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] - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Discounted Cash Flow [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value	$ 11
Discounted cash flow method	10.00%
Sales Comparison Approach and Discounted Cash Flow Method [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value		$ 2,067
Weighted Average [Member] | Discounted Cash Flow [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Discounted cash flow method	10.00%
Weighted Average [Member] | Sales Comparison Approach and Discounted Cash Flow Method [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Sales comparison approach and discounted cash flow method		13.00%
Minimum [Member] | Sales Comparison Approach and Discounted Cash Flow Method [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Sales comparison approach and discounted cash flow method		10.00%
Maximum [Member] | Sales Comparison Approach and Discounted Cash Flow Method [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Sales comparison approach and discounted cash flow method		14.00%