Fair Value Measurement - Assets and Liabilities Measured at Fair Value on Non-recurring Basis (Detail) - USD ($)	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018
Mortgage Servicing Rights [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Mortgage servicing rights	$ 3,000,000		$ 282,000
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	668,000
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	668,000
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	467,000	$ 540,000
Other real estate owned, net	113,000	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	467,000	540,000
Other real estate owned, net	113,000	121,000
Fair Value, Measurements, Nonrecurring [Member] | Home Equity [Member] | Consumer Loans [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Impaired loans	19,000
Fair Value, Measurements, Nonrecurring [Member] | Home Equity [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	19,000
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	14,000	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	14,000	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	66,000
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	66,000
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	3,018,000	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	3,018,000	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	58,000
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	$ 58,000
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