Loans - Age Analysis of Past Due Loans, Segregated by Class of Loans (Detail) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	$ 123,012
Current Loans	16,473,314
Total loans	16,596,326	$ 17,481,309
Accruing Loans 90 or More Days Past Due	30,513
Commercial Portfolio Segment [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	31,884
Current Loans	4,723,461
Total loans	4,755,345	4,955,341
Accruing Loans 90 or More Days Past Due	9,282
Commercial and Industrial Total Energy Loans [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	16,185
Current Loans	1,007,017
Total loans	1,023,202	1,235,198
Accruing Loans 90 or More Days Past Due	1,017
Paycheck Protection Program [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	0
Current Loans	1,926,824
Total loans	1,926,824	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	59,314
Current Loans	5,830,423
Total loans	5,889,737	5,796,653
Accruing Loans 90 or More Days Past Due	18,135
Construction Loans [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	2,548
Current Loans	1,163,761
Total loans	1,166,309	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	9,738
Current Loans	1,342,240
Total loans	1,351,978	1,330,774
Accruing Loans 90 or More Days Past Due	1,791
Consumer Loan [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	3,343
Current Loans	479,588
Total loans	482,931	$ 505,680
Accruing Loans 90 or More Days Past Due	288
Financing Receivables, 30 to 89 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	67,265
Financing Receivables, 30 to 89 Days Past Due [Member] \| Commercial Portfolio Segment [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	20,612
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	1,717
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	31,764
Financing Receivables, 30 to 89 Days Past Due [Member] \| Construction Loans [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	2,548
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	7,569
Financing Receivables, 30 to 89 Days Past Due [Member] \| Consumer Loan [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	3,055
Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	55,747
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	11,272
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	14,468
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	27,550
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	2,169
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	$ 288