Loans - Loans (Detail) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 16,596,326	$ 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	$ 4,755,345	$ 4,955,341
Total commercial and industrial percentage of total loans	28.70%	28.40%
Energy Production [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 812,862	$ 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	$ 89,153	$ 116,825
Percentage of energy service loans to total loans	0.60%	0.70%
Energy Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 121,187	$ 141,900
Percentage of other energy loans to total loans	0.70%	0.80%
Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 1,023,202	$ 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	$ 1,926,824	$ 2,433,849
Percentage Of Payment Protection Program Loans To Loans	11.60%	13.90%
Commercial mortgage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 5,577,069	$ 5,478,806
Commercial mortgages percentage of total loans	33.60%	31.30%
Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 1,166,309	$ 1,223,814
Construction percentage of total loans	7.00%	7.00%
Commercial land loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 312,668	$ 317,847
Land percentage of total loans	1.90%	1.80%
Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 7,056,046	$ 7,020,467
Total commercial real estate percentage of total loans	42.50%	40.10%
Home Equity Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 313,891	$ 329,390
Home equity loans percentage of total loans	1.90%	1.90%
Home Equity Line of Credit [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 479,099	$ 452,854
Home equity lines of credit percentage of total loans	2.90%	2.60%
Other consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 558,988	$ 548,530
Other percentage of total loans	3.30%	3.10%
Total consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 1,351,978	$ 1,330,774
Total consumer real estate percentage of total loans	8.10%	7.60%
Real Estate Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 8,408,024	$ 8,351,241
Total real estate percentage of total loans	50.60%	47.70%
Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 482,931	$ 505,680
Total consumer and other percentage of total loans	2.90%	2.90%