Fair Value Measurement - Quantitative Information about Recurring Level 3 Fair Value Measurements (Detail) - Mortgage Servicing Rights [Member] $ in Thousands	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Mortgages Servicing Rights, Fair Value	$ 7,098	$ 6,687	$ 6,595	$ 7,618
Minimum [Member] | Valuation Technique, Discounted Cash Flow [Member] | Measurement Input, Constant Prepayment Rate [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Constant prepayment rate	0.050	0.062
Minimum [Member] | Valuation Technique, Discounted Cash Flow [Member] | Measurement Input, Discount Rate [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Constant prepayment rate	0.12	0.130
Maximum [Member] | Valuation Technique, Discounted Cash Flow [Member] | Measurement Input, Constant Prepayment Rate [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Constant prepayment rate	0.273	0.220
Maximum [Member] | Valuation Technique, Discounted Cash Flow [Member] | Measurement Input, Discount Rate [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Constant prepayment rate	0.13	0.150
Weighted Average [Member] | Valuation Technique, Discounted Cash Flow [Member] | Measurement Input, Constant Prepayment Rate [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Constant prepayment rate	0.076	0.089
Weighted Average [Member] | Valuation Technique, Discounted Cash Flow [Member] | Measurement Input, Discount Rate [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Constant prepayment rate	0.12	0.130