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Note 3 - Securities - Securities in a Continuous Loss Position (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Temporarily impaired securities, less than 12 months, fair value	$ 69,967	$ 270,457
Temporarily impaired securities, less than 12 months unrealized loss	498	2,769
Temporarily impaired securities, 12 months or more fair value	4,695	39,553
Temporarily impaired securities, 12 months or more unrealized loss	5	85
US Government Agencies Debt Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	28,798	53,244
Temporarily impaired securities, less than 12 months unrealized loss	173	738
Temporarily impaired securities, 12 months or more fair value		38,962
Temporarily impaired securities, 12 months or more unrealized loss		37
US States and Political Subdivisions Debt Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	32,353	35,934
Temporarily impaired securities, less than 12 months unrealized loss	249	596
Temporarily impaired securities, 12 months or more fair value	635	591
Temporarily impaired securities, 12 months or more unrealized loss	3	48
Commercial Mortgage Backed Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	8,816	181,279
Temporarily impaired securities, less than 12 months unrealized loss	76	$ 1,435
Temporarily impaired securities, 12 months or more fair value	4,060
Temporarily impaired securities, 12 months or more unrealized loss	$ 2

X

- Definition

Amount of the excess of amortized cost basis over fair value of securities that have been in a loss position for twelve months or longer for securities.

+ References

No definition available.

+ Details

Name:	nksh_ContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses
Namespace Prefix:	nksh_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Amount of the excess of amortized cost basis over fair value of securities that have been in a loss position for less than twelve months for securities.

+ References

No definition available.

+ Details

Name:	nksh_ContinuousUnrealizedLossPositionLessThan12MonthsAggregateLosses
Namespace Prefix:	nksh_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

This item represents the aggregate fair value of investments in debt and equity securities that have been in a continuous unrealized loss position for less than twelve months.

+ References

No definition available.

+ Details

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

X

- Definition

This item represents the aggregate fair value of investments in debt and equity securities that have been in a continuous unrealized loss position for twelve months or longer.

+ References

No definition available.

+ Details

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

X

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USGovernmentAgenciesDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USStatesAndPoliticalSubdivisionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CommercialMortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: