Loans - Age Analysis of Past Due Loans, Segregated by Class of Loans (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2013
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans 30-89 Days Past Due	$ 66,935
Loans 90 or More Days Past Due	34,182
Total Past Due Loans	101,117
Current Loans	10,645,667
Total loans	10,746,784	9,515,700	9,306,454
Accruing Loans 90 or More Days Past Due	20,287
Commercial and industrial, Energy [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans 30-89 Days Past Due	4,689
Loans 90 or More Days Past Due	0
Total Past Due Loans	4,689
Current Loans	1,595,777
Total loans	1,600,466
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	27,801
Loans 90 or More Days Past Due	14,772
Total Past Due Loans	42,573
Current Loans	3,867,848
Total loans	3,910,421
Accruing Loans 90 or More Days Past Due	10,740
Commercial real estate, Buildings, land and other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans 30-89 Days Past Due	19,129
Loans 90 or More Days Past Due	16,855
Total Past Due Loans	35,984
Current Loans	3,341,502
Total loans	3,377,486
Accruing Loans 90 or More Days Past Due	7,216
Commercial real estate, Construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans 30-89 Days Past Due	7,111
Loans 90 or More Days Past Due	0
Total Past Due Loans	7,111
Current Loans	635,881
Total loans	642,992
Accruing Loans 90 or More Days Past Due	1,503
Consumer Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans 30-89 Days Past Due	3,777
Loans 90 or More Days Past Due	1,727
Total Past Due Loans	5,504
Current Loans	841,564
Total loans	847,068
Accruing Loans 90 or More Days Past Due	828
Consumer and Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans 30-89 Days Past Due	4,428
Loans 90 or More Days Past Due	828
Total Past Due Loans	5,256
Current Loans	386,242
Total loans	391,498
Accruing Loans 90 or More Days Past Due	0
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	(23,147)
Total loans	(23,147)
Accruing Loans 90 or More Days Past Due	$ 0