Income Taxes - Significant Components of Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Deferred tax assets:
Loan loss reserves	$ 5,909	$ 6,190
Postretirement benefits	1,042	1,066
Depreciation	735	625
Other real estate owned valuation	418	498
Tax credits carryforward	736	513
Unrealized loss on securities available for sale	3,426	2,324
Interest on nonaccrual loans	933	943
Net operating loss carryforward	20,315	24,027
Purchase accounting adjustment	86	82
Accrued bonuses	405	459
Other	338	279
Deferred tax assets	34,343	37,006
Deferred tax liabilities:
Deferred loan fees	369	321
Federal Home Loan Bank stock dividends	4,585	4,585
Mortgage servicing rights	1,960	1,917
FHLB prepayment penalty	1,195	1,332
Postretirement benefits accrual	493	493
Prepaid expenses	248	201
Deferred tax liabilities	8,850	8,849
Net deferred tax asset	$ 25,493	$ 28,157