Loans (Summary of Loan Portfolio) (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018
Loans [Line Items]
Total loans, net of unearned income	$ 25,304,659	$ 23,820,691	$ 22,610,560
Total loans, net of unearned income, percentage	100.00%	100.00%	100.00%
Commercial
Loans [Line Items]
Total loans, net of unearned income	$ 8,270,774	$ 7,828,538	$ 7,289,060
Total loans, net of unearned income, percentage	33.00%	33.00%	32.00%
Commercial real estate
Loans [Line Items]
Total loans, net of unearned income	$ 7,276,244	$ 6,933,252	$ 6,575,084
Total loans, net of unearned income, percentage	29.00%	29.00%	29.00%
Home equity
Loans [Line Items]
Total loans, net of unearned income	$ 527,370	$ 552,343	$ 593,500
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	$ 1,118,178	$ 1,002,464	$ 895,470
Total loans, net of unearned income, percentage	4.00%	4.00%	4.00%
Premium finance receivables | Commercial
Loans [Line Items]
Total loans, net of unearned income	$ 3,368,423	$ 2,841,659	$ 2,833,452
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	$ 4,634,478	$ 4,541,794	$ 4,302,288
Total loans, net of unearned income, percentage	18.00%	19.00%	19.00%
Consumer and other
Loans [Line Items]
Total loans, net of unearned income	$ 109,192	$ 120,641	$ 121,706
Total loans, net of unearned income, percentage	1.00%	1.00%	1.00%