Loans - Age Analysis of Past Due Loans, Segregated by Class of Loans (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	$ 108,704
Current Loans	13,603,058
Total loans	13,711,762	$ 13,145,665
Accruing Loans 90 or More Days Past Due	23,000
Commercial Portfolio Segment [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	34,216
Current Loans	5,009,056
Total loans	5,043,272
Accruing Loans 90 or More Days Past Due	5,842
Commercial and industrial, Energy [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	23,689
Current Loans	1,504,339
Total loans	1,528,028
Accruing Loans 90 or More Days Past Due	5,878
Commercial real estate, Buildings, land and other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	37,508
Current Loans	4,365,332
Total loans	4,402,840
Accruing Loans 90 or More Days Past Due	9,055
Construction Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	615
Current Loans	1,106,384
Total loans	1,106,999
Accruing Loans 90 or More Days Past Due	0
Consumer Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	8,358
Current Loans	1,066,341
Total loans	1,074,699
Accruing Loans 90 or More Days Past Due	1,617
Consumer and Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	4,318
Current Loans	551,606
Total loans	555,924
Accruing Loans 90 or More Days Past Due	608
Financing Receivables, 30 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	48,267
Financing Receivables, 30 to 89 Days Past Due [Member] | Commercial Portfolio Segment [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	19,560
Financing Receivables, 30 to 89 Days Past Due [Member] | Commercial and industrial, Energy [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	3,775
Financing Receivables, 30 to 89 Days Past Due [Member] | Commercial real estate, Buildings, land and other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	14,208
Financing Receivables, 30 to 89 Days Past Due [Member] | Construction Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	615
Financing Receivables, 30 to 89 Days Past Due [Member] | Consumer Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	6,399
Financing Receivables, 30 to 89 Days Past Due [Member] | Consumer and Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	3,710
Financing Receivables, Equal to Greater than 90 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	60,437
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Commercial Portfolio Segment [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	14,656
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Commercial and industrial, Energy [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	19,914
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Commercial real estate, Buildings, land and other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	23,300
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Construction Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	0
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Consumer Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	1,959
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Consumer and Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	$ 608