Loans (Age Analysis of Past Due Loans, Segregated by Class of Loans) (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans 30-89 Days Past Due	$ 45,510
Loans 90 or More Days Past Due	49,257
Total Past Due Loans	94,767
Current Loans	10,892,768
Total loans	10,987,535	9,515,700
Accruing Loans 90 or More Days Past Due	20,941
Commercial and Industrial, Energy [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans 30-89 Days Past Due	7,278
Loans 90 or More Days Past Due	0
Total Past Due Loans	7,278
Current Loans	1,766,667
Total loans	1,773,945
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	16,350
Loans 90 or More Days Past Due	33,998
Total Past Due Loans	50,348
Current Loans	3,943,450
Total loans	3,993,798
Accruing Loans 90 or More Days Past Due	14,254
Commercial Real Estate, Buildings, Land and Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans 30-89 Days Past Due	6,535
Loans 90 or More Days Past Due	11,308
Total Past Due Loans	17,843
Current Loans	3,354,266
Total loans	3,372,109
Accruing Loans 90 or More Days Past Due	3,333
Commercial Real Estate, Construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans 30-89 Days Past Due	5,081
Loans 90 or More Days Past Due	1,327
Total Past Due Loans	6,408
Current Loans	623,580
Total loans	629,988
Accruing Loans 90 or More Days Past Due	1,042
Consumer Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans 30-89 Days Past Due	4,560
Loans 90 or More Days Past Due	2,148
Total Past Due Loans	6,708
Current Loans	842,343
Total loans	849,051
Accruing Loans 90 or More Days Past Due	1,910
Consumer and Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans 30-89 Days Past Due	5,706
Loans 90 or More Days Past Due	476
Total Past Due Loans	6,182
Current Loans	387,419
Total loans	393,601
Accruing Loans 90 or More Days Past Due	402
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	(24,957)
Total loans	(24,957)
Accruing Loans 90 or More Days Past Due	$ 0