Loans (Summary of Loan Portfolio) (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018
Loans [Line Items]
Total loans, net of unearned income	$ 25,710,171	$ 23,820,691	$ 23,123,951
Total loans, net of unearned income, percentage	100.00%	100.00%	100.00%
Commercial
Loans [Line Items]
Total loans, net of unearned income	$ 8,195,602	$ 7,828,538	$ 7,473,958
Total loans, net of unearned income, percentage	32.00%	33.00%	32.00%
Commercial real estate
Loans [Line Items]
Total loans, net of unearned income	$ 7,448,667	$ 6,933,252	$ 6,746,774
Total loans, net of unearned income, percentage	29.00%	29.00%	29.00%
Home equity
Loans [Line Items]
Total loans, net of unearned income	$ 512,303	$ 552,343	$ 578,844
Total loans, net of unearned income, percentage	2.00%	2.00%	3.00%
Residential real estate
Loans [Line Items]
Total loans, net of unearned income	$ 1,218,666	$ 1,002,464	$ 924,250
Total loans, net of unearned income, percentage	5.00%	4.00%	4.00%
Premium finance receivables | Commercial insurance loans
Loans [Line Items]
Total loans, net of unearned income	$ 3,449,950	$ 2,841,659	$ 2,885,327
Total loans, net of unearned income, percentage	13.00%	12.00%	12.00%
Premium finance receivables | Life insurance
Loans [Line Items]
Total loans, net of unearned income	$ 4,795,496	$ 4,541,794	$ 4,398,971
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	$ 89,487	$ 120,641	$ 115,827
Total loans, net of unearned income, percentage	0.00%	1.00%	1.00%