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No authoritative reference available. false 8 Net Changes in Price Risk Management Assets and Liabilities 3 us-gaap_IncreaseDecreaseInRiskManagementAssetsAndLiabilities us-gaap true credit duration No definition available. false false false false false false false false false false true negated false 1 false true false false 4400000 4.4 false false false 2 false true false false -10800000 -10.8 false false false xbrli:monetaryItemType monetary The net change during the reporting period in the aggregate market value of financial instruments that are purchased and held principally for the purpose of hedging or mitigating market risk, credit risk, legal risk, operational risk, and other types of risk. Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 95 -Paragraph 28 false 9 Deferred Income Taxes and Investment Tax Credits 3 us-gaap_DeferredIncomeTaxesAndTaxCredits us-gaap true debit duration No definition available. false false false false false false false false false false false verboselabel false 1 false true false false 38600000 38.6 false false false 2 false true false false 36400000 36.4 false false false xbrli:monetaryItemType monetary The net amount of deferred income taxes and income tax credits less the tax benefit from exercise of stock options. 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Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 13 -Section A -Subsection 1 false 11 Stock Compensation Expense 3 us-gaap_ShareBasedCompensation us-gaap true debit duration No definition available. false false false false false false false false false false false verboselabel false 1 false true false false 6000000 6.0 false false false 2 false true false false 4900000 4.9 false false false xbrli:monetaryItemType monetary The aggregate amount of noncash, equity-based employee remuneration. This may include the value of stock options, amortization of restricted stock, and adjustment for officers compensation. As noncash, this element is an add back when calculating net cash generated by operating activities using the indirect method. Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 95 -Paragraph 28 false 12 (Gain) Loss on Sale of Assets 3 us-gaap_GainLossOnSaleOfPropertyPlantEquipment us-gaap true credit duration No definition available. false false false false false false false false false false true negated false 1 false true false false 100000 0.1 false false false 2 false true false false -2000000 -2.0 false false false xbrli:monetaryItemType monetary The difference between the sale price or salvage price and the book value of a property, plant, and equipment asset that was sold or retired during the reporting period. This element refers to the gain (loss). 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Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 144 -Paragraph 43 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 144 -Paragraph 47 -Subparagraph b false 16 Loss from Discontinued Operations - Net of Taxes 3 us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxAttributableToReportingEntity us-gaap true credit duration No definition available. false false false false false false false false false false true negated false 1 false true false false 100000 0.1 false false false 2 false true false false 11400000 11.4 false false false xbrli:monetaryItemType monetary This element represents the overall income (loss) from a disposal group apportioned to the parent that is classified as a component of the entity, net of income tax, reported as a separate component of income before extraordinary items and the cumulative effect of accounting changes after deduction or consideration of the amount which may be allocable to noncontrolling interests, if any. Includes the following (net of tax): income (loss) from operations during the phase-out period, gain (loss) on disposal, provision (or any reversals of earlier provisions) for loss on disposal, and adjustments of a prior period gain (loss) on disposal. 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This element excludes distributions that constitute a return of investment, which are classified as investing activities. 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Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 95 -Paragraph 28 false 26 Taxes Accrued 4 us-gaap_IncreaseDecreaseInPropertyAndOtherTaxesPayable us-gaap true debit duration No definition available. false false false false false false false false false false false verboselabel false 1 false true false false 6200000 6.2 false false false 2 false true false false 84200000 84.2 false false false xbrli:monetaryItemType monetary The net change during the period in the amount of cash payments due to taxing authorities for non-income-related taxes. 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Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 95 -Paragraph 28 false 28 (Under) Overrecovered Gas and Fuel Costs 4 us-gaap_IncreaseDecreaseInDeferredGasCost us-gaap true credit duration No definition available. false false false false false false false false false false true negated false 1 false true false false -252300000 -252.3 false false false 2 false true false false 566800000 566.8 false false false xbrli:monetaryItemType monetary The net change during the reporting period in carrying amount of capitalized gas costs of a regulated entity that are expected to be recoverable through rate adjustments within one year or the normal operating cycle, if longer. Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 95 -Paragraph 28 false 29 Exchange Gas Receivable/Payable 4 ni_ExchangeGasReceivablePayable ni false credit duration Exchange gas receivable/payable. false false false false false false false false false false true negated false 1 false true false false -51300000 -51.3 false false false 2 false true false false -16000000 -16.0 false false false xbrli:monetaryItemType monetary Exchange gas receivable/payable. No authoritative reference available. false 30 Other Accruals 4 ni_OtherAccruals ni false credit duration Other accruals. false false false false false false false false false false true negated false 1 false true false false -30500000 -30.5 false false false 2 false true false false -103900000 -103.9 false false false xbrli:monetaryItemType monetary Other accruals. 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Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 95 -Paragraph 28 true 37 Net Operating Activities from Continuing Operations 2 us-gaap_NetCashProvidedByUsedInOperatingActivitiesContinuingOperations us-gaap true na duration No definition available. false false false false false false false false false false false verboselabel false 1 false true false false 447300000 447.3 false false false 2 false true false false 1302100000 1302.1 false false false xbrli:monetaryItemType monetary The net cash from (used in) the entity's continuing operations. This element specifically EXCLUDES the cash flows derived by the entity from its discontinued operations, if any. This element is only to be used when the entity reports its cash flows attributable to discontinued operations separately from the cash flow provided by or used in operating activities. Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 95 -Paragraph 26 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 95 -Paragraph 26 -Footnote 10 false 38 Net Operating Activities used for Discontinued Operations 2 us-gaap_CashProvidedByUsedInOperatingActivitiesDiscontinuedOperations us-gaap true debit duration No definition available. false false false false false false false false false false false totallabel false 1 false true false false -44800000 -44.8 false false false 2 false true false false -62300000 -62.3 false false false xbrli:monetaryItemType monetary This element represents cash provided by (used in) the operating activities of the entity's discontinued operations during the period. This element should only be used by those entities that separately report cash flows attributable to discontinued operations. If using this element, it is an indication that the cash flows of the entity which are detailed in reconciling to cash provided by or used in operating activities reflect only cash flows attributable to continuing operations. 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Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 95 -Paragraph 28 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 95 -Paragraph 26 true 40 Investing Activities 1 us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract us-gaap true na duration No definition available. false false false false false true false false false false false verboselabel false 1 false false false false 0 0 false false false 2 false false false false 0 0 false false false xbrli:stringItemType string No definition available. false 41 Capital Expenditures 2 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment us-gaap true credit duration No definition available. false false false false false false false false false false true negated false 1 false true false false -336900000 -336.9 false false false 2 false true false false -385800000 -385.8 false false false xbrli:monetaryItemType monetary The cash outflow associated with the acquisition of long-lived, physical assets that are used in the normal conduct of business to produce goods and services and not intended for resale; includes cash outflows to pay for construction of self-constructed assets. 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Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 95 -Paragraph 15 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 95 -Paragraph 16 -Subparagraph c false 44 Restricted Cash Deposits (Borrowings) 2 us-gaap_IncreaseDecreaseInRestrictedCash us-gaap true credit duration No definition available. false false false false false false false false false false true negated false 1 false true false false -38000000 -38.0 false false false 2 false true false false 31100000 31.1 false false false xbrli:monetaryItemType monetary The net cash inflow (outflow) for the net change associated with funds that are not available for withdrawal or use (such as funds held in escrow) and are associated with underlying transactions that are classified as investing activities. Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 95 -Paragraph 15, 16, 17 false 45 Contributions to Equity Investments 2 us-gaap_PaymentsToAcquireEquityMethodInvestments us-gaap true credit duration No definition available. false false false false false false false false false false true negated false 1 false true false false -300000 -0.3 false false false 2 false true false false -20200000 -20.2 false false false xbrli:monetaryItemType monetary The cash outflow associated with the purchase of or advances to an equity method investments, which are investments in joint ventures and entities in which the entity has an equity ownership interest normally of 20 to 50 percent and exercises significant influence. Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 95 -Paragraph 17 -Subparagraph b false 46 Other Investing Activities 2 us-gaap_PaymentsForProceedsFromOtherInvestingActivities us-gaap true credit duration No definition available. false false false false false false false false false false true negatedtotal false 1 false true false false -19900000 -19.9 false false false 2 false true false false -9200000 -9.2 false false false xbrli:monetaryItemType monetary The net cash outflow (inflow) from other investing activities. This element is used when there is not a more specific and appropriate element in the taxonomy. Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 95 -Paragraph 15 true 47 Net Investing Activities used for Continuing Operations 2 us-gaap_NetCashProvidedByUsedInInvestingActivitiesContinuingOperations us-gaap true debit duration No definition available. false false false false false false false false false false false verboselabel false 1 false true false false -394300000 -394.3 false false false 2 false true false false -327400000 -327.4 false false false xbrli:monetaryItemType monetary The net cash from (used in) the entity's investing activities specifically EXCLUDING the cash flows derived by the entity from its discontinued operations, if any. This element is only to be used when the entity reports its cash flows attributable to discontinued operations separately from the cash flow provided by or used in investing activities. Such reporting would necessitate the entity to use the Net Cash Provided by (Used in) Discontinued Operations, Total element provided in the taxonomy. Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 95 -Paragraph 26 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 95 -Paragraph 26 -Footnote 10 false 48 Net Investing Activities from Discontinued Operations 2 us-gaap_CashProvidedByUsedInInvestingActivitiesDiscontinuedOperations us-gaap true debit duration No definition available. false false false false false false false false false false false totallabel false 1 false true false false 400000 0.4 false false false 2 false true false false 7500000 7.5 false false false xbrli:monetaryItemType monetary This element represents cash provided by (used in) the investing activities of the entity's discontinued operations during the period. This element should only be used by those entities that separately report cash flows attributable to discontinued operations. If using this element, it is an indication that the cash flows of the entity which are detailed in reconciling to cash provided by or used in investing activities reflect only cash flows attributable to continuing operations. Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 95 -Paragraph 26 true 49 Net Cash Flow used for Investing Activities 2 us-gaap_NetCashProvidedByUsedInInvestingActivities us-gaap true debit duration No definition available. false false false false false false false false false false false totallabel false 1 false true false false -393900000 -393.9 false false false 2 false true false false -319900000 -319.9 false false false xbrli:monetaryItemType monetary The net cash inflow (outflow) from investing activity. Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 95 -Paragraph 26 true 50 Financing Activities 1 us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract us-gaap true na duration No definition available. false false false false false true false false false false false verboselabel false 1 false false false false 0 0 false false false 2 false false false false 0 0 false false false xbrli:stringItemType string No definition available. false 51 Issuance of Long-Term Debt 2 us-gaap_ProceedsFromIssuanceOfLongTermDebt us-gaap true debit duration No definition available. false false false false false false false false false false false verboselabel false 1 false false false false 0 0 false false false 2 false true false false 963500000 963.5 false false false xbrli:monetaryItemType monetary The cash inflow from a debt initially having maturity due after one year or beyond the operating cycle, if longer. 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Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 95 -Paragraph 18 false 54 Issuance of Common Stock 2 us-gaap_ProceedsFromIssuanceOfCommonStock us-gaap true debit duration No definition available. false false false false false false false false false false false verboselabel false 1 false true false false 6800000 6.8 false false false 2 false true false false 400000 0.4 false false false xbrli:monetaryItemType monetary The cash inflow from the additional capital contribution to the entity. 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Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 95 -Paragraph 18 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 95 -Paragraph 20 -Subparagraph a true 57 Net Cash Flow used for Financing Activities 2 us-gaap_NetCashProvidedByUsedInFinancingActivities us-gaap true debit duration No definition available. false false false false false false false false false false false totallabel false 1 false true false false -17600000 -17.6 false false false 2 false true false false -691600000 -691.6 false false false xbrli:monetaryItemType monetary The net cash inflow (outflow) from financing activity for the period. 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No authoritative reference available. false 59 Cash contributions to discontinued operations 1 us-gaap_NetCashProvidedByUsedInDiscontinuedOperations us-gaap true debit duration No definition available. false false false false false false false false false false false verboselabel false 1 false true false false -44400000 -44.4 false false false 2 false true false false -54800000 -54.8 false false false xbrli:monetaryItemType monetary Net change in cash associated with the entity's discontinued operations. Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 95 -Paragraph 26 false 60 Cash and cash equivalents at beginning of period 1 us-gaap_CashAndCashEquivalentsAtCarryingValue us-gaap true debit instant No definition available. false false false false false false false false true false false periodstartlabel false 1 false true false false 16400000 16.4 false false false 2 false true false false 20600000 20.6 false false false xbrli:monetaryItemType monetary Includes currency on hand as well as demand deposits with banks or financial institutions. It also includes other kinds of accounts that have the general characteristics of demand deposits in that the Entity may deposit additional funds at any time and also effectively may withdraw funds at any time without prior notice or penalty. Cash equivalents, excluding items classified as marketable securities, include short-term, highly liquid investments that are both readily convertible to known amounts of cash, and so near their maturity that they present minimal risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify under that definition. Original maturity means original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three years ago does not become a cash equivalent when its remaining maturity is three months. Compensating balance arrangements that do not legally restrict the withdrawal or usage of cash amounts may be reported as Cash and Cash Equivalents, while legally restricted deposits held as compensating balances against borrowing arrangements, contracts entered into with others, or company statements of intention with regard to particular deposits should not be reported as cash and cash equivalents. 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It also includes other kinds of accounts that have the general characteristics of demand deposits in that the Entity may deposit additional funds at any time and also effectively may withdraw funds at any time without prior notice or penalty. Cash equivalents, excluding items classified as marketable securities, include short-term, highly liquid investments that are both readily convertible to known amounts of cash, and so near their maturity that they present minimal risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify under that definition. Original maturity means original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three years ago does not become a cash equivalent when its remaining maturity is three months. Compensating balance arrangements that do not legally restrict the withdrawal or usage of cash amounts may be reported as Cash and Cash Equivalents, while legally restricted deposits held as compensating balances against borrowing arrangements, contracts entered into with others, or company statements of intention with regard to particular deposits should not be reported as cash and cash equivalents. 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