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Equity - Additional Information (Detail) (USD $) | 12 Months Ended | 0 Months Ended
--- | --- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012 | May 16, 2012 | Jan. 26, 2015 | Dec. 01, 2012
Class of Stock [Line Items] |  |  |  |  |  |
Common stock, shares outstanding | 13,916,026 us-gaap_CommonStockSharesOutstanding | 13,841,400 us-gaap_CommonStockSharesOutstanding |  |  |  |
Common stock, share authorized | 25,000,000 us-gaap_CommonStockSharesAuthorized | 25,000,000 us-gaap_CommonStockSharesAuthorized |  |  |  |
Proceed from issuance of common stock | $ 1,200,000 us-gaap_ProceedsFromIssuanceOfCommonStock | $ 1,100,000 us-gaap_ProceedsFromIssuanceOfCommonStock | $ 66,800,000 us-gaap_ProceedsFromIssuanceOfCommonStock |  |  |
Common stock, share issued | 38,020 us-gaap_StockIssuedDuringPeriodSharesNewIssues | 39,559 us-gaap_StockIssuedDuringPeriodSharesNewIssues | 41,752 us-gaap_StockIssuedDuringPeriodSharesNewIssues |  |  |
Common stock shares repurchase | 2,763 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesPurchasedForAward | 2,969 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesPurchasedForAward | 6,368 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesPurchasedForAward |  |  |
Repurchase expense | 100,000 utl_StockRepurchaseProgramFeesAndExpensesRelatedToTenderOffer | 100,000 utl_StockRepurchaseProgramFeesAndExpensesRelatedToTenderOffer | 200,000 utl_StockRepurchaseProgramFeesAndExpensesRelatedToTenderOffer |  |  |
Share based compensation expense | 1,400,000 us-gaap_RestrictedStockExpense | 700,000 us-gaap_RestrictedStockExpense | 1,300,000 us-gaap_RestrictedStockExpense |  |  |
Preferred Stock | 200,000 us-gaap_PreferredStockValue | 200,000 us-gaap_PreferredStockValue |  |  |  |
Redeemable, Cumulative Preferred Stock | 0 us-gaap_TemporaryEquityCarryingAmountIncludingPortionAttributableToNoncontrollingInterests | 0 us-gaap_TemporaryEquityCarryingAmountIncludingPortionAttributableToNoncontrollingInterests | 8,000 us-gaap_TemporaryEquityCarryingAmountIncludingPortionAttributableToNoncontrollingInterests |  |  |
Non Vested Restricted Stock |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |
Antidilutive securities excluded from computation of earnings per share | 0 us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount / us-gaap_RangeAxis = utl_NonVestedRestrictedStockMember | 4,481 us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount / us-gaap_RangeAxis = utl_NonVestedRestrictedStockMember | 24,325 us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount / us-gaap_RangeAxis = utl_NonVestedRestrictedStockMember |  |  |
Restricted Stock Units |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |
Percentage of fully-vested restricted stock units that directors will receive in common shares when settled | 70.00%utl_PercentageOfFullyVestedRestrictedStockUnitsThatDirectorsWillReceiveInCommonSharesWhenSettled / us-gaap_StatementClassOfStockAxis = utl_RestrictedStockUnitMember |  |  |  |  |
Percentage of fully-vested restricted stock units that directors will receive in cash when settled | 30.00%utl_PercentageOfFullyVestedRestrictedStockUnitsThatDirectorsWillReceiveInCashWhenSettled / us-gaap_StatementClassOfStockAxis = utl_RestrictedStockUnitMember |  |  |  |  |
Fair value of liabilities associated with fully vested RSUs that will be settled in cash | 400,000 utl_FairValueOfRestrictedStockUnitsSettledInCash / us-gaap_StatementClassOfStockAxis = utl_RestrictedStockUnitMember | 200,000 utl_FairValueOfRestrictedStockUnitsSettledInCash / us-gaap_StatementClassOfStockAxis = utl_RestrictedStockUnitMember |  |  |  |
Common Stock Offering |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |
Common stock, share issued |  |  |  | 2,760,000 us-gaap_CommonStockSharesIssued / us-gaap_StatementClassOfStockAxis = utl_CommonStockOfferingMember |  |
Common stock price per share |  |  |  | $ 25.25 us-gaap_EquityIssuancePerShareAmount / us-gaap_StatementClassOfStockAxis = utl_CommonStockOfferingMember |  |
Proceed from issuance of common stock |  |  |  | 65,700,000 us-gaap_ProceedsFromIssuanceOfCommonStock / us-gaap_StatementClassOfStockAxis = utl_CommonStockOfferingMember |  |
Dividend and Distribution Reinvestment and Share Purchase Plan |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |
Common stock price per share | $ 32.83 us-gaap_EquityIssuancePerShareAmount / us-gaap_StatementClassOfStockAxis = utl_DividendAndDistributionReinvestmentAndSharePurchasePlanMember |  |  |  |  |
Proceed from issuance of common stock | 1,200,000 us-gaap_ProceedsFromIssuanceOfCommonStock / us-gaap_StatementClassOfStockAxis = utl_DividendAndDistributionReinvestmentAndSharePurchasePlanMember | 1,100,000 us-gaap_ProceedsFromIssuanceOfCommonStock / us-gaap_StatementClassOfStockAxis = utl_DividendAndDistributionReinvestmentAndSharePurchasePlanMember | 1,100,000 us-gaap_ProceedsFromIssuanceOfCommonStock / us-gaap_StatementClassOfStockAxis = utl_DividendAndDistributionReinvestmentAndSharePurchasePlanMember |  |  |
Dividend and Distribution Reinvestment and Share Purchase Plan | Common Stock |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |
Common stock, share issued | 38,020 us-gaap_StockIssuedDuringPeriodSharesNewIssues / us-gaap_StatementClassOfStockAxis = utl_DividendAndDistributionReinvestmentAndSharePurchasePlanMember / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockMember | 39,559 us-gaap_StockIssuedDuringPeriodSharesNewIssues / us-gaap_StatementClassOfStockAxis = utl_DividendAndDistributionReinvestmentAndSharePurchasePlanMember / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockMember | 41,752 us-gaap_StockIssuedDuringPeriodSharesNewIssues / us-gaap_StatementClassOfStockAxis = utl_DividendAndDistributionReinvestmentAndSharePurchasePlanMember / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockMember |  |  |
Restricted Stock |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |
Restricted stock vesting period | 4 years |  |  |  |  |
Restricted stock vesting percentage annually | 25.00%us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember |  |  |  |  |
Restricted stock non-vested | 67,334 utl_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonvestedOptionsOutstandingNumber / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember | 53,480 utl_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonvestedOptionsOutstandingNumber / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember |  |  |  |
Restricted stock weighted average grant date fair value | $ 28.51 utl_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedInPeriodWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember | $ 25.99 utl_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedInPeriodWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember |  |  |  |
Unrecognized share based compensation | 800,000 us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember |  |  |  |  |
Share compensation recognition period | 2 years 6 months |  |  |  |  |
Forfeitures or cancellations under the stock plan | 0 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember |  |  |  |  |
Shares | 9,078 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember | 10,710 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember |  |  |  |
Restricted Stock | Maximum |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |
Restricted stock available for awards | 677,500 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / us-gaap_RangeAxis = us-gaap_MaximumMember |  |  |  |  |
Restricted stock that may be awarded in any one calendar year to any one participant | 20,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / us-gaap_RangeAxis = us-gaap_MaximumMember |  |  |  |  |
Restricted Stock | Subsequent Event |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |
Shares |  |  |  |  | 40,010 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember |
Aggregate Market Value |  |  |  |  | $ 1,500,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodIntrinsicValue / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember |
Unitil Energy Systems Inc | Series 6 | Nonredeemable Preferred Stock | Noncumulative Preferred Stock |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |
Preferred Stock | 200,000 us-gaap_PreferredStockValue / us-gaap_DebtConversionByUniqueDescriptionAxis = us-gaap_NoncumulativePreferredStockMember / us-gaap_ExtinguishmentOfDebtAxis = us-gaap_NonredeemablePreferredStockMember / dei_LegalEntityAxis = utl_UnitilEnergySystemsIncMember / us-gaap_StatementEquityComponentsAxis = utl_SeriesSixMember |  |  |  |  |
Dividend rate | 6.00%us-gaap_PreferredStockDividendRatePercentage / us-gaap_DebtConversionByUniqueDescriptionAxis = us-gaap_NoncumulativePreferredStockMember / us-gaap_ExtinguishmentOfDebtAxis = us-gaap_NonredeemablePreferredStockMember / dei_LegalEntityAxis = utl_UnitilEnergySystemsIncMember / us-gaap_StatementEquityComponentsAxis = utl_SeriesSixMember |  |  |  |  |
Fitchburg Gas and Electric Light Company | Series 8 |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |
Dividend rate |  |  |  |  |  | 8.00%us-gaap_PreferredStockDividendRatePercentage / dei_LegalEntityAxis = utl_FitchburgGasAndElectricLightCompanyMember / us-gaap_StatementClassOfStockAxis = utl_SeriesEightMember
Redeemable, Cumulative Preferred Stock |  |  |  |  |  | 1,000,000 us-gaap_TemporaryEquityCarryingAmountIncludingPortionAttributableToNoncontrollingInterests / dei_LegalEntityAxis = utl_FitchburgGasAndElectricLightCompanyMember / us-gaap_StatementClassOfStockAxis = utl_SeriesEightMember
Fitchburg Gas and Electric Light Company | Series 5 |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |
Dividend rate |  |  |  |  |  | 5.125%us-gaap_PreferredStockDividendRatePercentage / dei_LegalEntityAxis = utl_FitchburgGasAndElectricLightCompanyMember / us-gaap_StatementClassOfStockAxis = utl_SeriesFiveMember
Redeemable, Cumulative Preferred Stock |  |  |  |  |  | $ 800,000 us-gaap_TemporaryEquityCarryingAmountIncludingPortionAttributableToNoncontrollingInterests / dei_LegalEntityAxis = utl_FitchburgGasAndElectricLightCompanyMember / us-gaap_StatementClassOfStockAxis = utl_SeriesFiveMember
Preferred Stock premium percentage |  |  |  |  |  | 1.28%utl_PreferredStockRedemptionPercentage / dei_LegalEntityAxis = utl_FitchburgGasAndElectricLightCompanyMember / us-gaap_StatementClassOfStockAxis = utl_SeriesFiveMember

X
- Definition Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) or earnings per unit (EPU) in the future that were not included in the computation of diluted EPS or EPU because to do so would increase EPS or EPU amounts or decrease loss per share or unit amounts for the period presented. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Antidilution -URI http://asc.fasb.org/extlink&oid=6505113 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Diluted Earnings Per Share -URI http://asc.fasb.org/extlink&oid=6510752 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Contingent Stock Agreement -URI http://asc.fasb.org/extlink&oid=6508534 + Details
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- Definition The maximum number of common shares permitted to be issued by an entity's charter and bylaws. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.29) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 30 -Article 5 + Details
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- Definition Total number of common shares of an entity that have been sold or granted to shareholders (includes common shares that were issued, repurchased and remain in the treasury). These shares represent capital invested by the firm's shareholders and owners, and may be all or only a portion of the number of shares authorized. Shares issued include shares outstanding and shares held in the treasury. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.29) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 30 -Article 5 + Details
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- Definition Number of shares of common stock outstanding. Common stock represent the ownership interest in a corporation. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Article 3 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21463-112644 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.3-04) -URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.29) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 30 -Article 5 + Details
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- Definition Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Amount per share or per unit assigned to the consideration received of equity securities issued for development stage entities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 915 -SubTopic 215 -Section 45 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6472370&loc=d3e38297-110927 + Details
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- Definition Aggregate par or stated value of issued nonredeemable preferred stock (or preferred stock redeemable solely at the option of the issuer). This item includes treasury stock repurchased by the entity. Note: elements for number of nonredeemable preferred shares, par value and other disclosure concepts are in another section within stockholders' equity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Article 3 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.3-04) -URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.28) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 29 -Article 5 + Details
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- Definition Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Percentage of vesting of share-based compensation awards. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(iii)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Per share weighted average intrinsic value of equity-based compensation awards granted. Excludes stock and unit options. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(iii)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The maximum number of shares (or other type of equity) originally approved (usually by shareholders and board of directors), net of any subsequent amendments and adjustments, for awards under the equity-based compensation plan. As stock or unit options and equity instruments other than options are awarded to participants, the shares or units remain authorized and become reserved for issuance under outstanding awards (not necessarily vested). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(iv)(3) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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