Reclassifications Out of Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amounts reclassified from accumulated other comprehensive income (loss)	$ (33)	$ (1,260)	$ (39)	$ (189)
Unrealized Gains (Losses) on Securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amounts reclassified from accumulated other comprehensive income (loss), before tax amount	(77)	(1,931)	(112)	(244)
Amounts reclassified from accumulated other comprehensive income (loss), tax expense (benefit)	26	663	34	24
Amounts reclassified from accumulated other comprehensive income (loss)	(51)	(1,268)	(78)	(220)
Defined Benefit Pension Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amounts reclassified from accumulated other comprehensive income (loss), before tax amount	28	12	60	47
Amounts reclassified from accumulated other comprehensive income (loss), tax expense (benefit)	(10)	(4)	(21)	(16)
Amounts reclassified from accumulated other comprehensive income (loss)	$ 18	$ 8	$ 39	$ 31