Loans - Age Analysis of Past Due Loans, Segregated by Class of Loans (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans 30-89 Days Past Due	$ 49,374
Loans 90 or More Days Past Due	58,036
Total Past Due Loans	107,410
Current Loans	9,054,941
Total loans	9,162,351	9,223,848	8,126,713
Accruing Loans 90 or More Days Past Due	11,910
Commercial and Industrial, Energy [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans 30-89 Days Past Due	119
Loans 90 or More Days Past Due	1,916
Total Past Due Loans	2,035
Current Loans	1,068,561
Total loans	1,070,596
Accruing Loans 90 or More Days Past Due	1,916
Commercial and Industrial, Other Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans 30-89 Days Past Due	22,701
Loans 90 or More Days Past Due	28,924
Total Past Due Loans	51,625
Current Loans	3,586,444
Total loans	3,638,069
Accruing Loans 90 or More Days Past Due	5,939
Commercial Real Estate, Buildings, Land and Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans 30-89 Days Past Due	16,445
Loans 90 or More Days Past Due	23,801
Total Past Due Loans	40,246
Current Loans	2,703,401
Total loans	2,743,647
Accruing Loans 90 or More Days Past Due	1,173
Commercial Real Estate, Construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans 30-89 Days Past Due
Loans 90 or More Days Past Due	83
Total Past Due Loans	83
Current Loans	647,779
Total loans	647,862
Accruing Loans 90 or More Days Past Due	83
Consumer Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans 30-89 Days Past Due	6,531
Loans 90 or More Days Past Due	3,100
Total Past Due Loans	9,631
Current Loans	762,025
Total loans	771,656
Accruing Loans 90 or More Days Past Due	2,629
Consumer and Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans 30-89 Days Past Due	3,578
Loans 90 or More Days Past Due	212
Total Past Due Loans	3,790
Current Loans	307,558
Total loans	311,348
Accruing Loans 90 or More Days Past Due	170
Unearned Discounts [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans 30-89 Days Past Due
Loans 90 or More Days Past Due
Total Past Due Loans
Current Loans	(20,827)
Total loans	(20,827)
Accruing Loans 90 or More Days Past Due