Other Comprehensive Income - Changes in Each Component of Accumulated Other Comprehensive Income (Loss), Net of Tax (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income(loss), beginning balance			$ 125,173
Amounts reclassified from accumulated other comprehensive income (loss)	$ 43	$ 46	152	$ 138
Total other comprehensive income (loss)	(753)	826	1,666	195
Accumulated other comprehensive income(loss), ending balance	138,309		138,309
Unrealized Gains (Losses) on Available-for-Sale Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income(loss), beginning balance	5,863	3,007	3,554	3,730
Other comprehensive income (loss) before reclassifications	(796)	780	1,514	57
Amounts reclassified from accumulated other comprehensive income (loss)	(12)		(13)
Total other comprehensive income (loss)	(808)	780	1,501	57
Accumulated other comprehensive income(loss), ending balance	5,055	3,787	5,055	3,787
Defined Benefit Pension Items [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income(loss), beginning balance	(3,939)	(3,685)	(4,049)	(3,777)
Amounts reclassified from accumulated other comprehensive income (loss)	55	46	165	138
Total other comprehensive income (loss)	55	46	165	138
Accumulated other comprehensive income(loss), ending balance	(3,884)	(3,639)	(3,884)	(3,639)
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income(loss), beginning balance	1,924	(678)	(495)	(47)
Other comprehensive income (loss) before reclassifications	(796)	780	1,514	57
Amounts reclassified from accumulated other comprehensive income (loss)	43	46	152	138
Total other comprehensive income (loss)	(753)	826	1,666	195
Accumulated other comprehensive income(loss), ending balance	$ 1,171	$ 148	$ 1,171	$ 148