Disclosures about Fair Value of Assets and Liabilities - Other Assets Measured at Fair Value on Non-recurring Basis (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Significant Unobservable Inputs (Level 3) [Member] | Impaired Loans [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Impaired loans	$ 3,433	$ 2,246
Significant Unobservable Inputs (Level 3) [Member] | Mortgage Servicing Rights [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Impaired loans	11,485	11,174
Fair Value, Measurements, Nonrecurring [Member] | Impaired Loans [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Impaired loans	3,433	2,246
Fair Value, Measurements, Nonrecurring [Member] | Mortgage Servicing Rights [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Impaired loans	11,485	11,174
Fair Value, Measurements, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Impaired Loans [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Impaired loans	3,433	2,246
Fair Value, Measurements, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Mortgage Servicing Rights [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Impaired loans	$ 11,485	$ 11,174