Loans - Age Analysis of Past Due Loans, Segregated by Class of Loans (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	$ 169,642
Current Loans	18,054,235
Total loans	18,223,877	$ 14,750,332
Accruing Loans 90 or More Days Past Due	35,706
Commercial Portfolio Segment [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	61,182
Current Loans	4,758,836
Total loans	4,820,018	5,187,466
Accruing Loans 90 or More Days Past Due	19,392
Commercial and Industrial Total Energy Loans [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	40,659
Current Loans	1,322,709
Total loans	1,363,368	1,652,882
Accruing Loans 90 or More Days Past Due	559
Paycheck Protection Program [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	0
Current Loans	3,226,980
Total loans	3,226,980	0
Accruing Loans 90 or More Days Past Due	0
Commercial Real Estate Buildings Land And Other Loans [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	44,684
Current Loans	5,688,454
Total loans	5,733,138	4,883,580
Accruing Loans 90 or More Days Past Due	11,676
Construction Loans [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	0
Current Loans	1,274,080
Total loans	1,274,080	1,312,659
Accruing Loans 90 or More Days Past Due	0
Total consumer real estate loans [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	15,815
Current Loans	1,291,938
Total loans	1,307,753	1,194,413
Accruing Loans 90 or More Days Past Due	3,406
Consumer Loan [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	7,302
Current Loans	491,238
Total loans	498,540	$ 519,332
Accruing Loans 90 or More Days Past Due	673
Financing Receivables, 30 to 89 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	109,934
Financing Receivables, 30 to 89 Days Past Due [Member] | Commercial Portfolio Segment [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	36,067
Financing Receivables, 30 to 89 Days Past Due [Member] | Commercial and Industrial Total Energy Loans [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	31,071
Financing Receivables, 30 to 89 Days Past Due [Member] | Paycheck Protection Program [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	0
Financing Receivables, 30 to 89 Days Past Due [Member] | Commercial Real Estate Buildings Land And Other Loans [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	25,113
Financing Receivables, 30 to 89 Days Past Due [Member] | Construction Loans [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	0
Financing Receivables, 30 to 89 Days Past Due [Member] | Total consumer real estate loans [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	11,054
Financing Receivables, 30 to 89 Days Past Due [Member] | Consumer Loan [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	6,629
Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	59,708
Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Commercial Portfolio Segment [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	25,115
Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Commercial and Industrial Total Energy Loans [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	9,588
Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Paycheck Protection Program [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	0
Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Commercial Real Estate Buildings Land And Other Loans [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	19,571
Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Construction Loans [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	0
Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Total consumer real estate loans [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	4,761
Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Consumer Loan [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	$ 673