Income Taxes - Significant Components of Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Deferred tax assets:
Loan loss reserves	$ 6,382	$ 6,199
Postretirement benefits	104	564
Depreciation	742	611
Other real estate owned valuation	351	430
Tax credits carryforward	1,416	951
Unrealized loss on securities available for sale		1,341
Unrealized loss on securities held to maturity	454	517
Interest on nonaccrual loans	978	834
Net operating loss carryforward	10,817	16,903
Purchase accounting adjustment	89	90
Accrued bonuses	799	723
Other	59	50
Deferred tax assets	22,191	29,213
Deferred tax liabilities:
Deferred loan fees	1,002	510
Federal Home Loan Bank stock dividends	4,585	4,585
Mortgage servicing rights	1,839	1,976
FHLB prepayment penalty	854	1,059
Unrealized gains on securities available for sale	3,775
Postretirement benefits accrual	97	447
Prepaid expenses	155	215
Deferred tax liabilities	12,307	8,792
Net deferred tax asset	$ 9,884	$ 20,421