Restrictions on Cash and Due from Banks and Interest-earning Deposit Accounts - Additional Information (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Cash and Cash Equivalents [Abstract]
Reserve requirement at Federal Reserve Bank	$ 6,700	$ 6,600
Average balances at Federal Reserve Bank	24,500	10,200
Pledging requirement for credit derivatives and SWAP agreements - cash	$ 400	$ 50