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Note 3 - Securities - Securities in a Continuous Loss Position (Details) - USD ($) $ in Thousands	Mar. 31, 2023	Dec. 31, 2022
Temporarily impaired securities, less than 12 months, fair value	$ 79,750	$ 389,208
Temporarily impaired securities, less than 12 months unrealized loss	3,615	39,109
Temporarily impaired securities, 12 months or more fair value	570,128	259,070
Temporarily impaired securities, 12 months or more unrealized loss	84,606	64,043
US Treasury Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	0	936
Temporarily impaired securities, less than 12 months unrealized loss	0	56
Temporarily impaired securities, 12 months or more fair value	947	0
Temporarily impaired securities, 12 months or more unrealized loss	46	0
US Government Agencies Debt Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	50,869	144,574
Temporarily impaired securities, less than 12 months unrealized loss	2,032	12,699
Temporarily impaired securities, 12 months or more fair value	283,074	190,950
Temporarily impaired securities, 12 months or more unrealized loss	46,616	42,303
US States and Political Subdivisions Debt Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	9,432	94,657
Temporarily impaired securities, less than 12 months unrealized loss	1,214	18,373
Temporarily impaired securities, 12 months or more fair value	139,371	52,134
Temporarily impaired securities, 12 months or more unrealized loss	30,041	19,645
Commercial Mortgage-Backed Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	18,576	144,198
Temporarily impaired securities, less than 12 months unrealized loss	242	7,326
Temporarily impaired securities, 12 months or more fair value	141,868	15,165
Temporarily impaired securities, 12 months or more unrealized loss	7,269	1,913
Corporate Debt Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	873	4,843
Temporarily impaired securities, less than 12 months unrealized loss	127	655
Temporarily impaired securities, 12 months or more fair value	4,868	821
Temporarily impaired securities, 12 months or more unrealized loss	$ 634	$ 182

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

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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

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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

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X

- Details

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Data Type:	na
Balance Type:
Period Type:

X

- Details

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Namespace Prefix:
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Balance Type:
Period Type:

X

- Details

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Balance Type:
Period Type:

X

- Details

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