Loans - Age Analysis of Past Due Loans, Segregated by Class of Loans (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	$ 99,176
Current Loans	12,413,162
Total loans	12,512,338	$ 11,975,392
Accruing Loans 90 or More Days Past Due	16,003
Commercial Portfolio Segment [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	50,469
Current Loans	4,553,800
Total loans	4,604,269
Accruing Loans 90 or More Days Past Due	10,768
Commercial and industrial, Energy [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	12,421
Current Loans	1,397,191
Total loans	1,409,612
Accruing Loans 90 or More Days Past Due	2,902
Commercial real estate, Buildings, land and other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	25,323
Current Loans	3,917,692
Total loans	3,943,015
Accruing Loans 90 or More Days Past Due	944
Construction Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	0
Current Loans	1,050,837
Total loans	1,050,837
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	6,715
Current Loans	984,358
Total loans	991,073
Accruing Loans 90 or More Days Past Due	739
Consumer and Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	4,248
Current Loans	509,284
Total loans	513,532
Accruing Loans 90 or More Days Past Due	650
Financing Receivables, 30 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	59,669
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	24,320
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	5,991
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	21,157
Financing Receivables, 30 to 89 Days Past Due [Member] | Construction Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	0
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	4,693
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	3,508
Financing Receivables, Equal to Greater than 90 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	39,507
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	26,149
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	6,430
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	4,166
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	2,022
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	$ 740