Allowance for Credit Losses (Schedule of Aging of the Company's Loan Portfolio) (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019
Financing Receivable, Past Due [Line Items]
Nonaccrual	$ 160,415	$ 105,908	$ 110,612
90+ days and still accruing	18,945	34,376	15,251
Current	27,332,294	26,361,332	23,893,423
Total Loans	27,807,321	26,800,290	24,214,629
60-89 days past due
Financing Receivable, Past Due [Line Items]
Past Due	32,999	50,523	19,181
30-59 days past due
Financing Receivable, Past Due [Line Items]
Past Due	262,668	248,151	176,162
Commercial
Financing Receivable, Past Due [Line Items]
Nonaccrual	49,916	37,224	55,792
90+ days and still accruing	1,241	1,855	2,499
Current	8,879,727	8,166,242	7,884,413
Total Loans	9,025,886	8,285,920	7,994,191
Commercial | 60-89 days past due
Financing Receivable, Past Due [Line Items]
Past Due	8,873	3,275	1,787
Commercial | 30-59 days past due
Financing Receivable, Past Due [Line Items]
Past Due	86,129	77,324	49,700
Construction and development
Financing Receivable, Past Due [Line Items]
Nonaccrual	7,422	2,112	1,084
90+ days and still accruing	147	3,514	621
Current	1,274,987	1,223,567	946,614
Total Loans	1,301,353	1,283,449	956,694
Construction and development | 60-89 days past due
Financing Receivable, Past Due [Line Items]
Past Due	1,859	5,292	496
Construction and development | 30-59 days past due
Financing Receivable, Past Due [Line Items]
Past Due	16,938	48,964	7,879
Non-construction
Financing Receivable, Past Due [Line Items]
Nonaccrual	55,408	24,001	14,849
90+ days and still accruing	369	11,432	3,644
Current	6,761,918	6,626,537	5,946,209
Total Loans	6,884,178	6,736,827	6,016,811
Non-construction | 60-89 days past due
Financing Receivable, Past Due [Line Items]
Past Due	8,353	26,254	5,116
Non-construction | 30-59 days past due
Financing Receivable, Past Due [Line Items]
Past Due	58,130	48,603	46,993
Home equity
Financing Receivable, Past Due [Line Items]
Nonaccrual	7,243	7,363	7,885
90+ days and still accruing	0	0	0
Current	485,102	501,716	515,438
Total Loans	494,655	513,066	528,448
Home equity | 60-89 days past due
Financing Receivable, Past Due [Line Items]
Past Due	214	454	810
Home equity | 30-59 days past due
Financing Receivable, Past Due [Line Items]
Past Due	2,096	3,533	4,315
Residential real estate
Financing Receivable, Past Due [Line Items]
Nonaccrual	18,965	13,797	15,879
90+ days and still accruing	605	5,771	1,481
Current	1,328,491	1,313,523	1,024,543
Total Loans	1,377,389	1,354,221	1,053,524
Residential real estate | 60-89 days past due
Financing Receivable, Past Due [Line Items]
Past Due	345	3,089	509
Residential real estate | 30-59 days past due
Financing Receivable, Past Due [Line Items]
Past Due	28,983	18,041	11,112
Premium finance receivables - commercial
Financing Receivable, Past Due [Line Items]
Nonaccrual	21,058	20,590	14,797
90+ days and still accruing	16,505	11,517	6,558
Current	3,384,354	3,379,018	2,941,038
Total Loans	3,465,055	3,442,027	2,988,788
Premium finance receivables - commercial | 60-89 days past due
Financing Receivable, Past Due [Line Items]
Past Due	10,327	12,119	5,628
Premium finance receivables - commercial | 30-59 days past due
Financing Receivable, Past Due [Line Items]
Past Due	32,811	18,783	20,767
Premium finance receivables - life
Financing Receivable, Past Due [Line Items]
Nonaccrual	0	590	0
90+ days and still accruing	0	0	168
Current	5,181,862	5,041,453	4,515,367
Total Loans	5,221,639	5,074,602	4,555,369
Premium finance receivables - life | 60-89 days past due
Financing Receivable, Past Due [Line Items]
Past Due	2,403	0	4,788
Premium finance receivables - life | 30-59 days past due
Financing Receivable, Past Due [Line Items]
Past Due	37,374	32,559	35,046
Consumer and other
Financing Receivable, Past Due [Line Items]
Nonaccrual	403	231	326
90+ days and still accruing	78	287	280
Current	35,853	109,276	119,801
Total Loans	37,166	110,178	120,804
Consumer and other | 60-89 days past due
Financing Receivable, Past Due [Line Items]
Past Due	625	40	47
Consumer and other | 30-59 days past due
Financing Receivable, Past Due [Line Items]
Past Due	$ 207	$ 344	$ 350