Fair Value - Quantitative Information about Level 3 Fair Value Measurements for Financial Instruments Measured at Fair Value on Non Recurring Basis (Detail) - Impaired Loans - Commercial Mortgages [Member] - Discounted Cash Flow [Member] - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value	$ 60	$ 11
Valuation Technique	10.00%	10.00%
Weighted Average [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Valuation Technique	10.00%	10.00%