Loans (Summary of Loan Portfolio) (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Loans [Line Items]
Loans, net of unearned income, excluding covered loans	$ 22,062,134	$ 21,640,797	$ 19,931,058
Covered loans	0	0	52,359
Total loans	$ 22,062,134	$ 21,640,797	$ 19,983,417
Total loans, net of unearned income, excluding covered loans	100.00%	100.00%	100.00%
Covered loans	0.00%	0.00%	0.00%
Total loans	100.00%	100.00%	100.00%
Commercial
Loans [Line Items]
Loans, net of unearned income, excluding covered loans	$ 7,060,871	$ 6,787,677	$ 6,081,489
Total loans, net of unearned income, excluding covered loans	32.00%	31.00%	30.00%
Commercial real estate
Loans [Line Items]
Loans, net of unearned income, excluding covered loans	$ 6,633,520	$ 6,580,618	$ 6,261,682
Total loans, net of unearned income, excluding covered loans	30.00%	30.00%	31.00%
Home equity
Loans [Line Items]
Loans, net of unearned income, excluding covered loans	$ 626,547	$ 663,045	$ 708,258
Total loans, net of unearned income, excluding covered loans	3.00%	3.00%	4.00%
Residential real estate
Loans [Line Items]
Loans, net of unearned income, excluding covered loans	$ 869,104	$ 832,120	$ 720,608
Total loans, net of unearned income, excluding covered loans	4.00%	4.00%	4.00%
Premium finance receivables—commercial | Commercial insurance loans
Loans [Line Items]
Loans, net of unearned income, excluding covered loans	$ 2,576,150	$ 2,634,565	$ 2,446,946
Total loans, net of unearned income, excluding covered loans	12.00%	12.00%	12.00%
Premium finance receivables—commercial | Life insurance
Loans [Line Items]
Loans, net of unearned income, excluding covered loans	$ 4,189,961	$ 4,035,059	$ 3,593,563
Total loans, net of unearned income, excluding covered loans	19.00%	19.00%	18.00%
Consumer and other
Loans [Line Items]
Loans, net of unearned income, excluding covered loans	$ 105,981	$ 107,713	$ 118,512
Total loans, net of unearned income, excluding covered loans	0.00%	1.00%	1.00%