Loans - Age Analysis of Past Due Loans, Segregated by Class of Loans (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	$ 116,865
Current Loans	15,715,805
Total loans	15,832,670	$ 17,481,309
Accruing Loans 90 or More Days Past Due	23,725
Commercial Portfolio Segment [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	51,339
Current Loans	4,878,317
Total loans	4,929,656	4,955,341
Accruing Loans 90 or More Days Past Due	7,522
Commercial and Industrial Total Energy Loans [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	14,470
Current Loans	963,741
Total loans	978,211	1,235,198
Accruing Loans 90 or More Days Past Due	965
Paycheck Protection Program [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	0
Current Loans	827,820
Total loans	827,820	2,433,849
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	40,100
Current Loans	5,944,290
Total loans	5,984,390	5,796,653
Accruing Loans 90 or More Days Past Due	12,224
Construction Loans [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	0
Current Loans	1,266,633
Total loans	1,266,633	1,223,814
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	6,249
Current Loans	1,356,790
Total loans	1,363,039	1,330,774
Accruing Loans 90 or More Days Past Due	1,985
Consumer Loan [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	4,707
Current Loans	478,214
Total loans	482,921	$ 505,680
Accruing Loans 90 or More Days Past Due	1,029
Financing Receivables, 30 to 89 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	72,678
Financing Receivables, 30 to 89 Days Past Due [Member] | Commercial Portfolio Segment [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	42,705
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	1,237
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	20,910
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	4,148
Financing Receivables, 30 to 89 Days Past Due [Member] | Consumer Loan [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	3,678
Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	44,187
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	8,634
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	13,233
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	19,190
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	2,101
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	$ 1,029