Loans and Leases - Narrative for Impaired Loans (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Receivables [Abstract]
Interest income recognized on a cash basis for nonaccrual loans	$ 4	$ 8	$ 37
Interest income recognized on accrual method for accruing impaired loans	1,100	1,400	$ 1,500
Reserve for uncollected interest	3	10
Off-balance Sheet Risks, Amount, Liability	$ 390	$ 385