Estimated Fair Value of Financial Instruments and Fair Value Measurement - Fair Value of Assets Measured on Non-recurring Basis (Detail) - Fair Value, Measurements, Non-recurring - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Assets:
Impaired loans and leases	$ 18,380	$ 22,591
Loans held for sale	1,340	456
Other real estate owned and other repossessed assets	2,754	843
(Level 1)
Assets:
Impaired loans and leases	0	0
Loans held for sale	0	0
Other real estate owned and other repossessed assets	0	0
(Level 2)
Assets:
Impaired loans and leases	0	0
Loans held for sale	1,340	456
Other real estate owned and other repossessed assets	0	0
(Level 3)
Assets:
Impaired loans and leases	18,380	22,591
Loans held for sale	0	0
Other real estate owned and other repossessed assets	$ 2,754	$ 843