Income Taxes - Significant Components of Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Deferred tax assets:
Loan loss reserves	$ 6,010	$ 6,199
Postretirement benefits	354	564
Depreciation	704	611
Other real estate owned valuation	351	430
Tax credits carryforward	1,246	951
Unrealized loss on securities available for sale		1,341
Unrealized loss on securities held to maturity	479	517
Interest on nonaccrual loans	908	834
Net operating loss carryforward	13,441	16,903
Purchase accounting adjustment	92	90
Accrued bonuses	571	723
Other	73	50
Deferred tax assets	24,229	29,213
Deferred tax liabilities:
Deferred loan fees	711	510
Federal Home Loan Bank stock dividends	4,585	4,585
Mortgage servicing rights	1,767	1,976
FHLB prepayment penalty	922	1,059
Unrealized gains on securities available for sale	4,512
Postretirement benefits accrual	253	447
Prepaid expenses	337	215
Deferred tax liabilities	13,087	8,792
Net deferred tax asset	$ 11,142	$ 20,421