Loans (Summary of Loan Portfolio) (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019
Loans [Line Items]
Total loans, net of unearned income	$ 27,807,321	$ 26,800,290	$ 24,214,629
Total loans, net of unearned income, percentage	100.00%	100.00%	100.00%
Commercial
Loans [Line Items]
Total loans, net of unearned income	$ 9,025,886	$ 8,285,920	$ 7,994,191
Total loans, net of unearned income, percentage	32.00%	31.00%	33.00%
Commercial real estate
Loans [Line Items]
Total loans, net of unearned income	$ 8,185,531	$ 8,020,276	$ 6,973,505
Total loans, net of unearned income, percentage	29.00%	30.00%	29.00%
Home equity
Loans [Line Items]
Total loans, net of unearned income	$ 494,655	$ 513,066	$ 528,448
Total loans, net of unearned income, percentage	2.00%	2.00%	2.00%
Residential real estate
Loans [Line Items]
Total loans, net of unearned income	$ 1,377,389	$ 1,354,221	$ 1,053,524
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	$ 3,465,055	$ 3,442,027	$ 2,988,788
Total loans, net of unearned income, percentage	13.00%	13.00%	12.00%
Premium finance receivables | Life insurance
Loans [Line Items]
Total loans, net of unearned income	$ 5,221,639	$ 5,074,602	$ 4,555,369
Total loans, net of unearned income, percentage	19.00%	19.00%	19.00%
Consumer and other
Loans [Line Items]
Total loans, net of unearned income	$ 37,166	$ 110,178	$ 120,804
Total loans, net of unearned income, percentage	0.00%	0.00%	1.00%