Fair Value - Quantitative Information about Level 3 (Detail) - Fair Value Measurement on Recurring Basis - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Commercial mortgages
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Impaired loans – commercial mortgages	$ 321	$ 11
Commercial mortgages | Significant Unobservable Inputs (Level 3)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Impaired loans – commercial mortgages	$ 321	$ 11
Commercial mortgages | Significant Unobservable Inputs (Level 3) | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value, measurement input	15.00%	10.00%
Commercial mortgages | Significant Unobservable Inputs (Level 3) | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value, measurement input	10.00%	10.00%
Commercial mortgages | Significant Unobservable Inputs (Level 3) | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value, measurement input	15.00%	10.00%
Commercial, Industrial, and Agricultural
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Impaired loans – commercial mortgages	$ 3,004
Commercial, Industrial, and Agricultural | Significant Unobservable Inputs (Level 3)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Impaired loans – commercial mortgages	$ 3,004
Commercial, Industrial, and Agricultural | Significant Unobservable Inputs (Level 3) | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value, measurement input	25.00%
Commercial, Industrial, and Agricultural | Significant Unobservable Inputs (Level 3) | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value, measurement input	25.00%
Commercial, Industrial, and Agricultural | Significant Unobservable Inputs (Level 3) | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value, measurement input	25.00%