Borrowings - Long-term Borrowings (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Schedule Of Borrowings [Line Items]
Fixed rates	$ 1,154,737	$ 1,158,216
Agreements to repurchase securities	1,084,694	1,899,281
Other	12,010	20,902
Long-term borrowings	9,493,835	10,653,858
Variable Rate Notes Due 2016 [Member]
Schedule Of Borrowings [Line Items]
Variable rates		300,000
Variable Rate Term Loans Due 2017 [Member]
Schedule Of Borrowings [Line Items]
Variable rates	550,000	550,000
1.25% Due 2017 [Member]
Schedule Of Borrowings [Line Items]
Variable rates	499,999	499,984
1.40% Due 2017 [Member]
Schedule Of Borrowings [Line Items]
Variable rates	749,946	749,851
1.45% Due 2018 [Member]
Schedule Of Borrowings [Line Items]
Variable rates	501,829	503,527
2.25% Due 2019 [Member]
Schedule Of Borrowings [Line Items]
Variable rates	649,012	648,628
2.30% Due 2019 [Member]
Schedule Of Borrowings [Line Items]
Variable rates	749,473	749,219
2.10% Due 2020 [Member]
Schedule Of Borrowings [Line Items]
Variable rates	749,735	749,650
2.90% Due 2025 [Member]
Schedule Of Borrowings [Line Items]
Variable rates	749,320	749,236
8.5% due 2018 [Member]
Schedule Of Borrowings [Line Items]
Subordinated notes	207,651	213,417
6.625% due 2017 [Member]
Schedule Of Borrowings [Line Items]
Subordinated notes	409,526	419,800
5.585% due 2020, Variable Rate Commenced 2015 [Member]
Schedule Of Borrowings [Line Items]
Subordinated notes	409,361	409,361
5.629% due 2021, Variable Rate Commenced 2016 [Member]
Schedule Of Borrowings [Line Items]
Subordinated notes	500,000	518,797
BSB Capital Trust I - 8.125%, due 2028 [Member]
Schedule Of Borrowings [Line Items]
Junior subordinated debentures associated with preferred capital securities	15,659	15,635
Provident Trust I - 8.29%, due 2028 [Member]
Schedule Of Borrowings [Line Items]
Junior subordinated debentures associated with preferred capital securities	26,293	25,817
Southern Financial Statutory Trust I - 10.60%, due 2030 [Member]
Schedule Of Borrowings [Line Items]
Junior subordinated debentures associated with preferred capital securities	6,620	6,583
First Maryland Capital I - due 2027 [Member]
Schedule Of Borrowings [Line Items]
Junior subordinated debentures associated with preferred capital securities	146,256	145,717
First Maryland Capital II - due 2027 [Member]
Schedule Of Borrowings [Line Items]
Junior subordinated debentures associated with preferred capital securities	147,954	147,291
Allfirst Asset Trust - due 2029 [Member]
Schedule Of Borrowings [Line Items]
Junior subordinated debentures associated with preferred capital securities	96,494	96,349
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	53,834	53,244
Southern Financial Capital Trust III - due 2033 [Member]
Schedule Of Borrowings [Line Items]
Junior subordinated debentures associated with preferred capital securities	$ 7,968	$ 7,889