Loans (Summary of Loan Portfolio) (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019
Loans [Line Items]
Total loans, net of unearned income	$ 32,135,555	$ 26,800,290	$ 25,710,171
Total loans, net of unearned income, percentage	100.00%	100.00%	100.00%
Commercial
Loans [Line Items]
Total loans, net of unearned income	$ 12,276,999	$ 8,285,920	$ 8,195,602
Total loans, net of unearned income, percentage	39.00%	31.00%	32.00%
Commercial real estate
Loans [Line Items]
Total loans, net of unearned income	$ 8,423,142	$ 8,020,276	$ 7,448,667
Total loans, net of unearned income, percentage	26.00%	30.00%	29.00%
Home equity
Loans [Line Items]
Total loans, net of unearned income	$ 446,274	$ 513,066	$ 512,303
Total loans, net of unearned income, percentage	1.00%	2.00%	2.00%
Residential real estate
Loans [Line Items]
Total loans, net of unearned income	$ 1,384,810	$ 1,354,221	$ 1,218,666
Total loans, net of unearned income, percentage	4.00%	5.00%	5.00%
Premium finance receivables | Commercial insurance loans
Loans [Line Items]
Total loans, net of unearned income	$ 4,060,144	$ 3,442,027	$ 3,449,950
Total loans, net of unearned income, percentage	13.00%	13.00%	13.00%
Premium finance receivables | Life insurance
Loans [Line Items]
Total loans, net of unearned income	$ 5,488,832	$ 5,074,602	$ 4,795,496
Total loans, net of unearned income, percentage	17.00%	19.00%	19.00%
Consumer and other
Loans [Line Items]
Total loans, net of unearned income	$ 55,354	$ 110,178	$ 89,487
Total loans, net of unearned income, percentage	0.00%	0.00%	0.00%