Income Taxes - Components of Net Deferred Tax Asset (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Allowance for loan losses	$ 2,840	$ 4,679
Fair value adjustments - business combination	1,135	2,530
Deferred compensation	1,903	2,891
Impaired security valuation	0	379
Net operating loss carryover	71	627
Post-retirement benefits	892	1,688
Unrealized loss on interest rate swap	34	161
Nonaccrual loan interest	390	649
Accrued expenses	699	1,282
Deferred fees and costs	598	629
Other	344	465
Deferred tax assets	8,906	15,980
Deferred tax liabilities:
Unrealized gain on securities available for sale	126	750
Premises and equipment	1,413	2,258
Unrealized gain on trading securities	156	118
Intangibles - section 197	2,593	4,735
Mortgage servicing rights	291	487
Other	78	23
Deferred tax liabilities	4,657	8,371
Net deferred tax asset	$ 4,249	$ 7,609