Allowance for Credit Losses (Schedule of Aging of the Company's Loan Portfolio) (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020
Credit Loss [Abstract]
Financial assets held at amortized cost, accrued interest	$ 119,200	$ 121,900	$ 116,900
Financing Receivable, Past Due [Line Items]
Nonaccrual	83,940	114,150	160,751
90+ days and still accruing	6,101	13,363	12,352
Loans, net of unearned income	33,264,043	32,079,073	32,135,555
60-89 days past due
Financing Receivable, Past Due [Line Items]
Loans, net of unearned income	32,863	41,585	49,910
30-59 days past due
Financing Receivable, Past Due [Line Items]
Loans, net of unearned income	128,764	139,128	186,549
Current
Financing Receivable, Past Due [Line Items]
Loans, net of unearned income	33,012,375	31,770,847	31,725,993
Commercial
Financing Receivable, Past Due [Line Items]
Loans, net of unearned income	11,187,972	11,955,967	12,276,999
Commercial | Commercial, industrial and other
Financing Receivable, Past Due [Line Items]
Nonaccrual	26,468	21,743	42,036
90+ days and still accruing	0	307	0
Loans, net of unearned income	10,105,984	9,240,046	8,897,986
Commercial | Commercial, industrial and other | 60-89 days past due
Financing Receivable, Past Due [Line Items]
Loans, net of unearned income	9,768	6,900	2,168
Commercial | Commercial, industrial and other | 30-59 days past due
Financing Receivable, Past Due [Line Items]
Loans, net of unearned income	24,086	44,345	48,271
Commercial | Commercial, industrial and other | Current
Financing Receivable, Past Due [Line Items]
Loans, net of unearned income	10,045,662	9,166,751	8,805,511
Commercial | Commercial PPP loans
Financing Receivable, Past Due [Line Items]
Nonaccrual	0	0	0
90+ days and still accruing	0	0	0
Loans, net of unearned income	1,081,988	2,715,921	3,379,013
Commercial | Commercial PPP loans | 60-89 days past due
Financing Receivable, Past Due [Line Items]
Loans, net of unearned income	0	0	0
Commercial | Commercial PPP loans | 30-59 days past due
Financing Receivable, Past Due [Line Items]
Loans, net of unearned income	1,138	36	0
Commercial | Commercial PPP loans | Current
Financing Receivable, Past Due [Line Items]
Loans, net of unearned income	1,080,850	2,715,885	3,379,013
Commercial real estate
Financing Receivable, Past Due [Line Items]
Loans, net of unearned income	8,885,714	8,494,132	8,423,142
Commercial real estate | Construction and development
Financing Receivable, Past Due [Line Items]
Nonaccrual	1,030	5,633	10,980
90+ days and still accruing	0	0	0
Loans, net of unearned income	1,343,715	1,371,802	1,333,149
Commercial real estate | Construction and development | 60-89 days past due
Financing Receivable, Past Due [Line Items]
Loans, net of unearned income	0	0	0
Commercial real estate | Construction and development | 30-59 days past due
Financing Receivable, Past Due [Line Items]
Loans, net of unearned income	12,631	5,344	12,150
Commercial real estate | Construction and development | Current
Financing Receivable, Past Due [Line Items]
Loans, net of unearned income	1,330,054	1,360,825	1,310,019
Commercial real estate | Non-construction
Financing Receivable, Past Due [Line Items]
Nonaccrual	22,676	40,474	57,835
90+ days and still accruing	0	0	0
Loans, net of unearned income	7,541,999	7,122,330	7,089,993
Commercial real estate | Non-construction | 60-89 days past due
Financing Receivable, Past Due [Line Items]
Loans, net of unearned income	5,395	5,178	8,299
Commercial real estate | Non-construction | 30-59 days past due
Financing Receivable, Past Due [Line Items]
Loans, net of unearned income	67,187	26,772	41,312
Commercial real estate | Non-construction | Current
Financing Receivable, Past Due [Line Items]
Loans, net of unearned income	7,446,741	7,049,906	6,982,547
Home equity
Financing Receivable, Past Due [Line Items]
Nonaccrual	3,449	6,529	6,329
90+ days and still accruing	164	0	0
Loans, net of unearned income	347,662	425,263	446,274
Home equity | 60-89 days past due
Financing Receivable, Past Due [Line Items]
Loans, net of unearned income	340	47	70
Home equity | 30-59 days past due
Financing Receivable, Past Due [Line Items]
Loans, net of unearned income	867	637	1,148
Home equity | Current
Financing Receivable, Past Due [Line Items]
Loans, net of unearned income	342,842	418,050	438,727
Residential real estate
Financing Receivable, Past Due [Line Items]
Nonaccrual	22,633	26,071	22,069
90+ days and still accruing	0	0	0
Loans, net of unearned income	1,547,736	1,259,598	1,384,810
Residential real estate | 60-89 days past due
Financing Receivable, Past Due [Line Items]
Loans, net of unearned income	1,540	1,635	814
Residential real estate | 30-59 days past due
Financing Receivable, Past Due [Line Items]
Loans, net of unearned income	1,076	12,584	2,443
Residential real estate | Current
Financing Receivable, Past Due [Line Items]
Loans, net of unearned income	1,522,487	1,219,308	1,359,484
Premium finance receivables | Commercial insurance loans
Financing Receivable, Past Due [Line Items]
Nonaccrual	7,300	13,264	21,080
90+ days and still accruing	5,811	12,792	12,177
Loans, net of unearned income	4,616,977	4,054,489	4,060,144
Premium finance receivables | Commercial insurance loans | 60-89 days past due
Financing Receivable, Past Due [Line Items]
Loans, net of unearned income	10,642	6,798	7,495
Premium finance receivables | Commercial insurance loans | 30-59 days past due
Financing Receivable, Past Due [Line Items]
Loans, net of unearned income	14,614	18,809	18,839
Premium finance receivables | Commercial insurance loans | Current
Financing Receivable, Past Due [Line Items]
Loans, net of unearned income	4,578,610	4,002,826	4,000,553
Premium finance receivables | Life insurance loans
Financing Receivable, Past Due [Line Items]
Nonaccrual	0	0	0
90+ days and still accruing	0	0	0
Loans, net of unearned income	6,655,453	5,857,436	5,488,832
Premium finance receivables | Life insurance loans | 60-89 days past due
Financing Receivable, Past Due [Line Items]
Loans, net of unearned income	5,162	21,003	30,791
Premium finance receivables | Life insurance loans | 30-59 days past due
Financing Receivable, Past Due [Line Items]
Loans, net of unearned income	7,040	30,465	61,893
Premium finance receivables | Life insurance loans | Current
Financing Receivable, Past Due [Line Items]
Loans, net of unearned income	6,643,251	5,805,968	5,396,148
Consumer and other
Financing Receivable, Past Due [Line Items]
Nonaccrual	384	436	422
90+ days and still accruing	126	264	175
Loans, net of unearned income	22,529	32,188	55,354
Consumer and other | 60-89 days past due
Financing Receivable, Past Due [Line Items]
Loans, net of unearned income	16	24	273
Consumer and other | 30-59 days past due
Financing Receivable, Past Due [Line Items]
Loans, net of unearned income	125	136	493
Consumer and other | Current
Financing Receivable, Past Due [Line Items]
Loans, net of unearned income	$ 21,878	$ 31,328	$ 53,991