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Note 8 - Fair Value Measurements (Details) - Impaired Loans and Other Real Estate Owned Measured at Fair Value on Nonrecurring Basis - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Note 8 - Fair Value Measurements (Details) - Impaired Loans and Other Real Estate Owned Measured at Fair Value on Nonrecurring Basis [Line Items]
Impaired loans	$ 2,352	$ 7,224
Other real estate owned	4,441	4,744
Fair Value, Inputs, Level 3 [Member]
Note 8 - Fair Value Measurements (Details) - Impaired Loans and Other Real Estate Owned Measured at Fair Value on Nonrecurring Basis [Line Items]
Impaired loans	2,352	7,224
Other real estate owned	$ 4,441	$ 4,744

X

- 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

X

- References

No definition available.

+ Details

Name:	nksh_Note8FairValueMeasurementsDetailsImpairedLoansandOtherRealEstateOwnedMeasuredatFairValueonNonrecurringBasisLineItems
Namespace Prefix:	nksh_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- 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

X

- Details

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