Loans - Loans (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 15,338,260	$ 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	$ 5,536,760	$ 5,187,466
Total commercial and industrial percentage of total loans	36.10%	35.20%
Energy Production [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 1,268,832	$ 1,348,900
Percentage of energy production loans to total loans	8.30%	9.20%
Energy Service [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 185,210	$ 192,996
Percentage of energy service loans to total loans	1.20%	1.30%
Energy Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 114,812	$ 110,986
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	$ 1,568,854	$ 1,652,882
Percentage of total energy loans to total loans	10.20%	11.20%
Commercial mortgage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 4,920,882	$ 4,594,113
Commercial mortgages percentage of total loans	32.10%	31.10%
Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 1,265,441	$ 1,312,659
Construction percentage of total loans	8.30%	8.90%
Commercial land loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 286,876	$ 289,467
Land percentage of total loans	1.90%	2.00%
Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 6,473,199	$ 6,196,239
Total commercial real estate percentage of total loans	42.30%	42.00%
Home Equity Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 389,844	$ 375,596
Home equity loans percentage of total loans	2.50%	2.60%
Home Equity Line of Credit [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 350,330	$ 354,671
Home equity lines of credit percentage of total loans	2.30%	2.40%
Other consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 480,933	$ 464,146
Other percentage of total loans	3.10%	3.10%
Total consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 1,221,107	$ 1,194,413
Total consumer real estate percentage of total loans	7.90%	8.10%
Real Estate Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 7,694,306	$ 7,390,652
Total real estate percentage of total loans	50.20%	50.10%
Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 538,340	$ 519,332
Total consumer and other percentage of total loans	3.50%	3.50%