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Allowance for Loan Losses, Nonperforming Assets and Impaired Loans, Non-impaired loans by credit quality indicator (Details) (USD $) In Thousands, unless otherwise specified | 9 Months Ended |  |  |  |  |  |  |  |  |  |  |  |  | 9 Months Ended |  |  |  |  |  |  |  |  |  |  |  | 9 Months Ended |  |  |  |  |  |  |  |  |  |  |
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Sep. 30, 2012 | Sep. 30, 2012 Pass [Member] | Dec. 31, 2011 Pass [Member] Scenario, Previously Reported [Member] | Dec. 31, 2011 Pass [Member] Commercial and Industrial [Member] Scenario, Previously Reported [Member] | Sep. 30, 2012 Pass [Member] Real Estate Construction [Member] Construction, 1-4 Family Residential [Member] | Sep. 30, 2012 Pass [Member] Real Estate Construction [Member] Construction other [Member] | Sep. 30, 2012 Pass [Member] Consumer Real Estate [Member] Equity lines [Member] | Dec. 31, 2011 Pass [Member] Consumer Real Estate [Member] Equity lines [Member] Scenario, Previously Reported [Member] | Dec. 31, 2011 Pass [Member] Consumer Real Estate [Member] Closed end Consumer Real Estate [Member] Scenario, Previously Reported [Member] | Dec. 31, 2011 Pass [Member] Consumer Real Estate [Member] Consumer Construction [Member] Scenario, Previously Reported [Member] | Sep. 30, 2012 Pass [Member] Consumer Real Estate [Member] Residential closed end first liens [Member] | Sep. 30, 2012 Pass [Member] Consumer Real Estate [Member] Residential closed end junior liens [Member] | Sep. 30, 2012 Pass [Member] Commercial Real Estate [Member] Commercial real estate, other [Member] | Dec. 31, 2011 Pass [Member] Commercial Real Estate [Member] Commercial real estate, other [Member] Scenario, Previously Reported [Member] | Sep. 30, 2012 Pass [Member] Commercial Real Estate [Member] Commercial real estate, owner occupied [Member] | Dec. 31, 2011 Pass [Member] Commercial Real Estate [Member] College Housing [Member] Scenario, Previously Reported [Member] | Dec. 31, 2011 Pass [Member] Commercial Real Estate [Member] Office/Retail [Member] Scenario, Previously Reported [Member] | Dec. 31, 2011 Pass [Member] Commercial Real Estate [Member] Nursing Homes [Member] Scenario, Previously Reported [Member] | Dec. 31, 2011 Pass [Member] Commercial Real Estate [Member] Hotel [Member] Scenario, Previously Reported [Member] | Dec. 31, 2011 Pass [Member] Commercial Real Estate [Member] Municipalities [Member] Scenario, Previously Reported [Member] | Sep. 30, 2012 Pass [Member] Commercial Real Estate [Member] Multifamily real estate [Member] | Dec. 31, 2011 Pass [Member] Commercial Real Estate [Member] Medical Professionals [Member] Scenario, Previously Reported [Member] | Dec. 31, 2011 Pass [Member] Commercial Real Estate [Member] Religious Organizations [Member] Scenario, Previously Reported [Member] | Dec. 31, 2011 Pass [Member] Commercial Real Estate [Member] Convenience Stores [Member] Scenario, Previously Reported [Member] | Dec. 31, 2011 Pass [Member] Commercial Real Estate [Member] Entertainment and Sports [Member] Scenario, Previously Reported [Member] | Dec. 31, 2011 Pass [Member] Commercial Real Estate [Member] Nonprofits [Member] Scenario, Previously Reported [Member] | Dec. 31, 2011 Pass [Member] Commercial Real Estate [Member] Restaurants [Member] Scenario, Previously Reported [Member] | Dec. 31, 2011 Pass [Member] Commercial Real Estate [Member] General Contractors [Member] Scenario, Previously Reported [Member] | Sep. 30, 2012 Pass [Member] Commercial Non Real Estate [Member] Commercial and Industrial [Member] | Sep. 30, 2012 Pass [Member] Public Sector and IDA [Member] States & Political Subdivisions [Member] | Sep. 30, 2012 Pass [Member] Consumer Non Real Estate [Member] Automobile [Member] | Sep. 30, 2012 Pass [Member] Consumer Non Real Estate [Member] Other Consumer [Member] | Sep. 30, 2012 Pass [Member] Consumer Non Real Estate [Member] Credit cards [Member] | Dec. 31, 2011 Pass [Member] Consumer Non Real Estate [Member] Credit cards [Member] Scenario, Previously Reported [Member] | Dec. 31, 2011 Pass [Member] Consumer Non Real Estate [Member] Consumer General [Member] Scenario, Previously Reported [Member] | Dec. 31, 2011 Pass [Member] Consumer Non Real Estate [Member] Consumer Overdraft [Member] Scenario, Previously Reported [Member] | Dec. 31, 2011 Pass [Member] Construction Development and Other land [Member] Residential [Member] Scenario, Previously Reported [Member] | Dec. 31, 2011 Pass [Member] Construction Development and Other land [Member] Commercial [Member] Scenario, Previously Reported [Member] | Sep. 30, 2012 Special Mention [Member] | Dec. 31, 2011 Special Mention [Member] Scenario, Previously Reported [Member] | Dec. 31, 2011 Special Mention [Member] Commercial and Industrial [Member] Scenario, Previously Reported [Member] | Sep. 30, 2012 Special Mention [Member] Real Estate Construction [Member] Construction, 1-4 Family Residential [Member] | Sep. 30, 2012 Special Mention [Member] Real Estate Construction [Member] Construction other [Member] | Sep. 30, 2012 Special Mention [Member] Consumer Real Estate [Member] Equity lines [Member] | Dec. 31, 2011 Special Mention [Member] Consumer Real Estate [Member] Equity lines [Member] Scenario, Previously Reported [Member] | Dec. 31, 2011 Special Mention [Member] Consumer Real Estate [Member] Closed end Consumer Real Estate [Member] Scenario, Previously Reported [Member] | Dec. 31, 2011 Special Mention [Member] Consumer Real Estate [Member] Consumer Construction [Member] Scenario, Previously Reported [Member] | Sep. 30, 2012 Special Mention [Member] Consumer Real Estate [Member] Residential closed end first liens [Member] | Sep. 30, 2012 Special Mention [Member] Consumer Real Estate [Member] Residential closed end junior liens [Member] | Sep. 30, 2012 Special Mention [Member] Commercial Real Estate [Member] Commercial real estate, other [Member] | Dec. 31, 2011 Special Mention [Member] Commercial Real Estate [Member] Commercial real estate, other [Member] Scenario, Previously Reported [Member] | Sep. 30, 2012 Special Mention [Member] Commercial Real Estate [Member] Commercial real estate, owner occupied [Member] | Dec. 31, 2011 Special Mention [Member] Commercial Real Estate [Member] College Housing [Member] Scenario, Previously Reported [Member] | Dec. 31, 2011 Special Mention [Member] Commercial Real Estate [Member] Office/Retail [Member] Scenario, Previously Reported [Member] | Dec. 31, 2011 Special Mention [Member] Commercial Real Estate [Member] Nursing Homes [Member] Scenario, Previously Reported [Member] | Dec. 31, 2011 Special Mention [Member] Commercial Real Estate [Member] Hotel [Member] Scenario, Previously Reported [Member] | Dec. 31, 2011 Special Mention [Member] Commercial Real Estate [Member] Municipalities [Member] Scenario, Previously Reported [Member] | Sep. 30, 2012 Special Mention [Member] Commercial Real Estate [Member] Multifamily real estate [Member] | Dec. 31, 2011 Special Mention [Member] Commercial Real Estate [Member] Medical Professionals [Member] Scenario, Previously Reported [Member] | Dec. 31, 2011 Special Mention [Member] Commercial Real Estate [Member] Religious Organizations [Member] Scenario, Previously Reported [Member] | Dec. 31, 2011 Special Mention [Member] Commercial Real Estate [Member] Convenience Stores [Member] Scenario, Previously Reported [Member] | Dec. 31, 2011 Special Mention [Member] Commercial Real Estate [Member] Entertainment and Sports [Member] Scenario, Previously Reported [Member] | Dec. 31, 2011 Special Mention [Member] Commercial Real Estate [Member] Nonprofits [Member] Scenario, Previously Reported [Member] | Dec. 31, 2011 Special Mention [Member] Commercial Real Estate [Member] Restaurants [Member] Scenario, Previously Reported [Member] | Dec. 31, 2011 Special Mention [Member] Commercial Real Estate [Member] General Contractors [Member] Scenario, Previously Reported [Member] | Sep. 30, 2012 Special Mention [Member] Commercial Non Real Estate [Member] Commercial and Industrial [Member] | Sep. 30, 2012 Special Mention [Member] Public Sector and IDA [Member] States & Political Subdivisions [Member] | Sep. 30, 2012 Special Mention [Member] Consumer Non Real Estate [Member] Automobile [Member] | Sep. 30, 2012 Special Mention [Member] Consumer Non Real Estate [Member] Other Consumer [Member] | Sep. 30, 2012 Special Mention [Member] Consumer Non Real Estate [Member] Credit cards [Member] | Dec. 31, 2011 Special Mention [Member] Consumer Non Real Estate [Member] Credit cards [Member] Scenario, Previously Reported [Member] | Dec. 31, 2011 Special Mention [Member] Consumer Non Real Estate [Member] Consumer General [Member] Scenario, Previously Reported [Member] | Dec. 31, 2011 Special Mention [Member] Consumer Non Real Estate [Member] Consumer Overdraft [Member] Scenario, Previously Reported [Member] | Dec. 31, 2011 Special Mention [Member] Construction Development and Other land [Member] Residential [Member] Scenario, Previously Reported [Member] | Dec. 31, 2011 Special Mention [Member] Construction Development and Other land [Member] Commercial [Member] Scenario, Previously Reported [Member] | Sep. 30, 2012 Classified (Excluding Impaired) [Member] | Dec. 31, 2011 Classified (Excluding Impaired) [Member] Scenario, Previously Reported [Member] | Dec. 31, 2011 Classified (Excluding Impaired) [Member] Commercial and Industrial [Member] Scenario, Previously Reported [Member] | Sep. 30, 2012 Classified (Excluding Impaired) [Member] Real Estate Construction [Member] Construction, 1-4 Family Residential [Member] | Sep. 30, 2012 Classified (Excluding Impaired) [Member] Real Estate Construction [Member] Construction other [Member] | Sep. 30, 2012 Classified (Excluding Impaired) [Member] Consumer Real Estate [Member] Equity lines [Member] | Dec. 31, 2011 Classified (Excluding Impaired) [Member] Consumer Real Estate [Member] Equity lines [Member] Scenario, Previously Reported [Member] | Dec. 31, 2011 Classified (Excluding Impaired) [Member] Consumer Real Estate [Member] Closed end Consumer Real Estate [Member] Scenario, Previously Reported [Member] | Dec. 31, 2011 Classified (Excluding Impaired) [Member] Consumer Real Estate [Member] Consumer Construction [Member] Scenario, Previously Reported [Member] | Sep. 30, 2012 Classified (Excluding Impaired) [Member] Consumer Real Estate [Member] Residential closed end first liens [Member] | Sep. 30, 2012 Classified (Excluding Impaired) [Member] Consumer Real Estate [Member] Residential closed end junior liens [Member] | Sep. 30, 2012 Classified (Excluding Impaired) [Member] Commercial Real Estate [Member] Commercial real estate, other [Member] | Dec. 31, 2011 Classified (Excluding Impaired) [Member] Commercial Real Estate [Member] Commercial real estate, other [Member] Scenario, Previously Reported [Member] | Sep. 30, 2012 Classified (Excluding Impaired) [Member] Commercial Real Estate [Member] Commercial real estate, owner occupied [Member] | Dec. 31, 2011 Classified (Excluding Impaired) [Member] Commercial Real Estate [Member] College Housing [Member] Scenario, Previously Reported [Member] | Dec. 31, 2011 Classified (Excluding Impaired) [Member] Commercial Real Estate [Member] Office/Retail [Member] Scenario, Previously Reported [Member] | Dec. 31, 2011 Classified (Excluding Impaired) [Member] Commercial Real Estate [Member] Nursing Homes [Member] Scenario, Previously Reported [Member] | Dec. 31, 2011 Classified (Excluding Impaired) [Member] Commercial Real Estate [Member] Hotel [Member] Scenario, Previously Reported [Member] | Dec. 31, 2011 Classified (Excluding Impaired) [Member] Commercial Real Estate [Member] Municipalities [Member] Scenario, Previously Reported [Member] | Sep. 30, 2012 Classified (Excluding Impaired) [Member] Commercial Real Estate [Member] Multifamily real estate [Member] | Dec. 31, 2011 Classified (Excluding Impaired) [Member] Commercial Real Estate [Member] Medical Professionals [Member] Scenario, Previously Reported [Member] | Dec. 31, 2011 Classified (Excluding Impaired) [Member] Commercial Real Estate [Member] Religious Organizations [Member] Scenario, Previously Reported [Member] | Dec. 31, 2011 Classified (Excluding Impaired) [Member] Commercial Real Estate [Member] Convenience Stores [Member] Scenario, Previously Reported [Member] | Dec. 31, 2011 Classified (Excluding Impaired) [Member] Commercial Real Estate [Member] Entertainment and Sports [Member] Scenario, Previously Reported [Member] | Dec. 31, 2011 Classified (Excluding Impaired) [Member] Commercial Real Estate [Member] Nonprofits [Member] Scenario, Previously Reported [Member] | Dec. 31, 2011 Classified (Excluding Impaired) [Member] Commercial Real Estate [Member] Restaurants [Member] Scenario, Previously Reported [Member] | Dec. 31, 2011 Classified (Excluding Impaired) [Member] Commercial Real Estate [Member] General Contractors [Member] Scenario, Previously Reported [Member] | Sep. 30, 2012 Classified (Excluding Impaired) [Member] Commercial Non Real Estate [Member] Commercial and Industrial [Member] | Sep. 30, 2012 Classified (Excluding Impaired) [Member] Public Sector and IDA [Member] States & Political Subdivisions [Member] | Sep. 30, 2012 Classified (Excluding Impaired) [Member] Consumer Non Real Estate [Member] Automobile [Member] | Sep. 30, 2012 Classified (Excluding Impaired) [Member] Consumer Non Real Estate [Member] Other Consumer [Member] | Sep. 30, 2012 Classified (Excluding Impaired) [Member] Consumer Non Real Estate [Member] Credit cards [Member] | Dec. 31, 2011 Classified (Excluding Impaired) [Member] Consumer Non Real Estate [Member] Credit cards [Member] Scenario, Previously Reported [Member] | Dec. 31, 2011 Classified (Excluding Impaired) [Member] Consumer Non Real Estate [Member] Consumer General [Member] Scenario, Previously Reported [Member] | Dec. 31, 2011 Classified (Excluding Impaired) [Member] Consumer Non Real Estate [Member] Consumer Overdraft [Member] Scenario, Previously Reported [Member] | Dec. 31, 2011 Classified (Excluding Impaired) [Member] Construction Development and Other land [Member] Residential [Member] Scenario, Previously Reported [Member] | Dec. 31, 2011 Classified (Excluding Impaired) [Member] Construction Development and Other land [Member] Commercial [Member] Scenario, Previously Reported [Member]
Financing Receivable, Recorded Investment [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Threshold period for considering loans as special mention or classified (in days) | 75 days |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase in percentage of allocation for internal and external factors used for estimating credit risk for non-impaired loans (in hundredths) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 50.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 100.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Non-impaired loans |  | $ 563,498 | $ 563,687 | [1] | $ 37,252 | [1] | $ 15,284 | $ 27,842 | $ 18,265 | $ 17,971 | [1] | $ 87,882 | [1] | $ 2,050 | [1] | $ 112,188 | $ 6,778 | $ 91,482 | $ 63,422 | [1] | $ 165,685 | $ 88,157 | [1] | $ 73,106 | [1] | $ 16,173 | [1] | $ 24,498 | [1] | $ 19,230 | [1] | $ 32,342 | $ 18,577 | [1] | $ 15,852 | [1] | $ 10,519 | [1] | $ 7,346 | [1] | $ 3,265 | [1] | $ 6,138 | [1] | $ 4,550 | [1] | $ 34,721 | $ 26,589 | $ 12,913 | $ 12,908 | $ 6,501 | $ 6,594 | [1] | $ 22,679 | [1] | $ 285 | [1] | $ 15,732 | [1] | $ 22,409 | [1] | $ 11,696 | $ 7,945 | [1] | $ 196 | [1] | $ 0 | $ 2,961 | $ 98 | $ 0 | [1] | $ 595 | [1] | $ 0 | [1] | $ 702 | $ 124 | $ 3,132 | $ 0 | [1] | $ 986 | $ 452 | [1] | $ 420 | [1] | $ 0 | [1] | $ 0 | [1] | $ 0 | [1] | $ 3,533 | $ 0 | [1] | $ 0 | [1] | $ 0 | [1] | $ 0 | [1] | $ 3,170 | [1] | $ 0 | [1] | $ 109 | [1] | $ 13 | $ 0 | $ 81 | $ 66 | $ 0 | $ 0 | [1] | $ 42 | [1] | $ 0 | [1] | $ 0 | [1] | $ 2,961 | [1] | $ 4,302 | $ 4,242 | [1] | $ 137 | [1] | $ 0 | $ 89 | $ 72 | $ 14 | [1] | $ 1,332 | [1] | $ 0 | [1] | $ 2,096 | $ 140 | $ 0 | $ 790 | [1] | $ 1,087 | $ 215 | [1] | $ 267 | [1] | $ 0 | [1] | $ 616 | [1] | $ 0 | [1] | $ 262 | $ 0 | [1] | $ 0 | [1] | $ 0 | [1] | $ 0 | [1] | $ 0 | [1] | $ 387 | [1] | $ 247 | [1] | $ 461 | $ 0 | $ 51 | $ 44 | $ 0 | $ 1 | [1] | $ 105 | [1] | $ 1 | [1] | $ 0 | [1] | $ 130 | [1]
[1] | Segments at December 31, 2011 are reported using the segmentation method in effect for 2011. The Company began reporting under revised segments beginning in 2012.

X
- Definition Refers to increase in percentage of allocation for internal and external factors used for estimating credit risk for non-impaired loans. + References No definition available. + Details
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Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents amount of loan receivable that are not deemed to be impaired. + References No definition available. + Details
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Namespace Prefix: | nksh_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Represents the threshold period for considering loans as special mention or classified. + References No definition available. + Details
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Namespace Prefix: | nksh_
Data Type: | xbrli:durationItemType
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Period Type: | duration

X
- Details
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Balance Type: | na
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