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Note 4 - Securities - Securities in a Continuous Loss Position (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Temporarily impaired securities, less than 12 months, fair value	$ 154,185	$ 69,967
Temporarily impaired securities, less than 12 months unrealized loss	3,641	498
Temporarily impaired securities, 12 months or more fair value	12,896	4,695
Temporarily impaired securities, 12 months or more unrealized loss	311	5
US Government Agencies Debt Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	90,558	28,798
Temporarily impaired securities, less than 12 months unrealized loss	2,101	173
Temporarily impaired securities, 12 months or more fair value		0
Temporarily impaired securities, 12 months or more unrealized loss		0
US States and Political Subdivisions Debt Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	57,581	32,353
Temporarily impaired securities, less than 12 months unrealized loss	1,538	249
Temporarily impaired securities, 12 months or more fair value	7,709	635
Temporarily impaired securities, 12 months or more unrealized loss	190	3
Commercial Mortgage Backed Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	6,046	8,816
Temporarily impaired securities, less than 12 months unrealized loss	2	76
Temporarily impaired securities, 12 months or more fair value	5,187	4,060
Temporarily impaired securities, 12 months or more unrealized loss	$ 121	$ 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: