Loans (Age Analysis of Past Due Loans, Segregated by Class of Loans) (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	$ 146,973
Current Loans	14,603,359
Total loans	14,750,332	$ 14,099,733
Accruing Loans 90 or More Days Past Due	7,421
Commercial Portfolio Segment [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	46,742
Current Loans	5,140,724
Total loans	5,187,466	5,111,957
Accruing Loans 90 or More Days Past Due	3,430
Commercial and Industrial Total Energy Loans [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	68,702
Current Loans	1,584,180
Total loans	1,652,882	1,602,598
Accruing Loans 90 or More Days Past Due	85
Commercial Real Estate, Buildings, Land and Other [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	15,109
Current Loans	4,868,471
Total loans	4,883,580	4,428,721
Accruing Loans 90 or More Days Past Due	967
Construction Loans [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	1,261
Current Loans	1,311,398
Total loans	1,312,659	1,267,717
Accruing Loans 90 or More Days Past Due	402
Total consumer real estate loans [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	9,571
Current Loans	1,184,842
Total loans	1,194,413	1,118,990
Accruing Loans 90 or More Days Past Due	1,425
Consumer Loan [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	5,588
Current Loans	513,744
Total loans	519,332	$ 569,750
Accruing Loans 90 or More Days Past Due	1,112
Financing Receivables, 30 to 89 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	56,107
Financing Receivables, 30 to 89 Days Past Due [Member] | Commercial Portfolio Segment [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	25,474
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	6,136
Financing Receivables, 30 to 89 Days Past Due [Member] | Commercial Real Estate, Buildings, Land and Other [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	12,384
Financing Receivables, 30 to 89 Days Past Due [Member] | Construction Loans [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	195
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	7,442
Financing Receivables, 30 to 89 Days Past Due [Member] | Consumer Loan [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	4,476
Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	90,866
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	21,268
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	62,566
Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Commercial Real Estate, Buildings, Land and Other [Member]
Financing Receivable, Past Due [Line Items]
Total Past Due Loans	2,725
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	1,066
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	2,129
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	$ 1,112