Borrowings - Long-term Borrowings (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Schedule Of Borrowings [Line Items]
Fixed rates advances from FHLB	$ 576,876	$ 1,154,737
Agreements to repurchase securities	421,771	1,084,694
Other	9,635	12,010
Long-term borrowings	8,141,430	9,493,835
Variable Rate Term Loans Due 2017 [Member]
Schedule Of Borrowings [Line Items]
Variable\fixed rates term loan amount		550,000
Variable Rate Term Loans Due 2022 [Member]
Schedule Of Borrowings [Line Items]
Variable\fixed rates term loan amount	249,558
Fixed Rate Term Loans 1.25% Due 2017 [Member]
Schedule Of Borrowings [Line Items]
Variable\fixed rates term loan amount		499,999
Fixed Rate Term Loans 1.40% Due 2017 [Member]
Schedule Of Borrowings [Line Items]
Variable\fixed rates term loan amount		749,946
Fixed Rate Term Loans 1.45% Due 2018 [Member]
Schedule Of Borrowings [Line Items]
Variable\fixed rates term loan amount	499,907	501,829
Fixed Rate Term Loans 2.25% Due 2019 [Member]
Schedule Of Borrowings [Line Items]
Variable\fixed rates term loan amount	644,977	649,012
Fixed Rate Term Loans 2.30% Due 2019 [Member]
Schedule Of Borrowings [Line Items]
Variable\fixed rates term loan amount	746,919	749,473
Fixed Rate Term Loans 2.10% Due 2020 [Member]
Schedule Of Borrowings [Line Items]
Variable\fixed rates term loan amount	743,788	749,735
Fixed Rate Term Loans 2.05% Due 2020 [Member]
Schedule Of Borrowings [Line Items]
Variable\fixed rates term loan amount	739,961
Fixed Rate Term Loans 2.50% Due 2022 [Member]
Schedule Of Borrowings [Line Items]
Variable\fixed rates term loan amount	638,872
Fixed Rate Term Loans 2.90% Due 2025 [Member]
Schedule Of Borrowings [Line Items]
Variable\fixed rates term loan amount	749,404	749,320
8.5% due 2018 [Member]
Schedule Of Borrowings [Line Items]
Subordinated notes	200,000	207,651
6.625% due 2017 [Member]
Schedule Of Borrowings [Line Items]
Subordinated notes		409,526
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	500,000
3.40% due 2027 [Member]
Schedule Of Borrowings [Line Items]
Subordinated notes	491,176
BSB Capital Trust I - 8.125%, due 2028 [Member]
Schedule Of Borrowings [Line Items]
Junior subordinated debentures associated with preferred capital securities	15,682	15,659
Provident Trust I - 8.29%, due 2028 [Member]
Schedule Of Borrowings [Line Items]
Junior subordinated debentures associated with preferred capital securities	26,847	26,293
Southern Financial Statutory Trust I - 10.60%, due 2030 [Member]
Schedule Of Borrowings [Line Items]
Junior subordinated debentures associated with preferred capital securities	6,664	6,620
First Maryland Capital I - due 2027 [Member]
Schedule Of Borrowings [Line Items]
Junior subordinated debentures associated with preferred capital securities	146,794	146,256
First Maryland Capital II - due 2027 [Member]
Schedule Of Borrowings [Line Items]
Junior subordinated debentures associated with preferred capital securities	148,617	147,954
Allfirst Asset Trust - due 2029 [Member]
Schedule Of Borrowings [Line Items]
Junior subordinated debentures associated with preferred capital securities	96,640	96,494
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	54,466	53,834
Southern Financial Capital Trust III - due 2033 [Member]
Schedule Of Borrowings [Line Items]
Junior subordinated debentures associated with preferred capital securities	$ 8,051	$ 7,968