Loans - Age Analysis of Past Due Loans, Segregated by Class of Loans (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	$ 148,448
Current Loans	13,215,581
Total loans	13,364,029	$ 13,145,665
Accruing Loans 90 or More Days Past Due	19,276
Commercial Portfolio Segment [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	49,647
Current Loans	4,826,876
Total loans	4,876,523
Accruing Loans 90 or More Days Past Due	10,357
Commercial and industrial, Energy [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	28,611
Current Loans	1,418,377
Total loans	1,446,988
Accruing Loans 90 or More Days Past Due	3,137
Commercial real estate, Buildings, land and other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	41,600
Current Loans	4,336,535
Total loans	4,378,135
Accruing Loans 90 or More Days Past Due	3,889
Construction Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	9,497
Current Loans	1,067,288
Total loans	1,076,785
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	11,389
Current Loans	1,028,274
Total loans	1,039,663
Accruing Loans 90 or More Days Past Due	1,680
Consumer and Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	7,704
Current Loans	538,231
Total loans	545,935
Accruing Loans 90 or More Days Past Due	213
Financing Receivables, 30 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	111,443
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	25,701
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	24,404
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	36,130
Financing Receivables, 30 to 89 Days Past Due [Member] | Construction Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	9,497
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	8,225
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	7,486
Financing Receivables, Equal to Greater than 90 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	37,005
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	23,946
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	4,207
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	5,470
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	3,164
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	$ 218