Allowance for Credit Losses (Schedule of Aging of the Company's Loan Portfolio) (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019
Financing Receivable, Past Due [Line Items]
Nonaccrual	$ 160,751	$ 105,908	$ 103,619
90+ days and still accruing	12,352	34,376	19,347
Current	31,725,993	26,361,332	25,401,935
Total Loans	32,135,555	26,800,290	25,710,171
60-89 days past due
Financing Receivable, Past Due [Line Items]
Past Due	49,910	50,523	51,113
30-59 days past due
Financing Receivable, Past Due [Line Items]
Past Due	186,549	248,151	134,157
Commercial
Financing Receivable, Past Due [Line Items]
Nonaccrual	42,036	37,224	43,931
90+ days and still accruing	0	1,855	382
Current	8,805,511	8,166,242	8,086,942
Total Loans	8,897,986	8,285,920	8,195,602
Commercial | 60-89 days past due
Financing Receivable, Past Due [Line Items]
Past Due	2,168	3,275	12,860
Commercial | 30-59 days past due
Financing Receivable, Past Due [Line Items]
Past Due	48,271	77,324	51,487
Commercial PPP Loans
Financing Receivable, Past Due [Line Items]
Nonaccrual	0	0	0
90+ days and still accruing	0	0	0
Current	3,379,013	0	0
Total Loans	3,379,013	0	0
Commercial PPP Loans | 60-89 days past due
Financing Receivable, Past Due [Line Items]
Past Due	0	0	0
Commercial PPP Loans | 30-59 days past due
Financing Receivable, Past Due [Line Items]
Past Due	0	0	0
Construction and development
Financing Receivable, Past Due [Line Items]
Nonaccrual	10,980	2,112	2,024
90+ days and still accruing	0	3,514	576
Current	1,310,019	1,223,567	1,032,473
Total Loans	1,333,149	1,283,449	1,042,211
Construction and development | 60-89 days past due
Financing Receivable, Past Due [Line Items]
Past Due	0	5,292	88
Construction and development | 30-59 days past due
Financing Receivable, Past Due [Line Items]
Past Due	12,150	48,964	7,050
Non-construction
Financing Receivable, Past Due [Line Items]
Nonaccrual	57,835	24,001	19,533
90+ days and still accruing	0	11,432	4,416
Current	6,982,547	6,626,537	6,346,918
Total Loans	7,089,993	6,736,827	6,406,456
Non-construction | 60-89 days past due
Financing Receivable, Past Due [Line Items]
Past Due	8,299	26,254	9,541
Non-construction | 30-59 days past due
Financing Receivable, Past Due [Line Items]
Past Due	41,312	48,603	26,048
Home equity
Financing Receivable, Past Due [Line Items]
Nonaccrual	6,329	7,363	7,920
90+ days and still accruing	0	0	0
Current	438,727	501,716	501,188
Total Loans	446,274	513,066	512,303
Home equity | 60-89 days past due
Financing Receivable, Past Due [Line Items]
Past Due	70	454	95
Home equity | 30-59 days past due
Financing Receivable, Past Due [Line Items]
Past Due	1,148	3,533	3,100
Residential real estate
Financing Receivable, Past Due [Line Items]
Nonaccrual	22,069	13,797	13,447
90+ days and still accruing	0	5,771	3,244
Current	1,359,484	1,313,523	1,198,674
Total Loans	1,384,810	1,354,221	1,218,666
Residential real estate | 60-89 days past due
Financing Receivable, Past Due [Line Items]
Past Due	814	3,089	1,868
Residential real estate | 30-59 days past due
Financing Receivable, Past Due [Line Items]
Past Due	2,443	18,041	1,433
Premium finance receivables - commercial
Financing Receivable, Past Due [Line Items]
Nonaccrual	21,080	20,590	15,950
90+ days and still accruing	12,177	11,517	10,612
Current	4,000,553	3,379,018	3,397,563
Total Loans	4,060,144	3,442,027	3,449,950
Premium finance receivables - commercial | 60-89 days past due
Financing Receivable, Past Due [Line Items]
Past Due	7,495	12,119	8,853
Premium finance receivables - commercial | 30-59 days past due
Financing Receivable, Past Due [Line Items]
Past Due	18,839	18,783	16,972
Premium finance receivables - life
Financing Receivable, Past Due [Line Items]
Nonaccrual	0	590	590
90+ days and still accruing	0	0	0
Current	5,396,148	5,041,453	4,749,358
Total Loans	5,488,832	5,074,602	4,795,496
Premium finance receivables - life | 60-89 days past due
Financing Receivable, Past Due [Line Items]
Past Due	30,791	0	17,753
Premium finance receivables - life | 30-59 days past due
Financing Receivable, Past Due [Line Items]
Past Due	61,893	32,559	27,795
Consumer and other
Financing Receivable, Past Due [Line Items]
Nonaccrual	422	231	224
90+ days and still accruing	175	287	117
Current	53,991	109,276	88,819
Total Loans	55,354	110,178	89,487
Consumer and other | 60-89 days past due
Financing Receivable, Past Due [Line Items]
Past Due	273	40	55
Consumer and other | 30-59 days past due
Financing Receivable, Past Due [Line Items]
Past Due	$ 493	$ 344	$ 272