Loans - Age Analysis of Past Due Loans, Segregated by Class of Loans (Detail) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	$ 123,018
Current Loans	17,848,919
Total loans	17,971,937	$ 14,750,332
Accruing Loans 90 or More Days Past Due	20,997
Commercial Portfolio Segment [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	57,783
Current Loans	4,946,446
Total loans	5,004,229	5,187,466
Accruing Loans 90 or More Days Past Due	7,669
Commercial and Industrial Total Energy Loans [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	17,297
Current Loans	1,407,583
Total loans	1,424,880	1,652,882
Accruing Loans 90 or More Days Past Due	2,466
Paycheck Protection Program [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	0
Current Loans	3,162,279
Total loans	3,162,279	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	19,864
Current Loans	5,339,521
Total loans	5,359,385	4,883,580
Accruing Loans 90 or More Days Past Due	5,225
Construction Loans [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	3,164
Current Loans	1,262,386
Total loans	1,265,550	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	16,313
Current Loans	1,233,872
Total loans	1,250,185	1,194,413
Accruing Loans 90 or More Days Past Due	4,661
Consumer Loan [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	8,597
Current Loans	496,832
Total loans	505,429	$ 519,332
Accruing Loans 90 or More Days Past Due	976
Financing Receivables, 30 to 89 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	74,830
Financing Receivables, 30 to 89 Days Past Due [Member] | Commercial Portfolio Segment [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	34,018
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	9,319
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	10,271
Financing Receivables, 30 to 89 Days Past Due [Member] | Construction Loans [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	3,164
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	10,437
Financing Receivables, 30 to 89 Days Past Due [Member] | Consumer Loan [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	7,621
Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	48,188
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	23,765
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	7,978
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	9,593
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	5,876
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	$ 976