Loans (Summary of Loan Portfolio) (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Loans [Line Items]
Loans, net of unearned income, excluding covered loans	$ 23,123,951	$ 21,640,797	$ 20,912,781
Covered loans	0	0	46,601
Total loans	$ 23,123,951	$ 21,640,797	$ 20,959,382
Total loans, net of unearned income, excluding covered loans	100.00%	100.00%	100.00%
Covered loans	0.00%	0.00%	0.00%
Total loans	100.00%	100.00%	100.00%
Commercial
Loans [Line Items]
Loans, net of unearned income, excluding covered loans	$ 7,473,958	$ 6,787,677	$ 6,456,034
Total loans, net of unearned income, excluding covered loans	32.00%	31.00%	31.00%
Commercial real estate
Loans [Line Items]
Loans, net of unearned income, excluding covered loans	$ 6,746,774	$ 6,580,618	$ 6,400,781
Total loans, net of unearned income, excluding covered loans	29.00%	30.00%	31.00%
Home equity
Loans [Line Items]
Loans, net of unearned income, excluding covered loans	$ 578,844	$ 663,045	$ 672,969
Total loans, net of unearned income, excluding covered loans	3.00%	3.00%	3.00%
Residential real estate
Loans [Line Items]
Loans, net of unearned income, excluding covered loans	$ 924,250	$ 832,120	$ 789,499
Total loans, net of unearned income, excluding covered loans	4.00%	4.00%	3.00%
Premium finance receivables—commercial | Commercial insurance loans
Loans [Line Items]
Loans, net of unearned income, excluding covered loans	$ 2,885,327	$ 2,634,565	$ 2,664,912
Total loans, net of unearned income, excluding covered loans	12.00%	12.00%	13.00%
Premium finance receivables—commercial | Life insurance
Loans [Line Items]
Loans, net of unearned income, excluding covered loans	$ 4,398,971	$ 4,035,059	$ 3,795,474
Total loans, net of unearned income, excluding covered loans	19.00%	19.00%	18.00%
Consumer and other
Loans [Line Items]
Loans, net of unearned income, excluding covered loans	$ 115,827	$ 107,713	$ 133,112
Total loans, net of unearned income, excluding covered loans	1.00%	1.00%	1.00%