Fair Value - Quantitative Information about Level 3 (Detail) - Commercial mortgages - Fair Value Measurement on Recurring Basis - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Impaired loans – commercial mortgages	$ 324	$ 11
Significant Unobservable Inputs (Level 3)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Impaired loans – commercial mortgages	$ 324	$ 11
Measurement Input, Loss Severity | Significant Unobservable Inputs (Level 3)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value, measurement input		10.00%
Measurement Input, Loss Severity | Significant Unobservable Inputs (Level 3) | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value, measurement input	15.00%	10.00%
Measurement Input, Loss Severity | Significant Unobservable Inputs (Level 3) | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value, measurement input	10.00%
Measurement Input, Loss Severity | Significant Unobservable Inputs (Level 3) | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value, measurement input	15.00%