Loans and Allowance for Credit Losses - Schedule of Activity of Purchased Impaired Loans (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Financing Receivable, Impaired [Line Items]
Purchased impaired loans	$ 44,167	$ 47,370
Carrying amount, net of allowance	42,465	45,831
Allowance for loan losses	1,702	1,539
Commercial Loan [Member]
Financing Receivable, Impaired [Line Items]
Purchased impaired loans	2,735	2,770
Allowance for loan losses	486	406
Commercial Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Purchased impaired loans	35,068	37,394
Allowance for loan losses	1,139	1,049
Consumer Loan [Member]
Financing Receivable, Impaired [Line Items]
Purchased impaired loans	6,233	7,073
Allowance for loan losses	63	67
Residential [Member]
Financing Receivable, Impaired [Line Items]
Purchased impaired loans	131	133
Allowance for loan losses	$ 14	$ 17