Allowance for Credit Losses (Schedule of Aging of the Company's Loan Portfolio) (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020
Credit Loss [Abstract]
Financial assets held at amortized cost, accrued interest	$ 128,800	$ 121,900	$ 116,400
Financing Receivable, Past Due [Line Items]
Nonaccrual	94,115	114,150	160,415
90+ days and still accruing	4,944	13,363	18,945
Current	32,892,465	31,770,847	27,332,294
Total Loans	33,171,233	32,079,073	27,807,321
60-89 days past due
Financing Receivable, Past Due [Line Items]
Past Due	28,005	41,585	32,999
30-59 days past due
Financing Receivable, Past Due [Line Items]
Past Due	151,704	139,128	262,668
Commercial
Financing Receivable, Past Due [Line Items]
Total Loans	12,708,207	11,955,967	9,025,886
Commercial | Commercial, industrial and other, excluding PPP loans
Financing Receivable, Past Due [Line Items]
Nonaccrual	22,459	21,743	49,916
90+ days and still accruing	0	307	1,241
Current	9,343,939	9,166,751	8,879,727
Total Loans	9,415,225	9,240,046	9,025,886
Commercial | Commercial PPP loans
Financing Receivable, Past Due [Line Items]
Nonaccrual	0	0	0
90+ days and still accruing	0	0	0
Current	3,292,976	2,715,885	0
Total Loans	3,292,982	2,715,921	0
Commercial | 60-89 days past due | Commercial, industrial and other, excluding PPP loans
Financing Receivable, Past Due [Line Items]
Past Due	13,292	6,900	8,873
Commercial | 60-89 days past due | Commercial PPP loans
Financing Receivable, Past Due [Line Items]
Past Due	0	0	0
Commercial | 30-59 days past due | Commercial, industrial and other, excluding PPP loans
Financing Receivable, Past Due [Line Items]
Past Due	35,535	44,345	86,129
Commercial | 30-59 days past due | Commercial PPP loans
Financing Receivable, Past Due [Line Items]
Past Due	6	36	0
Commercial real estate
Financing Receivable, Past Due [Line Items]
Total Loans	8,544,779	8,494,132	8,185,531
Commercial real estate | Construction and development
Financing Receivable, Past Due [Line Items]
Nonaccrual	2,673	5,633	7,422
90+ days and still accruing	0	0	147
Current	1,326,185	1,360,825	1,274,987
Total Loans	1,353,324	1,371,802	1,301,353
Commercial real estate | Non-construction
Financing Receivable, Past Due [Line Items]
Nonaccrual	31,707	40,474	55,408
90+ days and still accruing	0	0	369
Current	7,105,890	7,049,906	6,761,918
Total Loans	7,191,455	7,122,330	6,884,178
Commercial real estate | 60-89 days past due | Construction and development
Financing Receivable, Past Due [Line Items]
Past Due	499	0	1,859
Commercial real estate | 60-89 days past due | Non-construction
Financing Receivable, Past Due [Line Items]
Past Due	7,657	5,178	8,353
Commercial real estate | 30-59 days past due | Construction and development
Financing Receivable, Past Due [Line Items]
Past Due	23,967	5,344	16,938
Commercial real estate | 30-59 days past due | Non-construction
Financing Receivable, Past Due [Line Items]
Past Due	46,201	26,772	58,130
Home equity
Financing Receivable, Past Due [Line Items]
Nonaccrual	5,536	6,529	7,243
90+ days and still accruing	0	0	0
Current	383,445	418,050	485,102
Total Loans	390,253	425,263	494,655
Home equity | 60-89 days past due
Financing Receivable, Past Due [Line Items]
Past Due	492	47	214
Home equity | 30-59 days past due
Financing Receivable, Past Due [Line Items]
Past Due	780	637	2,096
Residential real estate
Financing Receivable, Past Due [Line Items]
Nonaccrual	21,553	26,071	18,965
90+ days and still accruing	0	0	605
Current	1,385,708	1,219,308	1,328,491
Total Loans	1,421,973	1,259,598	1,377,389
Residential real estate | 60-89 days past due
Financing Receivable, Past Due [Line Items]
Past Due	944	1,635	345
Residential real estate | 30-59 days past due
Financing Receivable, Past Due [Line Items]
Past Due	13,768	12,584	28,983
Premium finance receivables | Commercial insurance loans
Financing Receivable, Past Due [Line Items]
Nonaccrual	9,690	13,264	21,058
90+ days and still accruing	4,592	12,792	16,505
Current	3,922,596	4,002,826	3,384,354
Total Loans	3,958,543	4,054,489	3,465,055
Premium finance receivables | Life insurance loans
Financing Receivable, Past Due [Line Items]
Nonaccrual	0	0	0
90+ days and still accruing	191	0	0
Current	6,096,483	5,805,968	5,181,862
Total Loans	6,111,495	5,857,436	5,221,639
Premium finance receivables | 60-89 days past due | Commercial insurance loans
Financing Receivable, Past Due [Line Items]
Past Due	5,113	6,798	10,327
Premium finance receivables | 60-89 days past due | Life insurance loans
Financing Receivable, Past Due [Line Items]
Past Due	0	21,003	2,403
Premium finance receivables | 30-59 days past due | Commercial insurance loans
Financing Receivable, Past Due [Line Items]
Past Due	16,552	18,809	32,811
Premium finance receivables | 30-59 days past due | Life insurance loans
Financing Receivable, Past Due [Line Items]
Past Due	14,821	30,465	37,374
Consumer and other
Financing Receivable, Past Due [Line Items]
Nonaccrual	497	436	403
90+ days and still accruing	161	264	78
Current	35,243	31,328	35,853
Total Loans	35,983	32,188	37,166
Consumer and other | 60-89 days past due
Financing Receivable, Past Due [Line Items]
Past Due	8	24	625
Consumer and other | 30-59 days past due
Financing Receivable, Past Due [Line Items]
Past Due	$ 74	$ 136	$ 207