Loans (Summary of Loan Portfolio) (Detail) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021	Mar. 31, 2021
Loans [Line Items]
Total loans, net of unearned income	$ 35,280,547	$ 34,789,104	$ 33,171,233
Total loans, net of unearned income, percentage	100.00%	100.00%	100.00%
Commercial
Loans [Line Items]
Total loans, net of unearned income	$ 11,583,963	$ 11,904,068	$ 12,708,207
Total loans, net of unearned income, percentage	33.00%	34.00%	38.00%
Commercial real estate
Loans [Line Items]
Total loans, net of unearned income	$ 9,235,074	$ 8,990,286	$ 8,544,779
Total loans, net of unearned income, percentage	26.00%	26.00%	26.00%
Home equity
Loans [Line Items]
Total loans, net of unearned income	$ 321,435	$ 335,155	$ 390,253
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	$ 1,799,985	$ 1,637,099	$ 1,421,973
Total loans, net of unearned income, percentage	5.00%	5.00%	4.00%
Premium finance receivables | Property and casualty insurance
Loans [Line Items]
Total loans, net of unearned income	$ 4,937,408	$ 4,855,487	$ 3,958,543
Total loans, net of unearned income, percentage	14.00%	14.00%	12.00%
Premium finance receivables | Life insurance
Loans [Line Items]
Total loans, net of unearned income	$ 7,354,163	$ 7,042,810	$ 6,111,495
Total loans, net of unearned income, percentage	21.00%	20.00%	19.00%
Consumer and other
Loans [Line Items]
Total loans, net of unearned income	$ 48,519	$ 24,199	$ 35,983
Total loans, net of unearned income, percentage	0.00%	0.00%	0.00%