Loans and Leases and the Allowance for Credit Losses - Summary of Loan grades applied various classes of Commercial and Real Estate Loans (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	$ 98,447,048	$ 90,922,869
Revolving Loans [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	16,208,710
Revolving Loans Converted to Term Loans [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	1,443,245
Commercial, Financial, Leasing, etc. [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	27,891,648	23,838,168
Commercial, Financial, Leasing, etc. [Member] | Pass [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	26,013,705	22,595,821
Commercial, Financial, Leasing, etc. [Member] | Criticized Accrual [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	1,527,315	895,790
Commercial, Financial, Leasing, etc. [Member] | Criticized Nonaccrual [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	350,628	346,557
Commercial, Financial, Leasing, etc. [Member] | Revolving Loans [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	11,368,511
Commercial, Financial, Leasing, etc. [Member] | Revolving Loans [Member] | Pass [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	10,444,292
Commercial, Financial, Leasing, etc. [Member] | Revolving Loans [Member] | Criticized Accrual [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	758,594
Commercial, Financial, Leasing, etc. [Member] | Revolving Loans [Member] | Criticized Nonaccrual [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	165,625
Commercial, Financial, Leasing, etc. [Member] | Revolving Loans Converted to Term Loans [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	44,237
Commercial, Financial, Leasing, etc. [Member] | Revolving Loans Converted to Term Loans [Member] | Pass [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	27,536
Commercial, Financial, Leasing, etc. [Member] | Revolving Loans Converted to Term Loans [Member] | Criticized Accrual [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	9,533
Commercial, Financial, Leasing, etc. [Member] | Revolving Loans Converted to Term Loans [Member] | Criticized Nonaccrual [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	7,168
Commercial [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	27,709,261	26,657,808
Commercial [Member] | Pass [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	23,817,295	25,728,725
Commercial [Member] | Criticized Accrual [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	3,639,451	770,609
Commercial [Member] | Criticized Nonaccrual [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	252,515	158,474
Commercial [Member] | Revolving Loans [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	862,836
Commercial [Member] | Revolving Loans [Member] | Pass [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	810,180
Commercial [Member] | Revolving Loans [Member] | Criticized Accrual [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	51,535
Commercial [Member] | Revolving Loans [Member] | Criticized Nonaccrual [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	1,121
Residential Builder and Developer [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	1,362,978	1,547,245
Residential Builder and Developer [Member] | Pass [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	1,292,307	1,419,162
Residential Builder and Developer [Member] | Criticized Accrual [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	68,838	124,101
Residential Builder and Developer [Member] | Criticized Nonaccrual [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	1,833	3,982
Residential Builder and Developer [Member] | Revolving Loans [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	236,749
Residential Builder and Developer [Member] | Revolving Loans [Member] | Pass [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	233,182
Residential Builder and Developer [Member] | Revolving Loans [Member] | Criticized Accrual [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	3,567
Other Commercial Construction [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	8,509,845	7,336,861
Other Commercial Construction [Member] | Pass [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	7,433,054	7,092,799
Other Commercial Construction [Member] | Criticized Accrual [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	1,039,164	211,292
Other Commercial Construction [Member] | Criticized Nonaccrual [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	37,627	32,770
Other Commercial Construction [Member] | Revolving Loans [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	82,993
Other Commercial Construction [Member] | Revolving Loans [Member] | Pass [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	77,873
Other Commercial Construction [Member] | Revolving Loans [Member] | Criticized Nonaccrual [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	5,120
Term Loans [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	18,549,250	15,936,862	$ 11,120,334	$ 8,351,948	$ 6,393,180
Term Loans [Member] | Commercial, Financial, Leasing, etc. [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	8,601,811	2,486,371	1,830,003	1,068,052	880,653
Term Loans [Member] | Commercial, Financial, Leasing, etc. [Member] | Pass [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	8,209,336	2,387,719	1,638,389	1,001,452	816,518
Term Loans [Member] | Commercial, Financial, Leasing, etc. [Member] | Criticized Accrual [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	389,682	75,106	141,774	50,144	45,345
Term Loans [Member] | Commercial, Financial, Leasing, etc. [Member] | Criticized Nonaccrual [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	2,793	23,546	49,840	16,456	18,790
Term Loans [Member] | Commercial [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	3,117,418	5,391,525	3,995,087	3,178,099	3,572,835
Term Loans [Member] | Commercial [Member] | Pass [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	2,773,454	4,911,992	3,458,071	2,851,599	2,823,120
Term Loans [Member] | Commercial [Member] | Criticized Accrual [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	343,722	436,191	531,848	294,994	728,208
Term Loans [Member] | Commercial [Member] | Criticized Nonaccrual [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	242	43,342	5,168	31,506	21,507
Term Loans [Member] | Residential Builder and Developer [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	432,219	413,185	222,443	29,799	11,668
Term Loans [Member] | Residential Builder and Developer [Member] | Pass [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	399,778	399,923	207,188	26,378	11,668
Term Loans [Member] | Residential Builder and Developer [Member] | Criticized Accrual [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	32,441	12,744	15,255	3,421
Term Loans [Member] | Residential Builder and Developer [Member] | Criticized Nonaccrual [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount		518
Term Loans [Member] | Other Commercial Construction [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	748,280	2,837,510	2,595,032	1,385,271	522,232
Term Loans [Member] | Other Commercial Construction [Member] | Pass [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	721,942	2,613,554	2,229,748	1,137,823	352,930
Term Loans [Member] | Other Commercial Construction [Member] | Criticized Accrual [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	26,338	$ 223,956	$ 365,284	243,108	166,048
Term Loans [Member] | Other Commercial Construction [Member] | Criticized Nonaccrual [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount				$ 4,340	$ 3,254
Term Loans Prior Period [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	20,443,519
Term Loans Prior Period [Member] | Commercial, Financial, Leasing, etc. [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	1,612,010
Term Loans Prior Period [Member] | Commercial, Financial, Leasing, etc. [Member] | Pass [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	1,488,463
Term Loans Prior Period [Member] | Commercial, Financial, Leasing, etc. [Member] | Criticized Accrual [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	57,137
Term Loans Prior Period [Member] | Commercial, Financial, Leasing, etc. [Member] | Criticized Nonaccrual [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	66,410
Term Loans Prior Period [Member] | Commercial [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	7,591,461
Term Loans Prior Period [Member] | Commercial [Member] | Pass [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	6,188,879
Term Loans Prior Period [Member] | Commercial [Member] | Criticized Accrual [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	1,252,953
Term Loans Prior Period [Member] | Commercial [Member] | Criticized Nonaccrual [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	149,629
Term Loans Prior Period [Member] | Residential Builder and Developer [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	16,915
Term Loans Prior Period [Member] | Residential Builder and Developer [Member] | Pass [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	14,190
Term Loans Prior Period [Member] | Residential Builder and Developer [Member] | Criticized Accrual [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	1,410
Term Loans Prior Period [Member] | Residential Builder and Developer [Member] | Criticized Nonaccrual [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	1,315
Term Loans Prior Period [Member] | Other Commercial Construction [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	338,527
Term Loans Prior Period [Member] | Other Commercial Construction [Member] | Pass [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	299,184
Term Loans Prior Period [Member] | Other Commercial Construction [Member] | Criticized Accrual [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	14,430
Term Loans Prior Period [Member] | Other Commercial Construction [Member] | Criticized Nonaccrual [Member]
Financing Receivable Recorded Investment [Line Items]
Loans and leases, net of unearned discount	$ 24,913