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Note 4 - Securities - Securities in a Continuous Loss Position (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Temporarily impaired securities, less than 12 months, fair value	$ 115,489	$ 87,068
Temporarily impaired securities, less than 12 months unrealized loss	4,027	1,065
Temporarily impaired securities, 12 months or more fair value	250,876	229,675
Temporarily impaired securities, 12 months or more unrealized loss	7,693	4,750
US Government Agencies Debt Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	68,197	68,380
Temporarily impaired securities, less than 12 months unrealized loss	2,570	871
Temporarily impaired securities, 12 months or more fair value	244,485	225,738
Temporarily impaired securities, 12 months or more unrealized loss	7,181	4,623
US States and Political Subdivisions Debt Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	42,422	18,688
Temporarily impaired securities, less than 12 months unrealized loss	1,293	194
Temporarily impaired securities, 12 months or more fair value	5,514	2,989
Temporarily impaired securities, 12 months or more unrealized loss	411	98
Corporate Debt Securities [Member]
Temporarily impaired securities, less than 12 months, fair value	4,870
Temporarily impaired securities, less than 12 months unrealized loss	164
Temporarily impaired securities, 12 months or more fair value	877	948
Temporarily impaired securities, 12 months or more unrealized loss	$ 101	$ 29

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

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