XML 73 R61.htm IDEA: XBRL DOCUMENT
v3.19.1

Fair Value Measurements - Narrative (Detail) - USD ($) | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2019 | Dec. 31, 2018
Loans held for sale | Nonrecurring basis |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Non recurring fair value adjustments on loans held for sale | $ 0 | $ 0
Maximum |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Period under consideration for valuation of real estate collateral | 12 months |
Period under consideration for valuation of business equipment | 1 year |
Minimum |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Period under consideration for valuation of house or building in the process of construction | 1 year |

X
- Definition Period of appraisal of property to consider different method to measure fair value. + References No definition available. + Details
Name: | fxnc_PeriodOfAppraisalOfPropertyToConsiderDifferentMethodToMeasureFairValue
Namespace Prefix: | fxnc_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Period used to determine value of business equipment. + References No definition available. + Details
Name: | fxnc_PeriodUsedToDetermineValueOfBusinessEquipment
Namespace Prefix: | fxnc_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Maximum period used to determine value of real estate collateral. + References No definition available. + Details
Name: | fxnc_PeriodUsedToDetermineValueOfRealEstateCollateral
Namespace Prefix: | fxnc_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of addition (reduction) to the amount at which an asset could be incurred (settled) in a current transaction between willing parties. + References No definition available. + Details
Name: | us-gaap_AssetsFairValueAdjustment
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_InvestmentTypeAxis=fxnc_LoansHeldForSaleMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FairValueByMeasurementFrequencyAxis=us-gaap_FairValueMeasurementsNonrecurringMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |