Income Taxes - Significant Components of Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Deferred tax assets:
Loan loss reserves	$ 5,916	$ 6,199
Postretirement benefits	459	564
Depreciation	671	611
Other real estate owned valuation	393	430
Tax credits carryforward	995	951
Unrealized loss on securities available for sale		1,341
Unrealized loss on securities held to maturity	499	517
Interest on nonaccrual loans	650	834
Net operating loss carryforward	16,259	16,903
Purchase accounting adjustment	91	90
Accrued bonuses	321	723
Other	106	50
Deferred tax assets	26,360	29,213
Deferred tax liabilities:
Deferred loan fees	581	510
Federal Home Loan Bank stock dividends	4,585	4,585
Mortgage servicing rights	1,839	1,976
FHLB prepayment penalty	990	1,059
Unrealized gains on securities available for sale	2,120
Postretirement benefits accrual	350	447
Prepaid expenses	292	215
Deferred tax liabilities	10,757	8,792
Net deferred tax asset	$ 15,603	$ 20,421