Accumulated Other Comprehensive Income - Balances in Accumulated Other Comprehensive Income (Loss) (Parenthetical) (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Unrealized holding gains arising during the period, tax expense (benefit)	$ 422	$ (550)
Reclassification for previously unrealized net (gains) losses recognized in income, tax benefit	(9)	1
Net gain (loss) pension and postretirement plans, tax expense	301	143
Net postretirement plan transition cost, tax expense		1
Holding gains (losses) on securities
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Unrealized holding gains arising during the period, tax expense (benefit)	422	(550)
Reclassification for previously unrealized net (gains) losses recognized in income, tax benefit	(9)	1
Pension and Post employment costs
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net gain (loss) pension and postretirement plans, tax expense	301	143
Net postretirement plan transition cost, tax expense		$ 1