Items]
Debt maturity date	2017	2017
Interest rate of debt instrument	1.40%	1.40%
Fixed Rate Term Loans 1.45% Due 2018 [Member]
Schedule Of Borrowings [Line Items]
Debt maturity date	2018	2018
Interest rate of debt instrument	1.45%	1.45%
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 2.30% Due 2019 [Member]
Schedule Of Borrowings [Line Items]
Debt maturity date	2019	2019
Interest rate of debt instrument	2.30%	2.30%
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 2.90% 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%
8.5% due 2018 [Member]
Schedule Of Borrowings [Line Items]
Debt maturity date	2018	2018
Interest rate of debt instrument	8.50%	8.50%
6.625% due 2017 [Member]
Schedule Of Borrowings [Line Items]
Debt maturity date	2017	2017
Interest rate of debt instrument	6.625%	6.625%
5.585% due 2020, Variable Rate Commenced 2015 [Member]
Schedule Of Borrowings [Line Items]
Interest rate of debt instrument	5.585%	5.585%
Debt Maturity, Start Date	Jan. 01, 2015	Jan. 01, 2015
Debt Maturity, End Date	Dec. 31, 2020	Dec. 31, 2020
5.629% due 2021, Variable Rate Commenced 2016 [Member]
Schedule Of Borrowings [Line Items]
Interest rate of debt instrument	5.629%	5.629%
Debt Maturity, Start Date	Jan. 01, 2016	Jan. 01, 2016
Debt Maturity, End Date	Dec. 31, 2021	Dec. 31, 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