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Note 7 - Fair Value Measurements - Impaired Loans and Other Real Estate Owned Measured at Fair Value on Nonrecurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Jun. 03, 2019	Mar. 31, 2019	Dec. 31, 2018
Impaired loans net of valuation allowance			$ 1,355	$ 1,014
Other real estate owned net of valuation allowance			2,025	2,052
Fair Value, Inputs, Level 1 [Member]
Impaired loans net of valuation allowance		0		0
Other real estate owned net of valuation allowance	$ 0
Fair Value, Inputs, Level 2 [Member]
Impaired loans net of valuation allowance		0		0
Other real estate owned net of valuation allowance	0
Fair Value, Inputs, Level 3 [Member]
Impaired loans net of valuation allowance			$ 1,355	1,014
Other real estate owned net of valuation allowance	$ 2,025			$ 2,052

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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 impaired loans net of valuation allowance.

+ References

No definition available.

+ Details

Name:	nksh_ImpairedLoansNetFairValueDisclosure
Namespace Prefix:	nksh_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

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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 other real estate owned by bank.

+ References

No definition available.

+ Details

Name:	nksh_OtherRealEstateFairValueDisclosure
Namespace Prefix:	nksh_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

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- Details

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Namespace Prefix:
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- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix:
Data Type:	na
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