Loans - Age Analysis of Past Due Loans, Segregated by Class of Loans (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	$ 128,987
Current Loans	17,760,664
Total loans	17,889,651	$ 17,481,309
Accruing Loans 90 or More Days Past Due	16,934
Commercial Portfolio Segment [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	34,758
Current Loans	4,735,470
Total loans	4,770,228	4,955,341
Accruing Loans 90 or More Days Past Due	7,547
Commercial and Industrial Total Energy Loans [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	15,043
Current Loans	1,087,166
Total loans	1,102,209	1,235,198
Accruing Loans 90 or More Days Past Due	1,086
Paycheck Protection Program [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	0
Current Loans	3,129,244
Total loans	3,129,244	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	43,589
Current Loans	5,823,447
Total loans	5,867,036	5,796,653
Accruing Loans 90 or More Days Past Due	5,045
Construction Loans [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	22,726
Current Loans	1,214,268
Total loans	1,236,994	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	9,227
Current Loans	1,312,927
Total loans	1,322,154	1,330,774
Accruing Loans 90 or More Days Past Due	2,826
Consumer Loan [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	3,644
Current Loans	458,142
Total loans	461,786	$ 505,680
Accruing Loans 90 or More Days Past Due	430
Financing Receivables, 30 to 89 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	95,439
Financing Receivables, 30 to 89 Days Past Due [Member] \| Commercial Portfolio Segment [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	24,542
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	11,125
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	27,876
Financing Receivables, 30 to 89 Days Past Due [Member] \| Construction Loans [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	22,726
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	5,973
Financing Receivables, 30 to 89 Days Past Due [Member] \| Consumer Loan [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	3,197
Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	33,548
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	10,216
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	3,918
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	15,713
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	3,254
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	$ 447