Fair Value - Quantitative Information about Level 3 Fair Value Measurements for Financial Instruments Measured at Fair Value on Non Recurring Basis (Detail) - Fair Value, Measurements, Non Recurring - Significant Unobservable Inputs (Level 3) $ in Thousands	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value		$ 11
Commercial, industrial, and agricultural
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	$ 783
Commercial, industrial, and agricultural | Measurement Input, Loss Severity | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Valuation technique	0.55
Commercial, industrial, and agricultural | Measurement Input, Loss Severity | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Valuation technique	0.60
Commercial, industrial, and agricultural | Measurement Input, Loss Severity | Weighted Average
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Valuation technique	0.57
Commercial mortgages
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	$ 224
Commercial mortgages | Measurement Input, Loss Severity
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Valuation technique		0.10
Commercial mortgages | Measurement Input, Loss Severity | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Valuation technique	0.15
Commercial mortgages | Measurement Input, Loss Severity | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Valuation technique	0.39
Commercial mortgages | Measurement Input, Loss Severity | Weighted Average
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Valuation technique	0.37	0.10