Allowance for Credit Losses - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Financing Receivable, Allowance for Credit Losses [Line Items]
Specific loss allocations are calculated for loans in excess	$ 500,000
Number of days for collecting principal or interest on consumer loans	90 days
Minimum collection period for home equity loan or high loan to value loan	180 days
Retail credits amount deemed unrecoverable charged-off period	60 days
Retail credits charged off period after discovery of the fraud	90 days
Reserve for lending-related commitments	$ 1,518,000	$ 2,143,000
Closed-End Retail Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Number of days delinquent from contractual due date	120 days
Open-End Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Number of days delinquent from contractual due date	180 days