Loans - Age Analysis of Past Due Loans, Segregated by Class of Loans (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2013
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans 30-89 Days Past Due	$ 40,910
Loans 90 or More Days Past Due	24,701
Total Past Due Loans	65,611
Current Loans	10,613,766
Total loans	10,679,377	9,515,700	9,232,543
Accruing Loans 90 or More Days Past Due	7,876
Commercial and industrial, Energy [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans 30-89 Days Past Due	1,808
Loans 90 or More Days Past Due	525
Total Past Due Loans	2,333
Current Loans	1,528,313
Total loans	1,530,646
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	17,242
Loans 90 or More Days Past Due	8,951
Total Past Due Loans	26,193
Current Loans	4,067,994
Total loans	4,094,187
Accruing Loans 90 or More Days Past Due	4,048
Commercial real estate, Buildings, land and other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans 30-89 Days Past Due	14,201
Loans 90 or More Days Past Due	11,936
Total Past Due Loans	26,137
Current Loans	3,232,006
Total loans	3,258,143
Accruing Loans 90 or More Days Past Due	1,039
Commercial real estate, Construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans 30-89 Days Past Due	301
Loans 90 or More Days Past Due	0
Total Past Due Loans	301
Current Loans	577,145
Total loans	577,446
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	4,305
Loans 90 or More Days Past Due	2,512
Total Past Due Loans	6,817
Current Loans	856,398
Total loans	863,215
Accruing Loans 90 or More Days Past Due	2,204
Consumer and Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans 30-89 Days Past Due	3,053
Loans 90 or More Days Past Due	777
Total Past Due Loans	3,830
Current Loans	374,399
Total loans	378,229
Accruing Loans 90 or More Days Past Due	585
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	(22,489)
Total loans	(22,489)
Accruing Loans 90 or More Days Past Due	$ 0