Accounting for Certain Loans Acquired in a Transfer - Additional Information (Detail) (USD $)	3 Months Ended							12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Business Acquisition [Line Items]
Total provision charged to operating expense	$ 978,000us-gaap_ProvisionForLoanAndLeaseLosses	$ 1,741,000us-gaap_ProvisionForLoanAndLeaseLosses	$ 339,000us-gaap_ProvisionForLoanAndLeaseLosses	$ (997,000)us-gaap_ProvisionForLoanAndLeaseLosses	$ 104,000us-gaap_ProvisionForLoanAndLeaseLosses	$ 729,000us-gaap_ProvisionForLoanAndLeaseLosses	$ 2,084,000us-gaap_ProvisionForLoanAndLeaseLosses	$ 3,058,000us-gaap_ProvisionForLoanAndLeaseLosses	$ 1,920,000us-gaap_ProvisionForLoanAndLeaseLosses	$ 3,524,000us-gaap_ProvisionForLoanAndLeaseLosses
Loans Purchased With Evidence Of Credit Deterioration [Member]
Business Acquisition [Line Items]
Total provision charged to operating expense								253,000	2,600,000
Allowances for loan losses								$ 283,000	$ 0