Loans - Loans (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 18,223,877	$ 14,750,332
Total loans percentage	100.00%	100.00%
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 4,820,018	$ 5,187,466
Total commercial and industrial percentage of total loans	26.40%	35.20%
Energy Production [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 1,108,753	$ 1,348,900
Percentage of energy production loans to total loans	6.10%	9.20%
Energy Service [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 163,197	$ 192,996
Percentage of energy service loans to total loans	0.90%	1.30%
Energy Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 91,418	$ 110,986
Percentage of other energy loans to total loans	0.40%	0.80%
Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 1,363,368	$ 1,652,882
Percentage of total energy loans to total loans	7.40%	11.20%
Paycheck Protection Program [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 3,226,980	$ 0
Percentage Of Payment Protection Program Loans To Loans	17.70%	0.00%
Commercial mortgage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 5,412,731	$ 4,594,113
Commercial mortgages percentage of total loans	29.70%	31.10%
Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 1,274,080	$ 1,312,659
Construction percentage of total loans	7.00%	8.90%
Commercial land loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 320,407	$ 289,467
Land percentage of total loans	1.80%	2.00%
Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 7,007,218	$ 6,196,239
Total commercial real estate percentage of total loans	38.50%	42.00%
Home Equity Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 340,226	$ 375,596
Home equity loans percentage of total loans	1.90%	2.60%
Home Equity Line of Credit [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 434,171	$ 354,671
Home equity lines of credit percentage of total loans	2.40%	2.40%
Other consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 533,356	$ 464,146
Other percentage of total loans	2.90%	3.10%
Total consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 1,307,753	$ 1,194,413
Total consumer real estate percentage of total loans	7.20%	8.10%
Real Estate Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 8,314,971	$ 7,390,652
Total real estate percentage of total loans	45.70%	50.10%
Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 498,540	$ 519,332
Total consumer and other percentage of total loans	2.80%	3.50%