Loans - Loans (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 15,832,670	$ 17,481,309
Total loans percentage	100.00%	100.00%
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 4,929,656	$ 4,955,341
Total commercial and industrial percentage of total loans	31.10%	28.40%
Energy Production [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 774,753	$ 976,473
Percentage of energy production loans to total loans	4.90%	5.60%
Energy Service [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 107,585	$ 116,825
Percentage of energy service loans to total loans	0.70%	0.70%
Energy Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 95,873	$ 141,900
Percentage of other energy loans to total loans	0.60%	0.80%
Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 978,211	$ 1,235,198
Percentage of total energy loans to total loans	6.20%	7.10%
Paycheck Protection Program [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 827,820	$ 2,433,849
Percentage Of Payment Protection Program Loans To Loans	5.20%	13.90%
Commercial mortgage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 5,648,541	$ 5,478,806
Commercial mortgages percentage of total loans	35.70%	31.30%
Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 1,266,633	$ 1,223,814
Construction percentage of total loans	8.00%	7.00%
Commercial land loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 335,849	$ 317,847
Land percentage of total loans	2.10%	1.80%
Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 7,251,023	$ 7,020,467
Total commercial real estate percentage of total loans	45.80%	40.10%
Home Equity Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 320,597	$ 329,390
Home equity loans percentage of total loans	2.00%	1.90%
Home Equity Line of Credit [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 478,978	$ 452,854
Home equity lines of credit percentage of total loans	3.00%	2.60%
Other consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 563,464	$ 548,530
Other percentage of total loans	3.60%	3.10%
Total consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 1,363,039	$ 1,330,774
Total consumer real estate percentage of total loans	8.60%	7.60%
Real Estate Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 8,614,062	$ 8,351,241
Total real estate percentage of total loans	54.40%	47.70%
Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 482,921	$ 505,680
Total consumer and other percentage of total loans	3.10%	2.90%