Income Taxes - Reconciliation of Deferred Taxes Included in Other Assets or Other Liabilities (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Deferred tax assets:
Allowance for loan losses	$ 3,130	$ 3,037	$ 3,011
Employee benefit plans' liabilities	4,490	4,326	4,135
Unrealized loss on available for sale securities, charged from equity	210	3,684
Earned retiree health benefits plan liability	1,638	1,638	1,673
General business and AMT credits	1,735
Tax net operating loss carryforward	651
Other	1,637	1,218	1,170
Valuation allowance	(8,140)
Deferred tax assets	5,351	13,903	9,989
Deferred tax liabilities:
Unrealized gain on available for sale securities, charged to equity			1,556
Unearned retiree health benefits plan asset	362	579	948
Bank premises and equipment	4,760	5,075	5,366
Other	229	129	92
Deferred tax liabilities	5,351	5,783	7,962
Net deferred taxes		$ 8,120	$ 2,027