Summary of Accounting Policies - Additional Information (Detail)		12 Months Ended
	Nov. 30, 2017	Dec. 31, 2017 USD ($) Segment	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Number of operating segments | Segment		1
Number of reportable segments | Segment		1
Principal and interest default period		90 days
Period of principal and interest payments in arrears for open-end consumer loans are reviewed for charge-off		4 months
Residential mortgage loans are placed on non-accrual status at the time principal and interest period		90 days
Non-accrual asset satisfactory payments period		6 months
Non-accrual loans with recorded investment, minimum amount		$ 500,000
Maximum amount of individual impaired commercial loans that places into homogeneous pool for collective evaluation		500,000
Amount of loans sold that are being serviced for others		23,000,000	$ 26,900,000
Originated mortgage servicing rights		88,000	124,000
Goodwill likelihood probability percentage	50.00%
Bank owned life insurance		107,489,000	72,384,000
Income on bank owned life insurance		2,354,000	$ 2,562,000	$ 2,017,000
Company's liability to trusts including investments		30,000,000
Cumulative effect of new accounting principle in period of adoption		$ (420,000)
Percentage of capital permitted for trust preferred securities		25.00%
Excess tax benefits from share-based compensation		$ 587,000
ASU 2018-02 | Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of new accounting principle in period of adoption		420,000
ASU 2018-02 | Accumulated Other Comprehensive Income
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of new accounting principle in period of adoption		$ 420,000