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Note 15 - Fair Value Measurements - Financial Instruments (Details) - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019
--- | --- | ---
Reported Value Measurement [Member] |  |
Cash and due from banks | $ 13,147 | $ 10,290
Interest-bearing deposits | 120,725 | 76,881
Securities | 546,742 | 435,263
Restricted securities | 1,279 | 1,220
Mortgage loans held for sale | 866 | 905
Loans, net | 760,318 | 726,588
Accrued interest receivable | 5,028 | 4,285
Bank-owned life insurance | 36,444 | 35,567
Deposits | 1,297,143 | 1,119,753
Accrued interest payable | 56 | 144
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member] |  |
Cash and due from banks | 13,147 | 10,290
Interest-bearing deposits | 120,725 | 76,881
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member] |  |
Securities | 546,742 | 435,263
Restricted securities | 1,279 | 1,220
Mortgage loans held for sale | 866 | 905
Accrued interest receivable | 5,028 | 4,285
Bank-owned life insurance | 36,444 | 35,567
Deposits | 1,207,561 | 991,725
Accrued interest payable | 56 | 144
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 3 [Member] |  |
Loans, net | 752,624 | 718,299
Deposits | $ 89,681 | $ 128,011

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- Definition This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This element represents the fair value of accrued interest receivable. + References No definition available. + Details
Name: | nksh_AccruedInterestReceivableFairValueDisclosure
Namespace Prefix: | nksh_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition For banks and other depository institutions: Includes fair value of cash on hand (currency and coin), cash items in process of collection, noninterest bearing deposits due from other financial institutions (including corporate credit unions), and noninterest bearing balances with the Federal Reserve Banks, Federal Home Loan Banks and central banks. + References No definition available. + Details
Name: | nksh_CashAndDueFromBanksFairValueDisclosure
Namespace Prefix: | nksh_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition For banks and other depository institutions (including Federal Reserve Banks, if applicable). Fair value of interest-bearing deposits in other financial institutions for relatively short periods of time including, for example, certificates of deposits, which are presented separately from cash on the balance sheet. + References No definition available. + Details
Name: | nksh_InterestBearingDepositsFairValueDisclosure
Namespace Prefix: | nksh_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition The fair value of marketable securities including held to maturity and available for sale. + References No definition available. + Details
Name: | nksh_MarketableSecuritiesFairValue
Namespace Prefix: | nksh_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Fair value portion of accrued expenses. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.20) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 + Details
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Namespace Prefix: | us-gaap_
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Balance Type: | credit
Period Type: | instant

X
- Definition Fair value portion of the amount that could be realized under a life insurance contract or contracts owned by the entity, commonly known as corporate-owned life insurance (COLI) or bank-owned life insurance (BOLI). + References No definition available. + Details
Name: | us-gaap_CashSurrenderValueFairValueDisclosure
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Fair value portion of deposit liabilities held by the entity, including, but not limited to, foreign and domestic, interest and noninterest bearing, demand deposits, saving deposits, negotiable orders of withdrawal (NOW) and time deposits. + References No definition available. + Details
Name: | us-gaap_DepositsFairValueDisclosure
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Fair value portion of loans held-for-sale including, but not limited to, mortgage loans and finance receivables. + References No definition available. + Details
Name: | us-gaap_LoansHeldForSaleFairValueDisclosure
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Fair value portion of loan receivable, including, but not limited to, mortgage loans held for investment, finance receivables held for investment, policy loans on insurance contracts. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -URI http://asc.fasb.org/subtopic&trid=2196772 + Details
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Namespace Prefix: | us-gaap_
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Balance Type: | debit
Period Type: | instant

X
- Definition The aggregate value of all restricted investments. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 946 -SubTopic 320 -Section S99 -Paragraph 1 -Subparagraph (SX 210.12-12.6(c)) -URI http://asc.fasb.org/extlink&oid=120401592&loc=d3e611133-123010 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 946 -SubTopic 320 -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-12B(Footnote 11(c))) -URI http://asc.fasb.org/extlink&oid=120401592&loc=d3e611197-123010 + Details
Name: | us-gaap_RestrictedInvestmentsAtFairValue
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Details
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