Fair Value Measurement - Assets and Liabilities Measured at Fair Value on Non-recurring Basis (Detail) - USD ($)	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018
Mortgage Servicing Rights [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Mortgage servicing rights	$ 3,100,000		$ 0
Fair Value, Measurements, Nonrecurring [Member] | Nonresidential [Member] | Commercial Loans [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Impaired loans	1,527,000
Fair Value, Measurements, Nonrecurring [Member] | Nonresidential [Member] | Significant Unobservable Inputs (Level 3) [Member] | Commercial Loans [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Impaired loans	1,527,000
Fair Value, Measurements, Nonrecurring [Member] | Secured Commercial [Member] | Commercial Loans [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Impaired loans	310,000
Fair Value, Measurements, Nonrecurring [Member] | Secured Commercial [Member] | Significant Unobservable Inputs (Level 3) [Member] | Commercial Loans [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Impaired loans	310,000
Fair Value, Measurements, Nonrecurring [Member] | One-to Four-Family [Member] | Residential Mortgage Loans [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Impaired loans	241,000	$ 540,000
Other real estate owned, net	102,000	121,000
Fair Value, Measurements, Nonrecurring [Member] | One-to Four-Family [Member] | Significant Unobservable Inputs (Level 3) [Member] | Residential Mortgage Loans [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Impaired loans	241,000	540,000
Other real estate owned, net	102,000	121,000
Fair Value, Measurements, Nonrecurring [Member] | Auto [Member] | Commercial Loans [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other real estate owned, net		181,000
Fair Value, Measurements, Nonrecurring [Member] | Auto [Member] | Consumer Loans [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Impaired loans	8,000	8,000
Fair Value, Measurements, Nonrecurring [Member] | Auto [Member] | Significant Unobservable Inputs (Level 3) [Member] | Commercial Loans [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other real estate owned, net		181,000
Fair Value, Measurements, Nonrecurring [Member] | Auto [Member] | Significant Unobservable Inputs (Level 3) [Member] | Consumer Loans [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Impaired loans	8,000	8,000
Fair Value, Measurements, Nonrecurring [Member] | Marine [Member] | Consumer Loans [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Impaired loans	105,000
Fair Value, Measurements, Nonrecurring [Member] | Marine [Member] | Significant Unobservable Inputs (Level 3) [Member] | Consumer Loans [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Impaired loans	105,000
Fair Value, Measurements, Nonrecurring [Member] | Recreational Vehicle [Member] | Consumer Loans [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Impaired loans	342,000
Fair Value, Measurements, Nonrecurring [Member] | Recreational Vehicle [Member] | Significant Unobservable Inputs (Level 3) [Member] | Consumer Loans [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Impaired loans	342,000
Fair Value, Measurements, Nonrecurring [Member] | Mortgage Servicing Rights [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Mortgage servicing rights	3,126,000	2,990,000
Fair Value, Measurements, Nonrecurring [Member] | Mortgage Servicing Rights [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Mortgage servicing rights	3,126,000	2,990,000
Fair Value, Measurements, Nonrecurring [Member] | Construction [Member] | Commercial Loans [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other real estate owned, net	73,000
Fair Value, Measurements, Nonrecurring [Member] | Construction [Member] | Significant Unobservable Inputs (Level 3) [Member] | Commercial Loans [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other real estate owned, net	$ 73,000
Fair Value, Measurements, Nonrecurring [Member] | Secured [Member] | Commercial Loans [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Impaired loans		419,000
Fair Value, Measurements, Nonrecurring [Member] | Secured [Member] | Significant Unobservable Inputs (Level 3) [Member] | Commercial Loans [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Impaired loans		$ 419,000