Loans and Allowance for Credit Losses - Schedule of Risk Category of Loans (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Commercial Loan [Member]
Risk Category Of Loans [Line Items]
Total	$ 1,668,275	$ 1,629,600
Commercial Loan [Member] | Pass [Member]
Risk Category Of Loans [Line Items]
Total	1,490,032	1,442,904
Commercial Loan [Member] | Criticized [Member]
Risk Category Of Loans [Line Items]
Total	77,490	89,775
Commercial Loan [Member] | Classified - Substandard [Member]
Risk Category Of Loans [Line Items]
Total	49,070	58,461
Commercial Loan [Member] | Classified - Nonaccrual [Member]
Risk Category Of Loans [Line Items]
Total	50,641	38,003
Commercial Loan [Member] | Classified - Doubtful [Member]
Risk Category Of Loans [Line Items]
Total	1,042	457
Commercial Real Estate - Construction [Member]
Risk Category Of Loans [Line Items]
Total	150,711	134,552
Commercial Real Estate - Construction [Member] | Pass [Member]
Risk Category Of Loans [Line Items]
Total	137,433	119,958
Commercial Real Estate - Construction [Member] | Criticized [Member]
Risk Category Of Loans [Line Items]
Total	3,495	2,229
Commercial Real Estate - Construction [Member] | Classified - Substandard [Member]
Risk Category Of Loans [Line Items]
Total	3,588	5,866
Commercial Real Estate - Construction [Member] | Classified - Nonaccrual [Member]
Risk Category Of Loans [Line Items]
Total	6,195	6,499
Commercial Real Estate - Other [Member]
Risk Category Of Loans [Line Items]
Total	1,662,868	1,576,558
Commercial Real Estate - Other [Member] | Pass [Member]
Risk Category Of Loans [Line Items]
Total	1,456,118	1,374,191
Commercial Real Estate - Other [Member] | Criticized [Member]
Risk Category Of Loans [Line Items]
Total	102,945	102,805
Commercial Real Estate - Other [Member] | Classified - Substandard [Member]
Risk Category Of Loans [Line Items]
Total	38,602	38,659
Commercial Real Estate - Other [Member] | Classified - Nonaccrual [Member]
Risk Category Of Loans [Line Items]
Total	63,031	59,771
Commercial Real Estate - Other [Member] | Classified - Doubtful [Member]
Risk Category Of Loans [Line Items]
Total	$ 2,172	$ 1,132