Disclosures about Fair Value of Assets and Liabilities - Other Assets Measured at Fair Value on Non-recurring Basis (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Impaired loans	$ 11,477
Significant Unobservable Inputs (Level 3) [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Impaired loans	11,477
Significant Unobservable Inputs (Level 3) [Member] | Impaired Loans [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Impaired loans	6,593	$ 6,957
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	11,477	11,602
Fair Value, Measurements, Nonrecurring [Member] | Impaired Loans [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Impaired loans	6,593	6,957
Fair Value, Measurements, Nonrecurring [Member] | Mortgage Servicing Rights [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Impaired loans		11,602
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	$ 6,593	6,957
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		$ 11,602