Loans (Narrative) (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2023 USD ($) Grade Rate	Dec. 31, 2022 USD ($) Grade	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
U.S. dollar denominated loans and commitments to borrowers in Mexico	$ 0	$ 0
Commercial and industrial loans related to overdraft for correspondent bank customer	$ 13,400	10,300
Financing Receivable, Threshold Period Past Due	90 days
Loans, net of unearned discounts	$ 18,824,251	$ 17,154,969
Loans Receivable, Basis Spread on Variable Rate | Rate	1.75%
Loan Receivable Variable Rate | Rate	10.25%
Loan Receivable Fixed Rate | Rate	6.74%
Number of days past due for payment of interest on loans, minimum	120 days
Texas Leading Index economic condition indicator	126.8	130.4
Financing Receivable, Change in Method, Credit Loss Expense (Reversal)	$ (45,000)
Financing Receivable, Allowance for Credit Loss	245,996	$ 227,621
Additional Funding Agreement Terms
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	0
Nonperforming Financial Instruments [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	25,974	6,301
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	359	1,510
Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Nonperforming Financial Instruments [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	24,302
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	6,537
Loans Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accrued interest receivable	$ 90,800	68,700
Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of total loans	5.00%
Loans, net of unearned discounts	$ 935,265	$ 925,729
Weighted Average Risk Grade | Grade	6.05	5.67
Financing Receivable, Allowance for Credit Loss	$ 17,814	$ 18,062	$ 17,217	$ 39,553
Financing Receivable, Year One, Originated, Current Fiscal Year	277,722	352,224
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	61,500	100,862
Commercial and Industrial Total Energy Loans [Member] | Nonperforming Financial Instruments [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	5,875	4,657
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	0	0
Commercial and Industrial Total Energy Loans [Member] | Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Nonperforming Financial Instruments [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	0
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	0
Commercial and Industrial Total Energy Loans [Member] | Risk Grade One To Eight [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 869,749	$ 887,182
Weighted Average Risk Grade | Grade	5.73	5.44
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 270,710	$ 338,050
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	54,185	99,089
Non Accrual Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Additional interest that would have been recognized on non-accrual loans, net of tax	$ 4,000	1,700	1,800
Grade 12 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
General percentage of valuation allowance in excess of principal balance, maximum	30.00%
Grade 13 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
General percentage of valuation allowance in excess of principal balance, minimum	30.00%
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 5,959,133	$ 5,674,798
Weighted Average Risk Grade | Grade	6.60	6.39
Financing Receivable, Allowance for Credit Loss	$ 74,006	$ 104,237	72,091	73,843
Financing Receivable, Year One, Originated, Current Fiscal Year	1,619,306	1,703,185
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	768,246	657,039
Commercial Portfolio Segment [Member] | Nonperforming Financial Instruments [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	0	0
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	359	1,252
Commercial Portfolio Segment [Member] | Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Nonperforming Financial Instruments [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	124
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	1,704
Commercial Portfolio Segment [Member] | Risk Grade One To Eight [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 5,499,829	$ 5,435,917
Weighted Average Risk Grade | Grade	6.32	6.24
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 1,489,026	$ 1,667,274
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	$ 706,210	618,756
Commercial and Industrial, Other Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of days past due for payment of interest on loans, minimum	180 days
Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 8,982,644	$ 8,183,325
Weighted Average Risk Grade | Grade	7.24	7.10
Financing Receivable, Allowance for Credit Loss	$ 130,598	$ 90,301	144,936	134,892
Financing Receivable, Year One, Originated, Current Fiscal Year	2,205,943	2,567,912
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	2,361,998	2,019,736
Commercial Real Estate Buildings Land And Other Financing Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 7,301,920	$ 6,706,078
Weighted Average Risk Grade | Grade	7.20	7.09
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 1,645,223	$ 1,901,888
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	1,785,624	1,528,173
Commercial Real Estate Buildings Land And Other Financing Receivable [Member] | Nonperforming Financial Instruments [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	20,099	1,644
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	0	0
Commercial Real Estate Buildings Land And Other Financing Receivable [Member] | Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Nonperforming Financial Instruments [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	0
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	6
Commercial Real Estate Buildings Land And Other Financing Receivable [Member] | Risk Grade One To Eight [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 6,895,358	$ 6,340,028
Weighted Average Risk Grade | Grade	7.03	6.94
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 1,601,175	$ 1,811,069
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	1,667,750	1,484,811
Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 1,680,724	$ 1,477,247
Weighted Average Risk Grade | Grade	7.45	7.12
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 560,720	$ 666,024
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	576,374	491,563
Construction Loans [Member] | Nonperforming Financial Instruments [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	0	0
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	0	0
Construction Loans [Member] | Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Nonperforming Financial Instruments [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	739
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	0
Construction Loans [Member] | Risk Grade One To Eight [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 1,526,086	$ 1,430,012
Weighted Average Risk Grade | Grade	7.27	7.04
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 538,072	$ 640,948
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	537,705	489,391
Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	476,962	492,726
Financing Receivable, Allowance for Credit Loss	10,040	7,017	7,837	$ 6,963
Consumer Loan [Member] | Nonperforming Financial Instruments [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	0	0
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	0	0
Consumer Loan [Member] | Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Nonperforming Financial Instruments [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	23,321
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	4,546
Paycheck Protection Program [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	9,521	34,852
Financing Receivable, Allowance for Credit Loss	0	$ 0	$ 0
Bank Overdrafts | Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Nonperforming Financial Instruments [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	23,100
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	$ 4,200
Automobile Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of total loans	5.90%
Commitments to Extend Credit [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unfunded balances	$ 1,100,000
Commitments to Extend Credit [Member] | Automobile Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unfunded balances	522,800
Standby Letters of Credit [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unfunded balances	86,900
Standby Letters of Credit [Member] | Automobile Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unfunded balances	$ 19,300