Fair Value Measurement - Additional Information (Detail) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Increase in fair value	$ 250,000
Transfers between level 1 and level 2	0		$ 0
Fair value of the collateral dependent loans, net carrying amount	2,500,000	$ 499,000	967,000
Specific allowance for collateral dependent loans	$ 105,000	491,000	69,000
Discount applied to appraisals for estimated selling costs percentage	10.00%
Net (recovery) impairment reflected in other income	$ 499,000	(9,000)
Mortgage servicing rights valuation allowance	569,000		70,000	$ 9,000
Other real estate owned carried at fair value	175,000	386,000	302,000
Valuation allowance related to other real estate owned	318,000	287,000	308,000
Additions charged to expense	19,000	23,000
Collateral Dependent Loans [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Increase (decrease) in provision for loan losses	36,000	42,000
Mortgage Servicing Rights [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Mortgage servicing rights	3,100,000	$ 0
Construction [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair value option, loans held as assets, aggregate amount in nonaccrual status	$ 0		$ 0