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Note 12 - Minimum Regulatory Capital Requirement (Details) - Minimum Capital Requirements (USD $) In Thousands, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013
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NBI Consolidated [Member] |  |
Total capital (to risk weighted assets) |  |
Total capital (to risk weighted assets) actual amount | $ 172,976 us-gaap_Capital / dei_LegalEntityAxis = nksh_NBIConsolidatedMember | $ 162,763 us-gaap_Capital / dei_LegalEntityAxis = nksh_NBIConsolidatedMember
Total capital (to risk weighted assets) actual ratio | 24.90%us-gaap_CapitalToRiskWeightedAssets / dei_LegalEntityAxis = nksh_NBIConsolidatedMember | 23.60%us-gaap_CapitalToRiskWeightedAssets / dei_LegalEntityAxis = nksh_NBIConsolidatedMember
Total capital (to risk weighted assets) minimum capital requirement amount | 55,615 us-gaap_CapitalRequiredForCapitalAdequacy / dei_LegalEntityAxis = nksh_NBIConsolidatedMember | 55,192 us-gaap_CapitalRequiredForCapitalAdequacy / dei_LegalEntityAxis = nksh_NBIConsolidatedMember
Total capital (to risk weighted assets) minimum capital requirement ratio | 8.00%us-gaap_CapitalRequiredForCapitalAdequacyToRiskWeightedAssets / dei_LegalEntityAxis = nksh_NBIConsolidatedMember | 8.00%us-gaap_CapitalRequiredForCapitalAdequacyToRiskWeightedAssets / dei_LegalEntityAxis = nksh_NBIConsolidatedMember
Tier 1 capital (to risk weighted assets) |  |
Tier 1 capital (to risk weighted assets) actual amount | 164,713 us-gaap_TierOneRiskBasedCapital / dei_LegalEntityAxis = nksh_NBIConsolidatedMember | 154,536 us-gaap_TierOneRiskBasedCapital / dei_LegalEntityAxis = nksh_NBIConsolidatedMember
Tier 1 capital (to risk weighted assets) actual ratio | 23.70%us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets / dei_LegalEntityAxis = nksh_NBIConsolidatedMember | 22.40%us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets / dei_LegalEntityAxis = nksh_NBIConsolidatedMember
Tier 1 capital (to risk weighted assets) minimum capital requirement amount | 27,807 us-gaap_TierOneRiskBasedCapitalRequiredForCapitalAdequacy / dei_LegalEntityAxis = nksh_NBIConsolidatedMember | 27,596 us-gaap_TierOneRiskBasedCapitalRequiredForCapitalAdequacy / dei_LegalEntityAxis = nksh_NBIConsolidatedMember
Tier 1 capital (to risk weighted assets) minimum capital requirement ratio | 4.00%us-gaap_TierOneRiskBasedCapitalRequiredForCapitalAdequacyToRiskWeightedAssets / dei_LegalEntityAxis = nksh_NBIConsolidatedMember | 4.00%us-gaap_TierOneRiskBasedCapitalRequiredForCapitalAdequacyToRiskWeightedAssets / dei_LegalEntityAxis = nksh_NBIConsolidatedMember
Tier 1 capital (to average assets) |  |
Tier 1 capital (to average assets) actual amount | 164,713 us-gaap_TierOneLeverageCapital / dei_LegalEntityAxis = nksh_NBIConsolidatedMember | 154,536 us-gaap_TierOneLeverageCapital / dei_LegalEntityAxis = nksh_NBIConsolidatedMember
Tier 1 capital (to average assets) actual ratio | 14.60%us-gaap_TierOneLeverageCapitalToAverageAssets / dei_LegalEntityAxis = nksh_NBIConsolidatedMember | 14.10%us-gaap_TierOneLeverageCapitalToAverageAssets / dei_LegalEntityAxis = nksh_NBIConsolidatedMember
Tier 1 capital (to average assets) minimum capital requirement amount | 45,122 us-gaap_TierOneLeverageCapitalRequiredForCapitalAdequacy / dei_LegalEntityAxis = nksh_NBIConsolidatedMember | 43,946 us-gaap_TierOneLeverageCapitalRequiredForCapitalAdequacy / dei_LegalEntityAxis = nksh_NBIConsolidatedMember
Tier 1 capital (to average assets) minimum capital requirement ratio | 4.00%us-gaap_TierOneLeverageCapitalRequiredForCapitalAdequacyToAverageAssets / dei_LegalEntityAxis = nksh_NBIConsolidatedMember | 4.00%us-gaap_TierOneLeverageCapitalRequiredForCapitalAdequacyToAverageAssets / dei_LegalEntityAxis = nksh_NBIConsolidatedMember
National Bank of Blacksburg [Member] |  |
Total capital (to risk weighted assets) |  |
Total capital (to risk weighted assets) actual amount | 170,665 us-gaap_Capital / dei_LegalEntityAxis = nksh_NationalBankOfBlacksburgMember | 160,049 us-gaap_Capital / dei_LegalEntityAxis = nksh_NationalBankOfBlacksburgMember
Total capital (to risk weighted assets) actual ratio | 24.60%us-gaap_CapitalToRiskWeightedAssets / dei_LegalEntityAxis = nksh_NationalBankOfBlacksburgMember | 23.30%us-gaap_CapitalToRiskWeightedAssets / dei_LegalEntityAxis = nksh_NationalBankOfBlacksburgMember
Total capital (to risk weighted assets) minimum capital requirement amount | 55,438 us-gaap_CapitalRequiredForCapitalAdequacy / dei_LegalEntityAxis = nksh_NationalBankOfBlacksburgMember | 54,980 us-gaap_CapitalRequiredForCapitalAdequacy / dei_LegalEntityAxis = nksh_NationalBankOfBlacksburgMember
Total capital (to risk weighted assets) minimum capital requirement ratio | 8.00%us-gaap_CapitalRequiredForCapitalAdequacyToRiskWeightedAssets / dei_LegalEntityAxis = nksh_NationalBankOfBlacksburgMember | 8.00%us-gaap_CapitalRequiredForCapitalAdequacyToRiskWeightedAssets / dei_LegalEntityAxis = nksh_NationalBankOfBlacksburgMember
Total capital (to risk weighted assets) minimum to be well capitalized under prompt corrective action provisions amount | 69,298 us-gaap_CapitalRequiredToBeWellCapitalized / dei_LegalEntityAxis = nksh_NationalBankOfBlacksburgMember | 68,726 us-gaap_CapitalRequiredToBeWellCapitalized / dei_LegalEntityAxis = nksh_NationalBankOfBlacksburgMember
Total capital (to risk weighted assets) minimum to be well capitalized under prompt corrective action provisions ratio | 10.00%us-gaap_CapitalRequiredToBeWellCapitalizedToRiskWeightedAssets / dei_LegalEntityAxis = nksh_NationalBankOfBlacksburgMember | 10.00%us-gaap_CapitalRequiredToBeWellCapitalizedToRiskWeightedAssets / dei_LegalEntityAxis = nksh_NationalBankOfBlacksburgMember
Tier 1 capital (to risk weighted assets) |  |
Tier 1 capital (to risk weighted assets) actual amount | 162,402 us-gaap_TierOneRiskBasedCapital / dei_LegalEntityAxis = nksh_NationalBankOfBlacksburgMember | 151,822 us-gaap_TierOneRiskBasedCapital / dei_LegalEntityAxis = nksh_NationalBankOfBlacksburgMember
Tier 1 capital (to risk weighted assets) actual ratio | 23.40%us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets / dei_LegalEntityAxis = nksh_NationalBankOfBlacksburgMember | 22.10%us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets / dei_LegalEntityAxis = nksh_NationalBankOfBlacksburgMember
Tier 1 capital (to risk weighted assets) minimum capital requirement amount | 27,719 us-gaap_TierOneRiskBasedCapitalRequiredForCapitalAdequacy / dei_LegalEntityAxis = nksh_NationalBankOfBlacksburgMember | 27,490 us-gaap_TierOneRiskBasedCapitalRequiredForCapitalAdequacy / dei_LegalEntityAxis = nksh_NationalBankOfBlacksburgMember
Tier 1 capital (to risk weighted assets) minimum capital requirement ratio | 4.00%us-gaap_TierOneRiskBasedCapitalRequiredForCapitalAdequacyToRiskWeightedAssets / dei_LegalEntityAxis = nksh_NationalBankOfBlacksburgMember | 4.00%us-gaap_TierOneRiskBasedCapitalRequiredForCapitalAdequacyToRiskWeightedAssets / dei_LegalEntityAxis = nksh_NationalBankOfBlacksburgMember
Tier 1 capital (to risk weighted assets) minimum to be well capitalized under prompt corrective action provisions amount | 41,579 us-gaap_TierOneRiskBasedCapitalRequiredToBeWellCapitalized / dei_LegalEntityAxis = nksh_NationalBankOfBlacksburgMember | 41,235 us-gaap_TierOneRiskBasedCapitalRequiredToBeWellCapitalized / dei_LegalEntityAxis = nksh_NationalBankOfBlacksburgMember
Tier 1 capital (to risk weighted assets) minimum to be well capitalized under prompt corrective action provisions ratio | 6.00%us-gaap_TierOneRiskBasedCapitalRequiredToBeWellCapitalizedToRiskWeightedAssets / dei_LegalEntityAxis = nksh_NationalBankOfBlacksburgMember | 6.00%us-gaap_TierOneRiskBasedCapitalRequiredToBeWellCapitalizedToRiskWeightedAssets / dei_LegalEntityAxis = nksh_NationalBankOfBlacksburgMember
Tier 1 capital (to average assets) |  |
Tier 1 capital (to average assets) actual amount | 162,402 us-gaap_TierOneLeverageCapital / dei_LegalEntityAxis = nksh_NationalBankOfBlacksburgMember | 151,822 us-gaap_TierOneLeverageCapital / dei_LegalEntityAxis = nksh_NationalBankOfBlacksburgMember
Tier 1 capital (to average assets) actual ratio | 14.40%us-gaap_TierOneLeverageCapitalToAverageAssets / dei_LegalEntityAxis = nksh_NationalBankOfBlacksburgMember | 13.80%us-gaap_TierOneLeverageCapitalToAverageAssets / dei_LegalEntityAxis = nksh_NationalBankOfBlacksburgMember
Tier 1 capital (to average assets) minimum capital requirement amount | 45,074 us-gaap_TierOneLeverageCapitalRequiredForCapitalAdequacy / dei_LegalEntityAxis = nksh_NationalBankOfBlacksburgMember | 43,904 us-gaap_TierOneLeverageCapitalRequiredForCapitalAdequacy / dei_LegalEntityAxis = nksh_NationalBankOfBlacksburgMember
Tier 1 capital (to average assets) minimum capital requirement ratio | 4.00%us-gaap_TierOneLeverageCapitalRequiredForCapitalAdequacyToAverageAssets / dei_LegalEntityAxis = nksh_NationalBankOfBlacksburgMember | 4.00%us-gaap_TierOneLeverageCapitalRequiredForCapitalAdequacyToAverageAssets / dei_LegalEntityAxis = nksh_NationalBankOfBlacksburgMember
Tier 1 capital (to average assets) minimum to be well capitalized under prompt corrective action provisions amount | $ 56,342 us-gaap_TierOneLeverageCapitalRequiredToBeWellCapitalized / dei_LegalEntityAxis = nksh_NationalBankOfBlacksburgMember | $ 54,880 us-gaap_TierOneLeverageCapitalRequiredToBeWellCapitalized / dei_LegalEntityAxis = nksh_NationalBankOfBlacksburgMember
Tier 1 capital (to average assets) minimum to be well capitalized under prompt corrective action provisions ratio | 5.00%us-gaap_TierOneLeverageCapitalRequiredToBeWellCapitalizedToAverageAssets / dei_LegalEntityAxis = nksh_NationalBankOfBlacksburgMember | 5.00%us-gaap_TierOneLeverageCapitalRequiredToBeWellCapitalizedToAverageAssets / dei_LegalEntityAxis = nksh_NationalBankOfBlacksburgMember

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- Definition The minimum amount of Total Capital required for capital adequacy purposes under the regulatory framework for prompt corrective action. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1H -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=35709795&loc=d3e65071-112826 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=35709795&loc=SL5958568-112826 + Details
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- Definition The minimum Total risk based capital ratio (total capital divided by risk weighted assets) required for capital adequacy purposes under the regulatory framework for prompt corrective action. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1H -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=35709795&loc=d3e65071-112826 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=35709795&loc=SL5958568-112826 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1B -URI http://asc.fasb.org/extlink&oid=35709795&loc=SL5958570-112826 + Details
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- Definition The amount of Total Capital required to be categorized as well as capitalized under the regulatory framework for prompt corrective action. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=35709795&loc=SL5958568-112826 + Details
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- Definition The Total risk based capital ratio (total capital divided by risk weighted assets) required to be categorized as "well capitalized", in the regulatory framework for prompt corrective action. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=35709795&loc=SL5958568-112826 + Details
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- Definition Total Capital divided by risk weighted assets as defined in the regulations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1H -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=35709795&loc=d3e65071-112826 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=35709795&loc=SL5958568-112826 + Details
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- Definition Tier 1 Leverage Capital as defined in the regulations. Also called Core Capital. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1H -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=35709795&loc=d3e65071-112826 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=35709795&loc=SL5958568-112826 + Details
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- Definition The minimum amount of Tier 1 Leverage Capital required for capital adequacy purposes under the regulatory framework for prompt corrective action. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1H -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=35709795&loc=d3e65071-112826 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=35709795&loc=SL5958568-112826 + Details
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- Definition The minimum amount of Tier 1 Leverage Capital for capital adequacy purposes divided by average assets as defined in the regulations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1H -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=35709795&loc=d3e65071-112826 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=35709795&loc=SL5958568-112826 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1B -URI http://asc.fasb.org/extlink&oid=35709795&loc=SL5958570-112826 + Details
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- Definition The amount of Tier 1 Leverage Capital required to be categorized as well capitalized under the regulatory framework for prompt corrective action. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=35709795&loc=SL5958568-112826 + Details
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- Definition The amount of Tier 1 Leverage Capital required to be categorized as well capitalized divided by average assets as defined in the regulatory framework for prompt corrective action. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=35709795&loc=SL5958568-112826 + Details
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- Definition Tier 1 Risk Based Capital as defined in the regulations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1H -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=35709795&loc=d3e65071-112826 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=35709795&loc=SL5958568-112826 + Details
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- Definition The minimum amount of Tier 1 Risk Based Capital required for capital adequacy purposes under the regulatory framework for prompt corrective action. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1H -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=35709795&loc=d3e65071-112826 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=35709795&loc=SL5958568-112826 + Details
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- Definition The minimum Tier One Capital Ratio (Tier one capital divided by risk-weighted assets) required for capital adequacy purposes under the regulatory framework for prompt corrective action. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1H -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=35709795&loc=d3e65071-112826 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=35709795&loc=SL5958568-112826 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1B -URI http://asc.fasb.org/extlink&oid=35709795&loc=SL5958570-112826 + Details
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- Definition The amount of Tier 1 Risk Based Capital required to be categorized as well capitalized under the regulatory framework for prompt corrective action. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=35709795&loc=SL5958568-112826 + Details
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- Definition The Tier 1 capital ratio (Tier 1 capital divided by risk weighted assets) required to be categorized as "well capitalized" under the regulatory framework for prompt corrective action. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=35709795&loc=SL5958568-112826 + Details
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- Definition Tier 1 capital divided by risk weighted assets as defined by regulations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1H -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=35709795&loc=d3e65071-112826 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=35709795&loc=SL5958568-112826 + Details
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