Accumulated Other Comprehensive Income - Balances in Accumulated Other Comprehensive Income (Loss) (Parenthetical) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income before reclassifications, tax expense (benefit)	$ 175	$ 46
Reclassification for previously unrealized net gains recognized in income, tax expense	148	14
Net gain pension and postretirement plans, tax expense	87	42
Net Unrealized Gains (Losses) on Securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income before reclassifications, tax expense (benefit)	175	46
Reclassification for previously unrealized net gains recognized in income, tax expense	148	14
Pension and Post employment costs
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net gain pension and postretirement plans, tax expense	$ 87	$ 42