Loans (Narrative) (Detail)	12 Months Ended
	Dec. 31, 2016 USD ($) Item	Dec. 31, 2015 USD ($) Item	Dec. 31, 2014 USD ($)	Nov. 30, 2016	Dec. 31, 2013 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration of loans in a single industry in excess of ten percent	0.00%	0.00%
Maximum percentage loan related to single industry	10.00%	10.00%
Commercial and industrial loans related to overdraft for correspondent bank customer	$ 6,300,000	$ 7,300,000
Minimum number of days to be considered past due	90 days
Troubled debt restructurings	$ 44,863,000	536,000	$ 8,339,000
Interest income recognized subsequent to classification as impaired	$ 0	$ 0	0
Number of loans restructured during the last year in excess of 90 days past due | Item	2	1
Previously restructured loans past due	$ 3,200,000	$ 259,000
Amount charged off related to loans restructured	9,500,000
Proceeds from sale of loans	30,470,000	0	0
Financing Receivables, Impaired, Troubled Debt Restructuring, Write-down	4,100,000	88,000
Financing Receivable, Modifications, Pre-Modification Recorded Investment	$ 91,418,000	$ 709,000	8,916,000
Number of days past due for payment of interest on loans, minimum	120 days
Texas Leading Index economic condition indicator		123.0		121.9
Maximum origination value of loans which were subject to further refinement of valuation methodology	$ 1,000,000
General valuation allowance allocations for loan agreement monitoring exceptions	$ 153,045,000	$ 135,859,000	99,542,000		$ 92,438,000
Sensitivity stress test price shock	75.00%
Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
General valuation allowance allocations for loan agreement monitoring exceptions	$ 30,213,000	$ 24,313,000	27,163,000		22,590,000
Commercial And Industrial Energy Financing Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of loans related to a single industry	11.60%	15.30%
Troubled debt restructurings	$ 43,841,000	$ 0	0
Financing Receivable, Modifications, Pre-Modification Recorded Investment	87,572,000	0	0
General valuation allowance allocations for loan agreement monitoring exceptions	60,653,000	54,696,000	14,919,000		6,090,000
Non Accrual Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Troubled debt restructurings	44,900,000	536,000
Additional interest that would have been recognized on non-accrual loans, net of tax	3,100,000	1,600,000	1,500,000
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Troubled debt restructurings	1,022,000	536,000	5,391,000
Financing Receivables, Impaired, Troubled Debt Restructuring, Write-down			627,000
Financing Receivable, Modifications, Pre-Modification Recorded Investment	2,148,000	709,000	5,795,000
General valuation allowance allocations for loan agreement monitoring exceptions	$ 52,915,000	42,993,000	44,273,000		$ 46,700,000
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 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 Buildings Land And Other Financing Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Troubled debt restructurings	$ 0	0	2,948,000
Amount charged off related to loans restructured			0
Financing Receivable, Modifications, Pre-Modification Recorded Investment	1,455,000	0	3,121,000
Previously Restructured Loans Moved to OREO			500,000
Commitments to Extend Credit [Member] | Commercial And Industrial Energy Financing Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unfunded balances	1,100,000,000
Commercial Borrower [Member] | Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Modifications, Pre-Modification Recorded Investment			2,900,000
Commercial Borrower [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Modifications, Pre-Modification Recorded Investment		$ 277,000	$ 2,700,000
Standby Letters of Credit [Member] | Commercial And Industrial Energy Financing Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unfunded balances	$ 37,400,000