Fair Value - Summary of Assets Measured at Fair Value on Non-recurring Basis (Detail) - Level 3 [Member] - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Impaired Loans [Member] | Commercial Real Estate [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Assets measured at fair value on non-recurring basis	$ 431	$ 1,439
Impaired Loans [Member] | Commercial and Industrial [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Assets measured at fair value on non-recurring basis	381	1,005
Impaired Loans [Member] | Residential Real Estate [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Assets measured at fair value on non-recurring basis	3,848	4,021
Impaired Loans [Member] | Agricultural Real Estate [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Assets measured at fair value on non-recurring basis	255	905
Impaired Loans [Member] | Other [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Assets measured at fair value on non-recurring basis	601	1,910
Other Real Estate Owned [Member] | Commercial Real Estate [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Assets measured at fair value on non-recurring basis	1,604	1,018
Other Real Estate Owned [Member] | Residential Real Estate [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Assets measured at fair value on non-recurring basis	$ 131	$ 157