Loans (Summary of Loan Portfolio) (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020
Loans [Line Items]
Total loans, net of unearned income	$ 33,264,043	$ 32,079,073	$ 32,135,555
Total loans, net of unearned income, percentage	100.00%	100.00%	100.00%
Commercial
Loans [Line Items]
Total loans, net of unearned income	$ 11,187,972	$ 11,955,967	$ 12,276,999
Total loans, net of unearned income, percentage	33.00%	37.00%	39.00%
Commercial real estate
Loans [Line Items]
Total loans, net of unearned income	$ 8,885,714	$ 8,494,132	$ 8,423,142
Total loans, net of unearned income, percentage	27.00%	26.00%	26.00%
Home equity
Loans [Line Items]
Total loans, net of unearned income	$ 347,662	$ 425,263	$ 446,274
Total loans, net of unearned income, percentage	1.00%	1.00%	1.00%
Residential real estate
Loans [Line Items]
Total loans, net of unearned income	$ 1,547,736	$ 1,259,598	$ 1,384,810
Total loans, net of unearned income, percentage	5.00%	5.00%	4.00%
Premium finance receivables | Commercial insurance loans
Loans [Line Items]
Total loans, net of unearned income	$ 4,616,977	$ 4,054,489	$ 4,060,144
Total loans, net of unearned income, percentage	14.00%	13.00%	13.00%
Premium finance receivables | Life insurance
Loans [Line Items]
Total loans, net of unearned income	$ 6,655,453	$ 5,857,436	$ 5,488,832
Total loans, net of unearned income, percentage	20.00%	18.00%	17.00%
Consumer and other
Loans [Line Items]
Total loans, net of unearned income	$ 22,529	$ 32,188	$ 55,354
Total loans, net of unearned income, percentage	0.00%	0.00%	0.00%