Loans - Age Analysis of Past Due Loans, Segregated by Class of Loans (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans 30-89 Days Past Due	$ 40,086
Loans 90 or More Days Past Due	54,145
Total Past Due Loans	94,231
Current Loans	9,212,223
Total loans	9,306,454	9,223,848	8,811,040
Accruing Loans 90 or More Days Past Due	11,221
Commercial and industrial, Energy [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans 30-89 Days Past Due	532
Loans 90 or More Days Past Due	228
Total Past Due Loans	760
Current Loans	1,078,139
Total loans	1,078,899
Accruing Loans 90 or More Days Past Due	0
Commercial and industrial, Other commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans 30-89 Days Past Due	18,105
Loans 90 or More Days Past Due	16,838
Total Past Due Loans	34,943
Current Loans	3,694,830
Total loans	3,729,773
Accruing Loans 90 or More Days Past Due	6,606
Commercial real estate, Buildings, land and other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans 30-89 Days Past Due	11,019
Loans 90 or More Days Past Due	33,578
Total Past Due Loans	44,597
Current Loans	2,913,843
Total loans	2,958,440
Accruing Loans 90 or More Days Past Due	1,683
Commercial real estate, Construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans 30-89 Days Past Due	0
Loans 90 or More Days Past Due	0
Total Past Due Loans	0
Current Loans	412,529
Total loans	412,529
Accruing Loans 90 or More Days Past Due	0
Consumer Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans 30-89 Days Past Due	6,248
Loans 90 or More Days Past Due	2,848
Total Past Due Loans	9,096
Current Loans	790,731
Total loans	799,827
Accruing Loans 90 or More Days Past Due	2,480
Consumer and Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans 30-89 Days Past Due	4,182
Loans 90 or More Days Past Due	653
Total Past Due Loans	4,835
Current Loans	345,277
Total loans	350,112
Accruing Loans 90 or More Days Past Due	452
Unearned discounts [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans 30-89 Days Past Due	0
Loans 90 or More Days Past Due	0
Total Past Due Loans	0
Current Loans	(23,126)
Total loans	(23,126)
Accruing Loans 90 or More Days Past Due	$ 0