NiSource (NI) 10-K 20 Feb 18 2017 FY Annual report Financial data

Document And Entity Information

Document And Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2017	Feb. 12, 2018	Jun. 30, 2017
Document Information [Line Items]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2017
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	FY
Entity Registrant Name	NISOURCE INC/DE
Entity Central Index Key	1,111,711
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Public Float			$ 8,237,384,461
Entity Common Stock, Shares Outstanding		337,410,827

Statements Of Consolidated Inco

Statements Of Consolidated Income - USD ($) shares in Thousands, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating Revenues
Gas Distribution	$ 2,063.2	$ 1,850.9	$ 2,081.9
Gas Transportation and Storage	1,021.5	964.6	969.8
Electric	1,785.5	1,660.8	1,572.9
Other	4.4	16.2	27.2
Total Operating Revenues	4,874.6	4,492.5	4,651.8
Operating Expenses
Cost of Sales (excluding depreciation and amortization)	1,518.7	1,390.2	1,643.7
Operation and maintenance	1,612.3	1,453.7	1,426.1
Depreciation and amortization	570.3	547.1	524.4
(Gain) Loss on sale of assets and impairments, net	5.5	(1)	1.6
Other taxes	257.2	244.3	256.1
Total Operating Expenses	3,964	3,634.3	3,851.9
Operating Income	910.6	858.2	799.9
Other Income (Deductions)
Interest expense, net	(353.2)	(349.5)	(380.2)
Other, net	(2.8)	1.5	17.4
Loss on early extinguishment of long-term debt	(111.5)	0	(97.2)
Total Other Deductions	(467.5)	(348)	(460)
Income from Continuing Operations before Income Taxes	443.1	510.2	339.9
Income Taxes	314.5	182.1	141.3
Income from Continuing Operations	128.6	328.1	198.6
Income (Loss) from Discontinued Operations - net of taxes	(0.1)	3.4	103.5
Net Income	128.5	331.5	302.1
Less: Net income attributable to noncontrolling interest	0	0	15.6
Net Income	128.5	331.5	286.5
Income (Loss) from Discontinued Operations - net of taxes	$ (0.1)	$ 3.4	$ 87.9
Basic Earnings Per Share
Continuing Operations	$ 0.39	$ 1.02	$ 0.63
Discontinued operations	0	0.01	0.27
Basic Earnings Per Share	0.39	1.03	0.90
Diluted Earnings Per Share
Continuing operations	0.39	1.01	0.63
Discontinued operations	0	0.01	0.27
Diluted Earnings Per Share	$ 0.39	$ 1.02	$ 0.90
Basic Average Common Shares Outstanding	329,388	321,805	317,746
Diluted Average Common Shares	330,756	323,524	319,836

Statements of Consolidated Comp

Statements of Consolidated Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net Income	$ 128.5	$ 331.5	$ 302.1
Other comprehensive income (loss):
Net unrealized gain (loss) on available-for-sale securities	0.8	(0.1)	(0.8)	[1]
Net unrealized gain (loss) on cash flow hedges	(22.5)	8.6	(7.8)	[2]
Unrecognized pension and OPEB benefit (costs)	3.4	1.5	(2.4)	[3]
Total other comprehensive income (loss)	(18.3)	10	(11)
Total Comprehensive Income	110.2	341.5	291.1
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Noncontrolling Interest	0	0	15.6
Total comprehensive income	$ 110.2	$ 341.5	$ 275.5
[1]	Net unrealized gain (loss) on available-for-sale securities, net of $0.4 million tax expense, $0.1 million tax benefit and $0.4 million tax benefit in 2017, 2016 and 2015, respectively.
[2]	Net unrealized gain (loss) on derivatives qualifying as cash flow hedges, net of $13.9 million tax benefit, $5.6 million tax expense and $4.8 million tax benefit in 2017, 2016 and 2015, respectively.
[3]	Unrecognized pension and OPEB benefit (costs), net of $2.1 million tax expense, $0.1 million tax expense and $4.6 million tax benefit in 2017, 2016 and 2015, respectively.

Statements of Consolidated Com4

Statements of Consolidated Comprehensive Income (Parenthetical) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Tax	$ 0.4	$ (0.1)	$ (0.4)
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Tax	(13.9)	5.6	(4.8)
Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, Tax	$ (2.1)	$ (0.1)	$ 4.6

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment
Utility Plant	$ 21,026.6	$ 19,368
Accumulated depreciation and amortization	(6,953.6)	(6,613.7)
Net utility plant	14,073	12,754.3
Other property, at cost, less accumulated depreciation	286.5	313.7
Net Property, Plant and Equipment	14,359.5	13,068
Investments and Other Assets
Unconsolidated affiliates	5.5	6.6
Other investments	204.1	193.3
Total Investments and Other Assets	209.6	199.9
Current Assets
Cash and cash equivalents	29	26.4
Restricted cash	9.4	9.6
Accounts receivable (less reserve of $18.3 and $23.3, respectively)	898.9	847
Gas inventory	285.1	279.9
Materials and supplies, at average cost	105.9	101.7
Electric production fuel, at average cost	80.1	112.8
Exchange gas receivable	45.8	5.4
Regulatory assets	176.3	248.7
Prepayments and other	132.8	130.6
Total Current Assets	1,763.3	1,762.1
Other Assets
Regulatory assets	1,624.9	1,636.7
Goodwill	1,690.7	1,690.7
Intangible assets	231.7	242.7
Deferred charges and other	82	91.8
Total Other Assets	3,629.3	3,661.9
Total Assets	19,961.7	18,691.9
Common Stockholders' Equity
Common stock - $0.01 par value, 400,000,000 shares authorized; 337,015,806 and 323,159,672 shares outstanding, respectively	3.4	3.3
Treasury stock	(95.9)	(88.7)
Additional paid-in capital	5,529.1	5,153.9
Retained deficit	(1,073.1)	(972.2)
Accumulated other comprehensive loss	(43.4)	(25.1)
Total Common Stockholders' Equity	4,320.1	4,071.2
Long-term debt, excluding amounts due within one year	7,512.2	6,058.2
Total Capitalization	11,832.3	10,129.4
Current Liabilities
Current portion of long-term debt	284.3	363.1
Short-term borrowings	1,205.7	1,488
Accounts payable	625.6	539.4
Customer deposits and credits	262.6	264.1
Taxes accrued	208.1	195.4
Interest accrued	112.3	120.3
Risk management liabilities	43.2	16.8
Exchange gas payable	59.6	83.7
Regulatory liabilities	58.7	116.7
Legal and environmental	32.1	37.4
Accrued compensation and employee benefits	195.4	161.4
Other accruals	90.8	65.9
Total Current Liabilities	3,178.4	3,452.2
Other Liabilities
Risk management liabilities	28.5	44.5
Deferred income taxes	1,292.9	2,528
Deferred investment tax credits	12.4	13.4
Accrued insurance liabilities	80.1	82.8
Accrued liability for postretirement and postemployment benefits	337.1	713.4
Regulatory liabilities	2,736.9	1,265.1
Asset retirement obligations	268.7	262.6
Other noncurrent liabilities	194.4	200.5
Total Other Liabilities	4,951	5,110.3
Commitments and Contingencies (Refer to Note 18, Other Commitments and Contingencies)	0	0
Total Capitalization and Liabilities	$ 19,961.7	$ 18,691.9

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Accounts receivable less reserve	$ 18.3	$ 23.3
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	400,000,000	400,000,000
Common stock, shares outstanding	337,015,806	323,159,672

Statements Of Consolidated Cash

Statements Of Consolidated Cash Flows - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net Income	$ 128.5	$ 331.5	$ 302.1
Adjustments to Reconcile Net Income to Net Cash from Continuing Operations:
Loss on early extinguishment of debt	111.5	0	97.2
Depreciation and amortization	570.3	547.1	524.4
Deferred income taxes and investment tax credits	306.7	182.3	135.3
Stock compensation expense and 401(k) profit sharing contribution	40.1	46.5	50.7
(Income) loss from discontinued operations - net of taxes	0.1	(3.4)	(103.5)
Amortization of discount/premium on debt	7.4	7.6	8.7
AFUDC equity	(12.6)	(11.6)	(11.5)
Other adjustments	6.5	(3.8)	13.1
Changes in Assets and Liabilities:
Accounts receivable	(52.3)	(188)	262.2
Inventories	19	38.9	46.9
Accounts payable	49	108.8	(190.5)
Customer deposits and credits	(2.5)	(52.3)	35.5
Taxes accrued	10.2	12.1	8.7
Interest accrued	(33.9)	(8.7)	(11.6)
Exchange gas receivable/payable	(64.5)	36.9	(31.7)
Other accruals	31.8	(6)	(55.1)
Prepayments and other current assets	(13.3)	(0.4)	0.1
Regulatory assets/liabilities	57.5	(187.9)	82
Postretirement and postemployment benefits	(380.9)	(44.8)	25.6
Deferred charges and other noncurrent assets	(2)	(1.2)	5.2
Other noncurrent liabilities	(34.5)	0.5	(30.4)
Net Operating Activities from Continuing Operations	742.1	804.1	1,163.4
Net Operating Activities from (used for) Discontinued Operations	0.1	(0.8)	293.4
Net Cash Flows from Operating Activities	742.2	803.3	1,456.8
Net Cash Flows from (used for) Investing Activities
Capital expenditures	(1,695.8)	(1,475.2)	(1,360.7)
Cash contributions from CPG	0	0	3,798.2
Cost of removal	(109)	(110.1)	(79.2)
Purchases of available-for-sale securities	(168.4)	(38.3)	(54.9)
Sales of available-for-sale securities	163.1	33	58.4
Other investing activities	1.6	(12.4)	18
Net Investing Activities from (used for) Continuing Operations	(1,808.5)	(1,603)	2,379.8
Net Investing Activities used for Discontinued Operations	0	0	(430.1)
Net Cash Flows from (used for) Investing Activities	(1,808.5)	(1,603)	1,949.7
Net Cash Flows from (used for) Financing Activities
Cash of CPG at Separation	0	0	(136.8)
Issuance of long-term debt	3,250	500	0
Repayments of long-term debt and capital lease obligations	(1,855)	(434.6)	(2,092.2)
Premiums and other debt related costs	(144.3)	(3.7)	(93.5)
Change in short-term borrowings, net	(282.4)	920.6	(936.4)
Issuance of common stock	336.7	23.1	22.5
Acquisition of treasury stock	(7.2)	(9.4)	(20.4)
Dividends paid - common stock	(229.1)	(205.5)	(263.4)
Net Financing Activities from (used for) Continuing Operations	1,068.7	790.5	(3,520.2)
Net Financing Activities from Discontinued Operations	0	0	108.6
Net Cash Flows from (used for) Financing Activities	1,068.7	790.5	(3,411.6)
Change in cash, cash equivalents and restricted cash from (used for) continuing operations	2.3	(8.4)	23
Change in cash, cash equivalents and restricted cash from (used for) discontinued operations	0.1	(0.8)	(28.1)
Change in cash included in discontinued operations	0	0	0.5
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	36	45.2	49.8
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	$ 38.4	$ 36	$ 45.2

Statements Of Consolidated Comm

Statements Of Consolidated Common Stockholders' Equity - USD ($) $ in Millions		Total	Common Stock	Treasury Stock	Additional Paid-in Capital	Retained Earnings (Deficit)	Accumulated Other Comprehensive Loss
Beginning Balance at Dec. 31, 2014		$ 6,175.3	$ 3.2	$ (58.9)	$ 4,787.6	$ 1,494	$ (50.6)
Comprehensive Income (Loss):
Net Income		286.5	0	0	0	286.5	0
Other comprehensive income, net of tax		(11)	0	0	0	0	(11)
Allocation of AOCI to Noncontrolling Interest	[1]	2	0	0	0	0	2
Sale of interest in Columbia OpCo to CPPL	[1],[2]	227.1	0	0	227.1	0	0
Dividends:
Common stock		(263.5)	0	0	0	(263.5)	0
Distribution of CPG stock to shareholders		(2,615.8)	0	0	0	(2,640.3)	24.5
Treasury stock acquired		(20.4)	0	(20.4)	0	0	0
Stock issuances:
Employee stock purchase plan		5.1	0	0	5.1	0	0
Long-term incentive plan		4.2	0	0	4.2	0	0
401(k) and profit sharing		46.7	0	0	46.7	0	0
Dividend reinvestment plan		7.3	0	0	7.3	0	0
Ending Balance at Dec. 31, 2015		3,843.5	3.2	(79.3)	5,078	(1,123.3)	(35.1)
Cumulative effect of change in accounting principle		25.3	0	0	0	25.3	0
Comprehensive Income (Loss):
Net Income		331.5	0	0	0	331.5	0
Other comprehensive income, net of tax		10	0	0	0	0	10
Dividends:
Common stock		(205.7)	0	0	0	(205.7)	0
Treasury stock acquired		(9.4)	0	(9.4)	0	0	0
Stock issuances:
Common stock		0.1	0.1	0	0	0
Employee stock purchase plan		4.7	0	0	4.7	0	0
Long-term incentive plan		20.9	0	0	20.9	0	0
401(k) and profit sharing		41.4	0	0	41.4	0	0
Dividend reinvestment plan		8.9	0	0	8.9	0	0
Ending Balance at Dec. 31, 2016		4,071.2	3.3	(88.7)	5,153.9	(972.2)	(25.1)
Comprehensive Income (Loss):
Net Income		128.5	0	0	0	128.5	0
Other comprehensive income, net of tax		(18.3)	0	0	0	0	(18.3)
Dividends:
Common stock		(229.4)	0	0	0	(229.4)	0
Treasury stock acquired		(7.2)	0	(7.2)	0	0	0
Stock issuances:
Common stock		314.7	0.1	0	314.6	0	0
Employee stock purchase plan		5	0	0	5	0	0
Long-term incentive plan		14.9	0	0	14.9	0	0
401(k) and profit sharing		34.3	0	0	34.3	0	0
Dividend reinvestment plan		6.4	0	0	6.4	0	0
Ending Balance at Dec. 31, 2017		$ 4,320.1	$ 3.4	$ (95.9)	$ 5,529.1	$ (1,073.1)	$ (43.4)
[1]	This transaction, which occurred prior to the Separation, was distributed through retained earnings as part of the Separation on July 1, 2015.
[2]	Represents the purchase of an additional 8.4% limited partner interest in Columbia OpCo by an affiliate of CPG, recorded at the historical carrying value of Columbia OpCo's net assets after giving effect to the $1,168.4 million equity contribution from CPPL's IPO completed on February 11, 2015.

Statements Of Consolidated Com9

Statements Of Consolidated Common Stockholders' Equity (Shares) (Parenthetical) - USD ($) shares in Thousands, $ in Millions	Feb. 11, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Beginning Balance		323,160	319,110	316,037
Treasury stock acquired		(293)	(433)	(472)
Issued:
Employee stock purchase plan		207	201	203
Long-term incentive plan		351	2,103	1,423
401(k) and profit sharing plan		1,396	1,793	1,644
Dividend reinvestment plan		264	386	275
ATM program		11,931
Ending Balance		337,016	323,160	319,110
Limited Liability Company or Limited Partnership, Members or Limited Partners, Ownership Interest	8.40%
Proceeds from Issuance Initial Public Offering	$ 1,168.4
Common Stock, Dividends, Per Share, Declared		$ 0.70	$ 0.83	$ 1.02
Common Stock
Beginning Balance		326,664	322,181	318,636
Issued:
Employee stock purchase plan		207	201	203
Long-term incentive plan		351	2,103	1,423
401(k) and profit sharing plan		1,396	1,793	1,644
Dividend reinvestment plan		264	386	275
ATM program		11,931
Ending Balance		340,813	326,664	322,181
Treasury Stock
Beginning Balance		3,504	3,071	2,599
Treasury stock acquired		(293)	(433)	(472)
Issued:
Employee stock purchase plan		0	0	0
Long-term incentive plan		0	0	0
401(k) and profit sharing plan		0	0	0
Dividend reinvestment plan		0	0	0
ATM program		0
Ending Balance		3,797	3,504	3,071

Nature of Operations And Summar

Nature of Operations And Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2017
Nature of Operations And Summary of Significant Accounting Policies [Abstract]
Nature of Operations and Summary of Significant Accounting Policies	Nature of Operations and Summary of Significant Accounting Policies A. Company Structure and Principles of Consolidation. NiSource, a Delaware corporation headquartered in Merrillville, Indiana, is an energy holding company whose subsidiaries are fully regulated natural gas and electric utility companies serving approximately 3.9 million customers in seven states. NiSource generates substantially all of its operating income through these rate-regulated businesses. The consolidated financial statements include the accounts of NiSource and its majority-owned subsidiaries after the elimination of all intercompany accounts and transactions. B. Use of Estimates. The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements, and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates. C. Cash, Cash Equivalents and Restricted Cash. NiSource considers all highly liquid investments with original maturities of three months or less to be cash equivalents. NiSource reports amounts deposited in brokerage accounts for margin requirements as restricted cash. In addition, NiSource has amounts deposited in trust to satisfy requirements for the provision of various property, liability, workers compensation, and long-term disability insurance, which is classified as restricted cash and disclosed as cash and cash equivalents on the Statements of Consolidated Cash Flows. D. Accounts Receivable and Unbilled Revenue. Accounts receivable on the Consolidated Balance Sheets includes both billed and unbilled amounts. Unbilled amounts of accounts receivable relate to a portion of a customer’s consumption of gas or electricity from the date of the last cycle billing date through the last day of the month (balance sheet date). Factors taken into consideration when estimating unbilled revenue include historical usage, customer rates and weather. Accounts receivable fluctuates from year to year depending in large part on weather impacts and price volatility. NiSource's accounts receivable on the Consolidated Balance Sheets include unbilled revenue, less reserves, in the amounts of $359.4 million and $329.7 million as of December 31, 2017 and 2016 , respectively. The reserve for uncollectible receivables is NiSource’s best estimate of the amount of probable credit losses in the existing accounts receivable. NiSource determined the reserve based on historical experience and in consideration of current market conditions. Account balances are charged against the allowance when it is anticipated the receivable will not be recovered. E. Investments in Debt Securities. NiSource’s investments in debt securities are carried at fair value and are designated as available-for-sale. These investments are included within “Other investments” on the Consolidated Balance Sheets. Unrealized gains and losses, net of deferred income taxes, are reflected as accumulated other comprehensive income. These investments are monitored for other than temporary declines in market value. Realized gains and losses and permanent impairments are reflected in the Statements of Consolidated Income. No material impairment charges were recorded for the years ended December 31, 2017 , 2016 or 2015 . Refer to Note 16 , "Fair Value," for additional information. F. Basis of Accounting for Rate-Regulated Subsidiaries. Rate-regulated subsidiaries account for and report assets and liabilities consistent with the economic effect of the way in which regulators establish rates, if the rates established are designed to recover the costs of providing the regulated service and it is probable that such rates can be charged and collected. Certain expenses and credits subject to utility regulation or rate determination normally reflected in income are deferred on the Consolidated Balance Sheets and are later recognized in income as the related amounts are included in customer rates and recovered from or refunded to customers. In the event that regulation significantly changes the opportunity for NiSource to recover its costs in the future, all or a portion of NiSource’s regulated operations may no longer meet the criteria for regulatory accounting. In such an event, a write-down of all or a portion of NiSource’s existing regulatory assets and liabilities could result. If transition cost recovery was approved by the appropriate regulatory bodies that would meet the requirements under GAAP for continued accounting as regulatory assets and liabilities during such recovery period, the regulatory assets and liabilities would be reported at the recoverable amounts. If unable to continue to apply the provisions of regulatory accounting, NiSource would be required to apply the provisions of ASC 980-20, Discontinuation of Rate-Regulated Accounting . In management’s opinion, NiSource’s regulated subsidiaries will be subject to regulatory accounting for the foreseeable future. Refer to Note 8 , "Regulatory Matters," for additional information. G. Plant and Other Property and Related Depreciation and Maintenance. Property, plant and equipment (principally utility plant) is stated at cost. The rate-regulated subsidiaries record depreciation using composite rates on a straight-line basis over the remaining service lives of the electric, gas and common properties as approved by the appropriate regulators. Non-utility property is generally depreciated on a straight-line basis over the life of the associated asset. Refer to Note 5 , "Property, Plant and Equipment," for additional information related to depreciation expense at Units 7 and 8 at Bailly Generating Station. For rate-regulated companies, AFUDC is capitalized on all classes of property except organization costs, land, autos, office equipment, tools and other general property purchases. The allowance is applied to construction costs for that period of time between the date of the expenditure and the date on which such project is placed in service. The pre-tax rate for AFUDC was 4.0% in 2017 , 4.5% in 2016 and 4.7% in 2015 . Generally, NiSource’s subsidiaries follow the practice of charging maintenance and repairs, including the cost of removal of minor items of property, to expense as incurred. When NiSource’s subsidiaries retire regulated property, plant and equipment, original cost plus the cost of retirement, less salvage value, is charged to accumulated depreciation. However, when it becomes probable a regulated asset will be retired substantially in advance of its original expected useful life or is abandoned, the cost of the asset and the corresponding accumulated depreciation is recognized as a separate asset. If the asset is still in operation, the net amount is classified as "Other property, at cost, less accumulated depreciation" on the Consolidated Balance Sheets. If the asset is no longer operating, the net amount is classified in "Regulatory assets" on the Consolidated Balance Sheets. If NiSource is able to recover a full return of and on investment, the carrying value of the asset is based on historical cost. If NiSource is not able to recover a full return on investment, a loss on impairment is recognized to the extent the net book value of the asset exceeds the present value of future revenues discounted at the incremental borrowing rate. When NiSource’s subsidiaries sell entire regulated operating units, or retire or sell nonregulated properties, the original cost and accumulated depreciation and amortization balances are removed from "Property, Plant and Equipment" on the Consolidated Balance Sheets. Any gain or loss is recorded in earnings, unless otherwise required by the applicable regulatory body. Refer to Note 5 , "Property, Plant and Equipment," for further information. External and internal costs associated with computer software developed for internal use are capitalized. Capitalization of such costs commences upon the completion of the preliminary stage of each project. Once the installed software is ready for its intended use, such capitalized costs are amortized on a straight-line basis generally over a period of five years, except for certain significant enterprise-wide technology investments which are amortized over a ten-year period. H. Goodwill and Other Intangible Assets. Substantially all of NiSource's goodwill relates to the excess of cost over the fair value of the net assets acquired in the Columbia acquisition on November 1, 2000. NiSource tests its goodwill for impairment annually as of May 1, or more frequently if events and circumstances indicate that goodwill might be impaired. Fair value of NiSource's reporting units is determined using a combination of income and market approaches. NiSource has other intangible assets consisting primarily of franchise rights apart from goodwill that were identified as part of the purchase price allocations associated with the acquisition of Columbia of Massachusetts which is being amortized on a straight-line basis over forty years from the date of acquisition. See Note 6 , "Goodwill and Other Intangible Assets," for additional information. I. Revenue Recognition. Revenue is recorded as products and services are delivered. Utility revenues are billed to customers monthly on a cycle basis. Revenues are recorded on the accrual basis and also include estimates for electricity and gas delivered but not billed. The accruals for unbilled revenues are reversed in the subsequent accounting period when meters are actually read and customers are billed. On occasion, NiSource's regulated subsidiaries are permitted to implement new rates that have not been formally approved by their state regulatory commissions, which are subject to refund. As permitted by accounting principles generally accepted in the United States, each regulated subsidiary recognizes this revenue and establishes a reserve for amounts that could be refunded based on its experience for the jurisdiction in which the rates were implemented. In connection with such revenues, estimated rate refund liabilities are recorded which reflect management’s current judgment of the ultimate outcomes of the proceedings. No provisions are made when, in the opinion of management, the facts and circumstances preclude a reasonable estimate of the outcome. J. Accounts Receivable Transfer Program. Certain of NiSource’s subsidiaries have agreements with third parties to sell certain accounts receivable without recourse. These transfers of accounts receivable are accounted for as secured borrowings. The entire gross receivables balance remains on the December 31, 2017 and 2016 Consolidated Balance Sheets and short-term debt is recorded in the amount of proceeds received from the purchasers involved in the transactions. Refer to Note 17 , "Transfers of Financial Assets," for further information. K. Gas Cost and Fuel Adjustment Clause. NiSource’s regulated subsidiaries defer most differences between gas and fuel purchase costs and the recovery of such costs in revenues, and adjust future billings for such deferrals on a basis consistent with applicable state-approved tariff provisions. These deferred balances are recorded as "Regulatory assets" or "Regulatory liabilities," as appropriate, on the Consolidated Balance Sheets. Refer to Note 8 , "Regulatory Matters," for additional information. L. Inventory. Both the LIFO inventory methodology and the weighted average cost methodology are used to value natural gas in storage, as approved by regulators for all of NiSource’s regulated subsidiaries. Inventory valued using LIFO was $45.5 million and $46.1 million at December 31, 2017 and 2016 , respectively. Based on the average cost of gas using the LIFO method, the estimated replacement cost of gas in storage was less than the stated LIFO cost by $17.4 million and $9.4 million at December 31, 2017 and 2016 , respectively. Gas inventory valued using the weighted average cost methodology was $239.6 million at December 31, 2017 and $233.8 million at December 31, 2016 . Electric production fuel is valued using the weighted average cost inventory methodology, as approved by NIPSCO's regulator. Materials and supplies are valued using the weighted average cost inventory methodology. M. Accounting for Exchange and Balancing Arrangements of Natural Gas. NiSource’s Gas Distribution Operations segment enters into balancing and exchange arrangements of natural gas as part of its operations and off-system sales programs. NiSource records a receivable or payable for any of its respective cumulative gas imbalances, as well as for any gas inventory borrowed or lent under a Gas Distribution Operations exchange agreement. Exchange gas is valued based on individual regulatory jurisdiction requirements (for example, historical spot rate, spot at the beginning of the month). These receivables and payables are recorded as “Exchange gas receivable” or “Exchange gas payable” on NiSource’s Consolidated Balance Sheets, as appropriate. N. Accounting for Risk Management Activities. NiSource accounts for its derivatives and hedging activities in accordance with ASC 815. NiSource recognizes all derivatives as either assets or liabilities on the Consolidated Balance Sheets at fair value, unless such contracts are exempted as a normal purchase normal sale under the provisions of the standard. The accounting for changes in the fair value of a derivative depends on the intended use of the derivative and resulting designation. NiSource has elected not to net fair value amounts for any of its derivative instruments or the fair value amounts recognized for its right to receive cash collateral or obligation to pay cash collateral arising from those derivative instruments recognized at fair value, which are executed with the same counterparty under a master netting arrangement. See Note 9 , "Risk Management Activities," for additional information. O. Income Taxes and Investment Tax Credits. NiSource records income taxes to recognize full interperiod tax allocations. Under the asset and liability method, deferred income taxes are provided for the tax consequences of temporary differences by applying enacted statutory tax rates applicable to future years to differences between the financial statement carrying amount and the tax basis of existing assets and liabilities. Previously recorded investment tax credits of the regulated subsidiaries were deferred on the balance sheet and are being amortized to book income over the regulatory life of the related properties to conform to regulatory policy. To the extent certain deferred income taxes of the regulated companies are recoverable or payable through future rates, regulatory assets and liabilities have been established. Regulatory assets for income taxes are primarily attributable to property-related tax timing differences for which deferred taxes had not been provided in the past, when regulators did not recognize such taxes as costs in the ratemaking process. Regulatory liabilities for income taxes are primarily attributable to the regulated companies’ obligation to refund to ratepayers deferred income taxes provided at rates higher than the current Federal income tax rate. Such property-related amounts are credited to ratepayers using either the average rate assumption method or the reverse South Georgia method. Non property-related amounts are credited to ratepayers consistent with state utility commission direction. Pursuant to the U.S. Internal Revenue Code and relevant state taxing authorities, NiSource and its subsidiaries file consolidated income tax returns for Federal and certain state jurisdictions. NiSource and its subsidiaries are parties to an agreement (the “Intercompany Income Tax Allocation Agreement”) that provides for the allocation of consolidated tax liabilities. The Intercompany Income Tax Allocation Agreement generally provides that each party is allocated an amount of tax similar to that which would be owed had the party been separately subject to tax. P. Environmental Expenditures. NiSource accrues for costs associated with environmental remediation obligations when the incurrence of such costs is probable and the amounts can be reasonably estimated, regardless of when the expenditures are actually made. The undiscounted estimated future expenditures are based on currently enacted laws and regulations, existing technology and estimated site-specific costs where assumptions may be made about the nature and extent of site contamination, the extent of cleanup efforts, costs of alternative cleanup methods and other variables. The liability is adjusted as further information is discovered or circumstances change. The reserves for estimated environmental expenditures are recorded on the Consolidated Balance Sheets in “Legal and environmental” for short-term portions of these liabilities and “Other noncurrent liabilities” for the respective long-term portions of these liabilities. Rate-regulated subsidiaries applying regulatory accounting establish regulatory assets on the Consolidated Balance Sheets to the extent that future recovery of environmental remediation costs is probable through the regulatory process. Refer to Note 18 , "Other Commitments and Contingencies," for further information. Q. Excise Taxes. NiSource accounts for excise taxes that are customer liabilities by separately stating on its invoices the tax to its customers and recording amounts invoiced as liabilities payable to the applicable taxing jurisdiction. Such balances are presented within "Other accruals" on the Consolidated Balance Sheets. These types of taxes collected from customers, comprised largely of sales taxes, are presented on a net basis affecting neither revenues nor cost of sales. NiSource accounts for excise taxes for which it is liable by recording a liability for the expected tax with a corresponding charge to “Other taxes” expense on the Statements of Consolidated Income. R. Accrued Insurance Liabilities. NiSource accrues for insurance costs related to workers compensation, automobile, property, general and employment practices liabilities based on the most probable value of each claim. Claim values are determined by professional, licensed loss adjusters who consider the facts of the claim, anticipated indemnification and legal expenses, and respective state rules. Claims are reviewed by NiSource at least quarterly and an adjustment is made to the accrual based on the most current information. NiSource’s actual exposure to liability is minimal due to coverage from its wholly-owned captive insurer who then transfers risk to third party insurance providers for the majority of costs paid to claimants above NiSource's deductible.

Recent Accounting Pronouncement

Recent Accounting Pronouncements	12 Months Ended
	Dec. 31, 2017
Recent Accounting Pronouncements [Abstract]
New Accounting Pronouncements and Changes in Accounting Principles	Recent Accounting Pronouncements Recently Issued Accounting Pronouncements NiSource is currently evaluating the impact of certain ASUs on its Consolidated Financial Statements or Notes to Consolidated Financial Statements, which are described below: Standard Description Effective Date Effect on the financial statements or other significant matters ASU 2016-13, Financial Instruments-Credit Losses (Topic 326) The pronouncement changes the impairment model for most financial assets, replacing the current "incurred loss" model. ASU 2016-13 will require the use of an "expected loss" model for instruments measured at amortized cost and will also require entities to record allowances for available-for-sale debt securities rather than reduce the carrying amount. Annual periods beginning after December 15, 2019, including interim periods therein. Early adoption is permitted for annual or interim periods beginning after December 15, 2018. NiSource is currently evaluating the impact of adoption, if any, on the Consolidated Financial Statements and Notes to Consolidated Financial Statements. Standard Description Effective Date Effect on the financial statements or other significant matters ASU 2018-01, Leases (Topic 842): Land Easement Practical Expedient for Transition to Topic 842 The pronouncement offers a practical expedient for accounting for land easements under ASU 2016-02. This practical expedient allows an entity the option of not evaluating existing land easements under ASC 842. New or modified land easements will still require evaluation under ASC 842 on a prospective basis beginning on the date of adoption. Annual periods beginning after December 15, 2018, including interim periods therein. Early adoption is permitted. NiSource has formed an internal stakeholder group that meets periodically to share information and gather data related to leasing activity at NiSource. This includes compiling a list of all contracts that could meet the definition of a lease under the new standard and evaluating the accounting for these contracts under the new standard to determine the ultimate impact the new standard will have on NiSource’s financial statements. Also, this procedure has identified process improvements to ensure data from newly initiated leases is captured to comply with the new standard. This work included the assistance of a third-party advisory firm. NiSource maintains a substantial number of easements and expects ASU 2018-01 will ease the process of implementation of ASC 842. NiSource plans to adopt these standards effective January 1, 2019. ASU 2016-02, Leases (Topic 842) The pronouncement introduces a lessee model that brings most leases on the balance sheet. The standard requires that lessees recognize the following for all leases (with the exception of short-term leases, as that term is defined in the standard) at the lease commencement date: (1) a lease liability, which is a lessee’s obligation to make lease payments arising from a lease, measured on a discounted basis; and (2) a right-of-use asset, which is an asset that represents the lessee’s right to use, or control the use of, a specified asset for the lease term. ASU 2018-02, Income Statement-Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income The pronouncement permits entities the option to reclassify tax effects that are stranded in accumulated other comprehensive income as a result of the implementation of the TCJA to retained earnings. Annual periods beginning after December 15, 2018, including interim periods therein. Early adoption is permitted for interim periods beginning after December 15, 2017. NiSource is currently evaluating the impact of adoption on the Consolidated Financial Statements and Notes to Consolidated Financial Statements. Recently Adopted Accounting Pronouncements Standard Adoption ASU 2017-12, Derivatives and Hedging (Topic 815): Targeted Improvements to accounting for Hedging Activities NiSource elected to adopt this ASU effective September 30, 2017. As a result, NiSource is no longer required to separately measure and report hedge ineffectiveness. The guidance also eases the requirements related to ongoing hedge effectiveness assessments at NiSource. The adoption of this standard did not have a material impact on the Consolidated Financial Statements or Notes to Consolidated Financial Statements. Standard Adoption ASU 2017-09, Compensation-Stock Compensation (Topic 718): Scope of Modification Accounting NiSource elected to adopt this ASU effective July 1, 2017. The adoption of this standard did not have a material impact on the Consolidated Financial Statements or Notes to Consolidated Financial Statements. ASU 2017-07, Compensation - Retirement Benefits (Topic 715): Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost NiSource adopted this ASU effective January 1, 2018. Beginning with NiSource's Form 10-Q for the first quarter of 2018, NiSource will continue to present the service cost component of net periodic benefit cost within "Operation and maintenance"; however, other components of the net periodic benefit cost (including regulatory deferrals) will be presented separately within "Other, net" in the Statement of Consolidated Income. This change in income statement presentation will be implemented on a retrospective basis. Beginning prospectively on the date of adoption, only the service cost component of NiSource's net periodic benefit cost is eligible for capitalization as "Property, Plant and Equipment" on the Consolidated Balance Sheets. NiSource's regulated subsidiaries have adopted this ASU for regulatory reporting purposes. ASU 2017-04, Intangibles - Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment NiSource elected to adopt this ASU effective January 1, 2017. The adoption of this standard did not have a material impact on the Consolidated Financial Statements or Notes to Consolidated Financial Statements. ASU 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash (a consensus of the FASB Emerging Issues Task Force) NiSource elected to adopt this ASU effective October 1, 2017. Restricted cash on the Statements of Consolidated Cash Flows is no longer presented as an investing activity and is instead included as a component of beginning and ending cash balances. The adoption of this standard is reflected in the Statements of Consolidated Cash Flows beginning with NiSource's Annual Report on Form 10-K for the year ended December 31, 2017 (including all prior periods presented). ASU 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments (a consensus of the Emerging Issues Task Force) NiSource adopted this ASU effective January 1, 2018. The adoption of this standard did not have a material impact on the Consolidated Financial Statements or Notes to Consolidated Financial Statements. ASU 2016-12, Revenue from Contracts with Customers (Topic 606): Narrow-Scope Improvements and Practical Expedients NiSource adopted the provisions of ASC 606 beginning on January 1, 2018 using a modified retrospective method, which was applied to all contracts. No material adjustments were made to January 1, 2018 opening balances as a result of the adoption. During the process of implementation, NiSource first separated its various revenue streams into high-level categories, which served as the basis for accounting analysis and documentation as it related to the pronouncement's impact on NiSource's revenues. Substantially all of NiSource’s revenues are tariff based, which NiSource concluded are in the scope of ASC 606. NiSource has identified its performance obligations created under tariff-based sales as the commodity (natural gas or electric, which includes generation and capacity) and delivery. Under ASC 606, NiSource's revenue from such tariff based sales continues to be equivalent to the natural gas or electricity supplied and billed each period (including unbilled revenues), and the adoption of the standards did not result in a material shift in the amount or timing of revenue recognition for such sales. In addition, the pattern and amount of revenue recognized for the remaining NiSource revenue streams were not materially affected as a result of the adoption of ASC 606. NiSource has outlined footnote disclosures intended to satisfy ASC 606's disclosure requirements, which will enhance its disclosures on revenue recognition policies and elections. Beginning prospectively upon date of adoption, NiSource will include revenue disaggregated by customer class and by operating segment in its footnote disclosures. In addition, NiSource will separately disclose those revenues that are not in scope of ASC 606, such as revenue earned under ASC 980 Alternative Revenue Programs. As required under the modified retrospective method of adoption, results for reporting periods beginning after January 1, 2018 will be presented under ASC 606, while prior period amounts will not be adjusted and will continue to be reported in accordance with historic accounting guidance. ASU 2016-08, Revenue from Contracts with Customers (Topic 606): Principal versus Agent Considerations ASU 2014-09, Revenue from Contracts with Customers (Topic 606)

Discontinued Operations

Discontinued Operations	12 Months Ended
	Dec. 31, 2017
Discontinued Operations and Disposal Groups [Abstract]
Discontinued Operations	Discontinued Operations On July 1, 2015, NiSource completed the Separation through a special pro rata stock dividend, distributing one share of CPG common stock for every one share of NiSource common stock held by any NiSource stockholder on June 19, 2015, the record date. The Separation resulted in two stand-alone energy infrastructure companies: NiSource, a fully regulated natural gas and electric utilities company, and CPG, a natural gas pipeline, midstream and storage company. As a stand-alone company, on the date of the Separation, CPG's operations consisted of NiSource's Columbia Pipeline Group Operations segment prior to the Separation. Following the Separation, NiSource retained no ownership interest in CPG. On the date of the Separation, CPG consisted of approximately $9.2 billion of assets, $5.6 billion of liabilities and $3.6 billion of equity. The results of operations and cash flows for the former Columbia Pipeline Group Operations segment have been reported as discontinued operations for all periods presented. Income (loss) from discontinued operations were immaterial for 2017. During 2016, NiSource recorded a $3.6 million tax benefit resulting from favorable estimate-to-actual adjustments related to non-deductible costs from the Separation. There were no other material results from discontinued operations during 2016. Results from discontinued operations for 2015 are provided in the following table. These results are primarily from NiSource's former Columbia Pipeline Group Operations segment. Year Ended December 31, 2015 (in millions) Columbia Pipeline Group Operations Corporate and Other Total Operating Revenues Transportation and storage $ 561.4 $ — $ 561.4 Other 94.3 — 94.3 Total Operating Revenues 655.7 — 655.7 Operating Expenses Cost of sales (excluding depreciation and amortization) 0.2 — 0.2 Operation and maintenance 375.8 (1) — 375.8 Depreciation and amortization 66.4 — 66.4 Gain on sale of assets (13.6 ) — (13.6 ) Other taxes 38.0 — 38.0 Total Operating Expenses 466.8 — 466.8 Equity Earnings in Unconsolidated Affiliates 29.1 — 29.1 Operating Income from Discontinued Operations 218.0 — 218.0 Other Income (Deductions) Interest expense, net (37.1 ) — (37.1 ) Other, net 7.8 0.4 8.2 Total Other Income (Deductions) (29.3 ) 0.4 (28.9 ) Income from Discontinued Operations before Income Taxes 188.7 0.4 189.1 Income Taxes 84.7 0.9 85.6 Income (Loss) from Discontinued Operations - net of taxes $ 104.0 $ (0.5 ) $ 103.5 (1) Includes approximately $55.4 million of transaction costs related to the Separation. CPG’s financing requirements prior to the private placement of senior notes on May 22, 2015 were satisfied through borrowings from NiSource Finance. Interest expense from discontinued operations primarily represents net interest charged to CPG from NiSource Finance, less AFUDC. Subsequent to May 22, 2015, interest expense from discontinued operations also includes interest incurred on CPG’s private placement of $2,750.0 million of senior notes. Continuing Involvement Natural gas transportation and storage services provided to NiSource by CPG were $ 151.6 million , $150.5 million and $147.6 million for the years ended December 31, 2017, 2016 and 2015, respectively. Prior to July 1, 2015, these costs were eliminated in consolidation. Beginning July 1, 2015, these costs and associated cash flows represent third-party transactions with CPG and are not eliminated in consolidation, as such services have continued subsequent to the Separation and are expected to continue for the foreseeable future. There were no material assets and liabilities of discontinued operations on the Consolidated Balance Sheets at December 31, 2017 and 2016.

Earnings Per Share

Earnings Per Share	12 Months Ended
	Dec. 31, 2017
Earnings Per Share [Abstract]
Earnings Per Share	Earnings Per Share Basic EPS is computed by dividing net income attributable to NiSource by the weighted-average number of shares of common stock outstanding for the period. The weighted-average shares outstanding for diluted EPS includes the incremental effects of the various long-term incentive compensation plans. The calculation of diluted earnings per share excludes the impact of forward agreements (see Note 12 , "Common Stock") which had an anti-dilutive effect for the periods indicated. The computation of diluted average common shares is as follows: Year Ended December 31, (in thousands) 2017 2016 2015 Denominator Basic average common shares outstanding 329,388 321,805 317,746 Dilutive potential common shares: Shares contingently issuable under employee stock plans 547 165 — Shares restricted under stock plans 821 1,554 2,090 Diluted Average Common Shares 330,756 323,524 319,836

Property, Plant And Equipment

Property, Plant And Equipment	12 Months Ended
	Dec. 31, 2017
Property, Plant and Equipment [Abstract]
Property, Plant And Equipment	Property, Plant and Equipment NiSource’s property, plant and equipment on the Consolidated Balance Sheets are classified as follows: At December 31, (in millions) 2017 2016 Property, Plant and Equipment Gas Distribution Utility (1) $ 12,531.0 $ 11,556.6 Electric Utility (1) 7,403.8 7,043.3 Corporate 141.3 105.0 Construction Work in Process 950.5 663.1 Non-Utility and Other (2) 623.3 681.7 Total Property, Plant and Equipment $ 21,649.9 $ 20,049.7 Accumulated Depreciation and Amortization Gas Distribution Utility (1) $ (3,227.8 ) $ (3,119.2 ) Electric Utility (1) (3,673.2 ) (3,442.0 ) Corporate (52.6 ) (52.5 ) Non-Utility and Other (2) (336.8 ) (368.0 ) Total Accumulated Depreciation and Amortization $ (7,290.4 ) $ (6,981.7 ) Net Property, Plant and Equipment $ 14,359.5 $ 13,068.0 (1) NIPSCO’s common utility plant and associated accumulated depreciation and amortization are allocated between Gas Distribution Utility and Electric Utility Property, Plant and Equipment. (2) Non-Utility and Other as of December 31, 2017 includes net book value of $247.8 million related to Bailly Generating Station (Units 7 and 8) which was reclassified from Electric Utility in the fourth quarter of 2016. Depreciation expense for the remaining net book value will continue to be recorded at the composite depreciation rate most recently approved by the IURC. See Note 18 -E, "Other Matters," and Note 25, "Subsequent Event," for additional information. The weighted average depreciation provisions for utility plant, as a percentage of the original cost, for the periods ended December 31, 2017 , 2016 and 2015 were as follows: 2017 2016 2015 Electric Operations 3.4 % 3.3 % 3.1 % Gas Distribution Operations 2.1 % 2.1 % 2.0 % NiSource recognized depreciation expense of $ 501.5 million , $ 475.1 million and $ 449.0 million for the years ended 2017 , 2016 and 2015 , respectively. Amortization of Software Costs. NiSource amortized $44.0 million in 2017 , $41.4 million in 2016 and $41.1 million in 2015 related to software costs. NiSource’s unamortized software balance was $189.0 million and $156.4 million at December 31, 2017 and 2016 , respectively.

Goodwill And Other Intangible A

Goodwill And Other Intangible Assets	12 Months Ended
	Dec. 31, 2017
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill And Other Intangible Assets	Goodwill and Other Intangible Assets Goodwill. Substantially all of NiSource's goodwill relates to the excess of cost over the fair value of the net assets acquired in the Columbia acquisition on November 1, 2000. The following presents NiSource's goodwill balance allocated by segment as of December 31, 2017 : (in millions) Gas Distribution Operations Electric Operations Corporate and Other Total Goodwill $ 1,690.7 $ — $ — $ 1,690.7 NiSource applied the qualitative "step 0" analysis to its reporting units for the annual impairment test performed as of May 1, 2017. For this test, NiSource assessed various assumptions, events and circumstances that would have affected the estimated fair value of the reporting units as compared to its base line May 1, 2016 "step 1" fair value measurement. The results of this assessment indicated that it was not more likely than not that its reporting unit fair values were less than the reporting unit carrying values, accordingly, no "step 1" analysis was required. Intangible Assets. NiSource's intangible assets, apart from goodwill, consist of franchise rights. Franchise rights were identified as part of the purchase price allocations associated with the acquisition in February 1999 of Columbia of Massachusetts. These amounts were $231.7 million and $242.7 million , net of accumulated amortization of $210.5 million and $199.5 million , at December 31, 2017 and 2016 , respectively, and are being amortized on a straight-line basis over forty years from the date of acquisition through 2039. NiSource recorded amortization expense of $11.0 million in 2017 , 2016 , and 2015 related to its franchise right intangible asset.

Asset Retirement Obligations

Asset Retirement Obligations	12 Months Ended
	Dec. 31, 2017
Asset Retirement Obligation [Abstract]
Asset Retirement Obligations	Asset Retirement Obligations NiSource has recognized asset retirement obligations associated with various legal obligations including costs to remove and dispose of certain construction materials located within many of NiSource’s facilities, certain costs to retire pipeline, removal costs for certain underground storage tanks, removal of certain pipelines known to contain PCB contamination, closure costs for certain sites including ash ponds, solid waste management units and a landfill, as well as some other nominal asset retirement obligations. NiSource also has a significant obligation associated with the decommissioning of its two hydro facilities located in Indiana. These hydro facilities have an indeterminate life, and as such, no asset retirement obligation has been recorded. Changes in NiSource’s liability for asset retirement obligations for the years 2017 and 2016 are presented in the table below: (in millions) 2017 2016 Beginning Balance $ 262.6 $ 254.0 Accretion recorded as a regulatory asset/liability 10.3 9.2 Additions 2.4 — Settlements (15.6 ) (7.5 ) Change in estimated cash flows 9.0 (1) 6.9 (2) Ending Balance $ 268.7 $ 262.6 (1) The change in estimated cash flows for 2017 is primarily attributed to changes in estimated costs and settlement timing for electric generating stations and the changes in estimated costs for retirement of gas mains. (2) The change in estimated cash flows for 2016 is primarily attributed to the changes in estimated costs for retirement of gas mains partially offset by revisions to estimated costs associated with the EPA's final rule for regulation of CCRs and changes to cost estimates for certain solid waste management units. See Note 18-D, "Environmental Matters," for additional information on CCRs. Certain non-legal costs of removal that have been, and continue to be, included in depreciation rates and collected in the customer rates of the rate-regulated subsidiaries are classified as "Regulatory liabilities" on the Consolidated Balance Sheets.

Regulatory Matters

Regulatory Matters	12 Months Ended
	Dec. 31, 2017
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory Matters	Regulatory Matters Regulatory Assets and Liabilities NiSource follows the accounting and reporting requirements of ASC Topic 980, which provides that regulated entities account for and report assets and liabilities consistent with the economic effect of regulatory rate-making procedures if the rates established are designed to recover the costs of providing the regulated service and it is probable that such rates can be charged and collected from customers. Certain expenses and credits subject to utility regulation or rate determination normally reflected in income or expense are deferred on the balance sheet and are recognized in the income statement as the related amounts are included in customer rates and recovered from or refunded to customers. Regulatory assets were comprised of the following items: At December 31, (in millions) 2017 2016 Regulatory Assets Unrecognized pension and other postretirement benefit costs (see Note 11) $ 733.5 $ 847.5 Deferred pension and other postretirement benefit costs (see Note 11) 70.7 59.6 Environmental costs (see Note 18-D) 63.4 62.6 Regulatory effects of accounting for income taxes (see Note 1-O and Note 10) 238.8 238.4 Underrecovered gas and fuel costs (see Note 1-K) 25.5 73.5 Depreciation 181.0 187.1 Post-in-service carrying charges 173.3 142.0 Safety activity costs 66.5 41.5 DSM programs 40.0 48.4 Other 208.5 184.8 Total Regulatory Assets $ 1,801.2 $ 1,885.4 Regulatory liabilities were comprised of the following items: At December 31, (in millions) 2017 2016 Regulatory Liabilities Overrecovered gas and fuel costs (see Note 1-K) $ 27.6 $ 54.8 Cost of removal (see Note 7) 1,096.8 1,174.5 Regulatory effects of accounting for income taxes (see Note 1-O and Note 10) 1,563.4 30.0 Deferred pension and other postretirement benefit costs (see Note 11) 59.0 41.2 Other 48.8 81.3 Total Regulatory Liabilities $ 2,795.6 $ 1,381.8 Regulatory assets, including underrecovered gas and fuel cost, of approximately $1,558.4 million as of December 31, 2017 are not earning a return on investment. Regulatory assets of approximately $1,402.8 million include expenses that are recovered as components of the cost of service and are covered by regulatory orders. These costs are recovered over a remaining life of up to 41 years. Regulatory assets of approximately $398.4 million at December 31, 2017 , require specific rate action. Assets: Unrecognized pension and other postretirement benefit costs. In 2007, NiSource adopted certain updates of ASC 715 which required, among other things, the recognition in other comprehensive income or loss of the actuarial gains or losses and the prior service costs or credits that arise during the period but that are not immediately recognized as components of net periodic benefit costs. Certain subsidiaries defer these gains or losses as a regulatory asset in accordance with regulatory orders or as a result of regulatory precedent, to be recovered through base rates. Deferred pension and other postretirement benefit costs. Primarily relates to the difference between postretirement expense recorded by certain subsidiaries due to regulatory orders and the postretirement expense recorded in accordance with GAAP. These costs are expected to be collected through future base rates, revenue riders or tracking mechanisms. Environmental costs. Includes certain recoverable costs of investigating, testing, remediating and other costs related to gas plant sites, disposal sites or other sites onto which material may have migrated. Certain companies defer the costs as a regulatory asset in accordance with regulatory orders, to be recovered in future base rates, billing riders or tracking mechanisms. Regulatory effects of accounting for income taxes. Represents the deferral and under collection of deferred taxes in the rate making process. In prior years, NiSource has lowered customer rates in certain jurisdictions for the benefits of accelerated tax deductions. Amounts are expensed for financial reporting purposes as NiSource recovers deferred taxes in the rate making process. Underrecovered gas and fuel costs. Represents the difference between the costs of gas and fuel and the recovery of such costs in revenue and is used to adjust future billings for such deferrals on a basis consistent with applicable state-approved tariff provisions. Recovery of these costs is achieved through tracking mechanisms. Depreciation. Represents differences between depreciation expense incurred on a GAAP basis and that prescribed through regulatory order. Significant components of this balance include: • Columbia of Ohio depreciation rates . Prior to 2005, the PUCO-approved depreciation rates for ratemaking had been lower than those which would have been utilized if Columbia of Ohio were not subject to regulation resulting in the creation of a regulatory asset. In 2005, the PUCO authorized Columbia of Ohio to revise its depreciation accrual rates for the period beginning January 1, 2005. The revised depreciation rates are now higher than those which would have been utilized if Columbia of Ohio were not subject to regulation allowing for amortization of the previously created regulatory asset. The amount of depreciation that would have been recorded from 2005 through 2017 had Columbia of Ohio not been subject to rate regulation is a cumulative $719.7 million , $82.3 million less than that reflected in rates. The resulting regulatory asset balance was $49.3 million and $57.6 million as of December 31, 2017 and 2016 , respectively. • Columbia of Ohio IRP and CEP. Columbia of Ohio also has PUCO approval to defer depreciation and debt-based post-in-service carrying charges (see " Post-in-service carrying charges" below) associated with its IRP and CEP. As of December 31, 2017 , depreciation of $26.5 million and $49.8 million was deferred for the respective programs. Depreciation deferral balances for the respective programs as of December 31, 2016 were $ 23.4 million and $ 31.8 million . Recovery of the IRP depreciation is approved annually through the IRP rider. The equivalent of annual depreciation expense, based on the average life of the related assets, is included in the calculation of the IRP rider approved by the PUCO and billed to customers. Deferred depreciation expense is recognized as the IRP rider is billed to customers. The recovery mechanism for depreciation associated with the CEP will be addressed in a separate rate proceeding as discussed below. • NIPSCO EERM. NIPSCO obtained approval from the IURC to recover certain environmental related costs including operation and maintenance and depreciation expense once the environmental facilities become operational. Recovery of these costs will continue until such assets are included in rate base through an electric base rate case. The EERM deferred charges represent expenses that will be recovered from customers through an annual EERM Cost Tracker (ECT) which authorizes the collection of deferred balances over a six month period. Depreciation of $ 13.9 million and $ 40.7 million was deferred to a regulatory asset as of December 31, 2017 and 2016, respectively. • NIPSCO TDSIC. NIPSCO obtained approval from the IURC to recover costs for certain system modernization projects outside of a base rate proceeding. Eighty percent of the related costs, including depreciation, property taxes, and debt and equity based carrying charges (see Post-in-service carrying charges below) are recovered through a semi-annual recovery mechanism. Recovery of these costs will continue until such assets are included in rate base through a gas or electric base rate case, respectively. The remaining twenty percent of the costs are deferred until the next base rate case. As of December 31, 2017 and 2016, depreciation of $ 10.3 million and $ 5.5 million , respectively, was deferred as a regulatory asset. Post-in-service carrying charges. Represents deferred debt-based carrying charges incurred on certain assets placed into service but not yet included in customer rates. This balance includes: • Columbia of Ohio IRP and CEP. See description of IRP and CEP programs above under the heading " Depreciation ." As of December 31, 2017 and 2016, Columbia of Ohio had deferred PISCC of $ 164.6 million and $ 134.9 million , respectively. • NIPSCO TDSIC. See description of TDSIC program above under the heading " Depreciation ." Deferral of equity-based carrying charges for the TDSIC program is allowed, however such amounts are not reflected in regulatory asset balances for financial reporting as equity-based returns do not meet the definition of incurred costs under ASC 980. As of December 31, 2017 and 2016, NIPSCO had deferred PISCC of $ 8.7 million and $ 7.1 million , respectively. Safety activity costs. Represents the difference between costs incurred in eligible safety programs in excess of those being recovered in rates. The eligible cost deferrals represent necessary business expenses incurred in compliance with PHMSA regulations and are targeted to enhance the safety of the pipeline systems. Certain subsidiaries defer the excess costs as a regulatory asset in accordance with regulatory orders and recovery of these costs will be address in future base rate proceedings. DSM programs. Represents costs associated with Gas Distribution Operations and Electric Operations segments' energy efficiency and conservation programs. Costs are recovered through tracking mechanisms. Liabilities: Overrecovered gas and fuel costs. Represents the difference between the cost of gas and fuel and the recovery of such costs in revenues, and is the basis to adjust future billings for such refunds on a basis consistent with applicable state-approved tariff provisions. Refunding of these revenues is achieved through tracking mechanisms. Cost of removal. Represents anticipated costs of removal that have been, and continue to be, included in depreciation rates and collected in customer rates of the rate-regulated subsidiaries for future costs to be incurred. Regulatory effects of accounting for income taxes. Represents amounts owed to customers for deferred taxes collected at a higher rate than the current statutory rates and liabilities associated with accelerated tax deductions owed to customers that are established during the rate making process. Balance includes excess deferred taxes recorded upon implementation of the TCJA in December 2017. Deferred pension and other postretirement benefit costs. Primarily represents cash contributions in excess of postretirement benefit expense that is deferred as a regulatory liability by certain subsidiaries in accordance with regulatory orders. Gas Distribution Operations Regulatory Matters Cost Recovery and Trackers . Comparability of Gas Distribution Operations line item operating results is impacted by regulatory trackers that allow for the recovery in rates of certain costs such as those described below. Increases in the expenses that are the subject of trackers result in a corresponding increase in operating revenues and therefore have essentially no impact on total operating income results. Certain operating costs of the NiSource distribution companies are significant, recurring in nature, and generally outside the control of the distribution companies. Some states allow the recovery of such costs through cost tracking mechanisms. Such tracking mechanisms allow for abbreviated regulatory proceedings in order for the distribution companies to implement charges and recover appropriate costs. Tracking mechanisms allow for more timely recovery of such costs as compared with more traditional cost recovery mechanisms. Examples of such mechanisms include GCR adjustment mechanisms, tax riders, and bad debt recovery mechanisms. A portion of the distribution companies' revenue is related to the recovery of gas costs, the review and recovery of which occurs through standard regulatory proceedings. All states in NiSource's operating area require periodic review of actual gas procurement activity to determine prudence and to permit the recovery of prudently incurred costs related to the supply of gas for customers. NiSource distribution companies have historically been found prudent in the procurement of gas supplies to serve customers. Certain of the NiSource distribution companies have completed rate proceedings involving infrastructure replacement or are embarking upon regulatory initiatives to replace significant portions of their operating systems that are nearing the end of their useful lives. Each LDC's approach to cost recovery may be unique, given the different laws, regulations and precedent that exist in each jurisdiction. Columbia of Ohio. On November 28, 2012, the PUCO approved Columbia of Ohio’s application to extend its IRP for an additional five years (2013-2017), allowing Columbia of Ohio to continue to invest and recover on its accelerated main replacements. Columbia of Ohio filed its most recent application to adjust rates associated with its IRP and DSM Riders on February 27, 2017, which requested authority to increase annual revenues by approximately $31.5 million that includes recovery of and return on approximately $ 235.9 million of incremental IRP capital investments in 2016. On March 23, 2017, the PUCO Staff filed comments which recommended approval of the application with only minor revisions. The PUCO issued an order on April 26, 2017, approving Columbia of Ohio's application. New rates went into effect on May 1, 2017. On February 27, 2017, Columbia of Ohio also filed an application requesting authority to extend its IRP for an additional five years (2018-2022). On July 10, 2017, the PUCO Staff recommended approval of Columbia of Ohio's IRP for the additional five years, with modifications to Columbia of Ohio's proposed IRP rates for the five-year period. A joint stipulation and recommendation, outlining annual maximum IRP rates for the five-year period, was filed on August 18, 2017 and was supported or not opposed by all parties except the OCC. A hearing on the stipulation was held on October 2, 2017 and briefing was completed on November 7, 2017. On January 31, 2018, the PUCO issued an order that approved the stipulation. On December 1, 2017, Columbia of Ohio filed an application that requested authority to implement a rider to begin recovering plant and associated deferrals related to the CEP. The application requested authority to increase annual revenues, through the requested rider, by approximately $ 29 million in 2018, with biennial increases up to approximately $ 98 million in 2022. The filing is pending at the PUCO and no procedural schedule has been established. The CEP was established in 2011 and allows for deferral of interest, depreciation and property taxes on certain plant investments not recovered through its IRP modernization tracker. NIPSCO Gas. On September 27, 2017, NIPSCO filed a base rate case with the IURC, seeking an annual revenue increase of $ 143.5 million (inclusive of amounts being recovered through various tracker programs). As part of this filing and among other items, NIPSCO proposed to update base rates for ongoing infrastructure improvements, revised depreciation rates and ongoing level of expenses to reflect the current costs of providing natural gas service. An order is expected in the second half of 2018. A supplemental filing to the base rate case was submitted on January 26, 2018 to reflect the impact of the TCJA, seeking a revised annual revenue increase of $ 117.9 million . On April 30, 2013, then Indiana Governor Pence signed Senate Enrolled Act 560, the TDSIC statute, into law. Among other provisions, this legislation provides for cost recovery outside of a base rate proceeding for new or replacement electric and gas transmission, distribution, and storage projects that a public utility undertakes for the purposes of safety, reliability, system modernization, or economic development. Provisions of the TDSIC statute require that, among other things, requests for recovery include a seven-year plan of eligible investments. Once the plan is approved by the IURC, eighty percent of eligible costs can be recovered using a periodic rate adjustment mechanism. The cost recovery mechanism is referred to as a TDSIC mechanism. Recoverable costs include a return on, and of, the investment, including AFUDC, post-in-service carrying charges, operation and maintenance expenses, depreciation and property taxes. The remaining twenty percent of recoverable costs are to be deferred for future recovery in the public utility’s next general rate case. The periodic rate adjustment mechanism is capped at an annual increase of no more than two percent of total retail revenues. On August 31, 2017, NIPSCO filed TDSIC-7 requesting to recover an incremental increase to revenue of $ 3.5 million associated with incremental capital investment of $ 59.0 million made in the first half of 2017. An order approving NIPSCO's filing was received from the IURC on December 28, 2017, and new rates went into effect on January 1, 2018. On November 8, 2017, NIPSCO filed a petition with the IURC seeking approval of NIPSCO’s federally mandated pipeline safety compliance plan. The four year compliance plan includes a total estimated $91 million of capital costs and $23 million of expected operating and maintenance costs. NIPSCO is requesting all associated accounting and ratemaking relief, including establishment of a periodic rate adjustment mechanism. Columbia of Massachusetts. On July 7, 2014, the Governor of Massachusetts signed into law Chapter 149 of the Acts of 2014, An Act Relative to Natural Gas Leaks (“the Act”). The Act authorizes natural gas distribution companies to file gas infrastructure replacement plans with the Massachusetts DPU to address the replacement of aging natural gas pipeline infrastructure. In addition, the Act provides that the Massachusetts DPU may, after review of the plans, allow the proposed estimated costs of the plan into rates as of May 1 of the subsequent year. On October 31, 2016, Columbia of Massachusetts filed its GSEP for the 2017 construction year. Columbia of Massachusetts proposed to recover incremental revenue of $ 8.1 million associated with incremental capital investment of $ 72.9 million made during calendar year 2017. An order was received from the Massachusetts DPU on April 28, 2017 approving the filing and rates went into effect on May 1, 2017. On October 31, 2017, Columbia of Massachusetts filed its GSEP for the 2018 construction year. Columbia of Massachusetts is proposing to recover incremental revenue of $ 9.7 million associated with incremental capital investment of $ 83.9 million to be made during calendar year 2018. The filing included a request for a waiver to allow collection of the $ 3.1 million revenue requirement that exceeds the GSEP cap provision as previously calculated. If the waiver is not approved, the incremental revenue will be $ 6.6 million . An order is expected from the Massachusetts DPU in the second quarter of 2018, with new rates effective May 1, 2018. Columbia of Virginia. On April 29, 2016, Columbia of Virginia filed a request with the VSCC, seeking an annual revenue increase of $ 37.0 million . On September 28, 2016, Columbia of Virginia implemented updated interim base rates subject to refund. On January 17, 2017, Columbia of Virginia presented a stipulation and proposed recommendation, representing a settlement by all parties to the proceeding that included a base revenue increase of $ 28.5 million . On March 17, 2017, by final order, the VSCC approved the settlement agreement without modification. In accordance with the terms of the final order, during 2017, Columbia of Virginia completed its refund of the difference between the interim customer rates implemented in 2016 and the rates approved by the final order. Columbia Gas of Kentucky. On October 13, 2017, Columbia of Kentucky filed its application to adjust rates associated with its AMRP, requesting authority to increase annual revenues by $ 4.5 million associated with incremental capital investment of $ 24.0 million to be made during calendar year 2018. On December 22, 2017, the Kentucky PSC issued an order approving Columbia of Kentucky’s request as filed, with rates effective January 2, 2018. Columbia of Maryland. On April 14, 2017, Columbia of Maryland filed a request with the MPSC to adjust base rates. On July 28, 2017, all parties filed a settlement agreement with the MPSC, under which Columbia of Maryland will receive an annual revenue increase of $ 2.4 million . The MPSC approved the settlement on September 19, 2017 and rates went into effect on October 27, 2017. Electric Operations Regulatory Matters Cost Recovery and Trackers . Comparability of Electric Operations line item operating results is impacted by regulatory trackers that allow for the recovery in rates of certain costs such as those described below. Increases in the expenses that are the subject of trackers result in a corresponding increase in operating revenues and therefore have essentially no impact on total operating income results. Certain operating costs of the Electric Operations are significant, recurring in nature, and generally outside the control of NIPSCO. The IURC allows for recovery of such costs through cost tracking mechanisms. Such tracking mechanisms allow for abbreviated regulatory proceedings in order for NIPSCO to implement charges and recover appropriate costs. Tracking mechanisms allow for more timely recovery of such costs as compared with more traditional cost recovery mechanisms. Examples of such mechanisms include electric energy efficiency programs, MISO non-fuel costs and revenues, resource capacity charges, federally mandated costs and environmental related costs. A portion of NIPSCO's revenue is related to the recovery of fuel costs to generate power and the fuel costs related to purchased power. These costs are recovered through a FAC, a quarterly regulatory proceeding in Indiana. NIPSCO made a TDSIC-2 rate adjustment mechanism filing on June 30, 2017 requesting revenues of $ 12.8 million to be billed over eight months, associated with $ 133.6 million of incremental capital expenditures from May 2016 through April 2017. An order approving the request was received from the IURC on October 31, 2017 and new rates went into effect with the first billing cycle of November 2017. NIPSCO made a TDSIC-3 rate adjustment mechanism filing on January 30, 2018 requesting a revenue decrease of $ 2.0 million to be billed over six months, associated with $ 75.0 million of incremental capital expenditures made from May 1, 2017 to November 30, 2017. This decreased revenue request reflects impacts of the TCJA. An order approving the request is expected in May 2018 with new rates expected to go into effect with the first billing cycle of June 2018. On November 1, 2016, NIPSCO filed a petition with the IURC for relief regarding the construction of additional environmental projects required to comply with the final rules for regulation of CCRs and the ELG. On June 9, 2017, a settlement agreement was filed with the IURC regarding the CCR projects and treatment of associated costs. An order approving the settlement agreement was received on December 13, 2017. Given the current postponement of the ELG rule, NIPSCO has agreed, with the settling parties, that the ELG projects and related costs would be addressed in a later proceeding. Refer to Note 18-D, “Environmental Matters,” for more information. Regulatory Impacts of the TCJA Since the passage of the TCJA, several of the public utility commissions in NiSource’s operating area have issued orders to examine the impact of the TCJA on rates charged by regulated utilities. The requirements in each jurisdiction vary but all will assess the appropriate pass back of excess deferred taxes and the need for reductions to current rates resulting from the decrease in the corporate tax rate. NiSource has implemented the requirements of these orders by, among other things, recognizing a regulatory liability for the expected impacts of the TCJA. See Note 10, “Income Taxes,” for additional information on the impacts of the implementation of the TCJA.

Risk Management Activities

Risk Management Activities	12 Months Ended
	Dec. 31, 2017
Risk Management Activities [Abstract]
Risk Management Activities	Risk Management Activities NiSource is exposed to certain risks relating to its ongoing business operations; namely commodity price risk and interest rate risk. NiSource recognizes that the prudent and selective use of derivatives may help to lower its cost of debt capital, manage its interest rate exposure and limit volatility in the price of natural gas. Risk management assets and liabilities on NiSource’s derivatives are presented on the Consolidated Balance Sheets as shown below: December 31, (in millions) 2017 2016 Risk Management Assets - Current (1) Interest rate risk programs $ 14.0 $ 17.0 Commodity price risk programs 0.5 7.4 Total $ 14.5 $ 24.4 Risk Management Assets - Noncurrent (2) Interest rate risk programs $ 5.6 $ 17.1 Commodity price risk programs 1.0 7.5 Total $ 6.6 $ 24.6 Risk Management Liabilities - Current Interest rate risk programs $ 38.6 $ 15.3 Commodity price risk programs 4.6 1.5 Total $ 43.2 $ 16.8 Risk Management Liabilities - Noncurrent Interest rate risk programs $ — $ 24.5 Commodity price risk programs 28.5 20.0 Total $ 28.5 $ 44.5 (1) Presented in "Prepayments and other" on the Consolidated Balance Sheets. (2) Presented in "Deferred charges and other" on the Consolidated Balance Sheets. Commodity Price Risk Management NiSource and NiSource’s utility customers are exposed to variability in cash flows associated with natural gas purchases and volatility in natural gas prices. NiSource purchases natural gas for sale and delivery to its retail, commercial and industrial customers, and for most customers the variability in the market price of gas is passed through in their rates. Some of NiSource’s utility subsidiaries offer programs whereby variability in the market price of gas is assumed by the respective utility. The objective of NiSource’s commodity price risk programs is to mitigate the gas cost variability, for NiSource or on behalf of its customers, associated with natural gas purchases or sales by economically hedging the various gas cost components using a combination of futures, options, forwards or other derivative contracts. NIPSCO received IURC approval to lock in a fixed price for its natural gas customers using long-term forward purchase instruments. In 2017 and 2016, the term of these instruments ranged from five to ten years and was limited to ten percent of NIPSCO’s average annual GCA purchase volume. During 2017, NIPSCO received IURC approval to increase the limit to twenty percent of NIPSCO's average annual GCA purchase volume in 2018 and 2019. Gains and losses on these derivative contracts are deferred as regulatory liabilities or assets and are remitted to or collected from customers through NIPSCO’s quarterly GCA mechanism. These instruments are not designated as accounting hedges. Interest Rate Risk Management As of December 31, 2017 , NiSource has forward-starting interest rate swaps with an aggregate notional value totaling $ 1.0 billion to hedge the variability in cash flows attributable to changes in the benchmark interest rate during the periods from the effective dates of the swaps to the anticipated dates of forecasted debt issuances, which are expected to take place by the end of 2019. These interest rate swaps are designated as cash flow hedges. The effective portions of the gains and losses related to these swaps are recorded to AOCI and are recognized in earnings concurrently with the recognition of interest expense on the associated debt, once issued. If it becomes probable that a hedged forecasted transaction will no longer occur, the accumulated gains or losses on the derivative will be recognized currently in earnings. On May 11, 2017, NiSource Finance settled $ 950.0 million of forward-starting interest rate swap agreements contemporaneously with the issuance of $ 2.0 billion of 3.49% and 4.375% senior notes, maturing in 2027 and 2047, respectively. These derivative contracts were accounted for as cash flow hedges. As part of the transaction, the associated net unrealized loss position of $ 6.9 million is being amortized from accumulated other comprehensive loss into interest expense over the term of the associated interest payments. On September 5, 2017, NiSource Finance settled $ 750.0 million of treasury lock agreements contemporaneously with the issuance of $ 750.0 million of 3.95% senior notes, maturing in 2048. These derivative contracts were accounted for as cash flow hedges. As part of the transaction, the associated net unrealized loss position of $ 19.0 million is being amortized from accumulated other comprehensive loss into interest expense over the term of the associated interest payments. On November 8, 2017, NiSource Finance settled $250.0 million of treasury lock agreements contemporaneously with the issuance of $500.0 million of 2.65% senior notes, maturing in 2022. These derivative contracts were accounted for as a cash flow hedges. NiSource Finance recognized an immaterial gain associated with this transaction. Cash associated with payments to settle interest rate swaps and treasury lock agreements are reflected within operating activities within the Statements of Consolidated Cash Flows for the year ended December 31, 2017 . Realized gains and losses from NiSource’s interest rate cash flow hedges are presented in “Interest expense, net” on the Statements of Consolidated Income. There were no amounts excluded from effectiveness testing for derivatives in cash flow hedging relationships at December 31, 2017 , 2016 and 2015. NiSource’s derivative instruments measured at fair value as of December 31, 2017 and 2016 do not contain any credit-risk-related contingent features.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes On December 22, 2017, the President signed into law the TCJA, which, among other things, enacted significant changes to the Internal Revenue Code of 1986, as amended, including a reduction in the maximum U.S. federal corporate income tax rate from 35% to 21% , and certain other provisions related specifically to the public utility industry, including the continuation of certain interest expense deductibility. These changes are effective January 1, 2018. Under GAAP, the effects of a change in tax law are recorded as a discrete item in the period of enactment. Rates for NiSource’s regulated customers include provisions for the collection of U.S. federal income taxes. Accordingly, accounting effects related to changes in tax rates at NiSource that would normally be recognized as a component of income tax expense may instead be deferred as a regulatory asset or liability and reflected in future ratemaking. In December 2017, NiSource remeasured its deferred tax assets and liabilities to the new federal corporate income tax rate. The result of this remeasurement was a reduction in the net deferred tax liability of approximately $1.3 billion , including approximately $0.4 billion of regulatory "gross up" to account for over-collection of past taxes from customers. Offsetting the reduction in net deferred tax liabilities was an increase in regulatory liabilities of approximately $1.5 billion and an increase in income tax expense of $0.2 billion . These changes are discussed in further detail below. On December 22, 2017, the SEC issued Staff Accounting Bulletin 118 (“SAB 118”), which provides guidance on accounting for tax effects of the TCJA. SAB 118 provides a measurement period that should not extend beyond one year from the TCJA enactment date for companies to complete the accounting under ASC 740. In accordance with SAB 118, a company must reflect the income tax effects of those aspects of the TCJA for which the accounting under ASC 740 is complete. To the extent that a company’s accounting for certain income tax effects of the TCJA is incomplete but it is able to determine a reasonable estimate, it must record a provisional estimate to be included in the financial statements. If a company cannot determine a provisional estimate to be included in the financial statements, it should continue to apply ASC 740 on the basis of the provision of the tax laws that were in effect immediately before the enactment of the TCJA. While NiSource was able to make reasonable estimates of the impact of the reduction in corporate rate on our net deferred income tax liability balances, the final impact of the TCJA may differ from these estimates, due to, among other things, changes in NiSource's interpretations and assumptions, additional guidance that may be issued by the IRS, and actions NiSource may take. NiSource is continuing to gather additional information to determine the final impact. The components of income tax expense (benefit) were as follows: Year Ended December 31, (in millions) 2017 2016 2015 Income Taxes Current Federal $ — $ — $ — State 7.8 (0.1 ) 6.0 Total Current 7.8 (0.1 ) 6.0 Deferred Federal 302.7 165.6 124.1 State 5.0 18.0 13.6 Total Deferred 307.7 183.6 137.7 Deferred Investment Credits (1.0 ) (1.4 ) (2.4 ) Income Taxes from Continuing Operations $ 314.5 $ 182.1 $ 141.3 Total income taxes from continuing operations were different from the amount that would be computed by applying the statutory federal income tax rate to book income before income tax. The major reasons for this difference were as follows: Year Ended December 31, (in millions) 2017 2016 2015 Book income from Continuing Operations before income taxes $ 443.1 $ 510.2 $ 339.9 Tax expense at statutory Federal income tax rate 155.0 35.0 % 178.6 35.0 % 118.9 35.0 % Increases (reductions) in taxes resulting from: State income taxes, net of Federal income tax benefit 6.9 1.5 11.3 2.2 14.8 4.4 Property and plant (including accelerated depreciation) (2.4 ) (0.5 ) (1.5 ) (0.3 ) (1.6 ) (0.4 ) Charitable contribution carryover (1.2 ) (0.3 ) 2.8 0.5 17.8 5.2 Remeasurement due to TCJA 161.1 36.4 — — — — Employee stock ownership plan dividends and other compensation (6.5 ) (1.5 ) (9.5 ) (1.9 ) (2.9 ) (0.9 ) Tax accrual adjustments and other, net 1.6 0.4 0.4 0.2 (5.7 ) (1.7 ) Income Taxes from Continuing Operations $ 314.5 71.0 % $ 182.1 35.7 % $ 141.3 41.6 % The effective income tax rates were 71.0% , 35.7% and 41.6% in 2017 , 2016 and 2015 , respectively. The 35.3% increase in the overall effective tax rate in 2017 versus 2016 was primarily the result of a $161.1 million increase in income taxes related to implementing the provisions of the TCJA. The charge to income tax expense resulting from implementation of the TCJA relates primarily to remeasurement of parent company deferred tax assets for NOL carryforwards. The 5.9% decrease in the overall effective tax rate in 2016 versus 2015 was primarily the result of a $7.2 million decrease in income taxes related to Federal tax benefits on stock compensation and the absence of $15.0 million of lost Federal tax benefit primarily related to charitable contribution carryforward adjustments recorded in the prior year. In March 2016, the FASB issued ASU 2016-09, Compensation-Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting. Among other provisions, the standard requires that all income tax effects of awards are recognized in the income statement when the awards vest and are distributed. On December 18, 2015, the President signed into law the PATH. PATH, among other provisions, extended and modified bonus depreciation through 2019. As a result of PATH and 50% bonus depreciation being extended, NiSource recorded tax expense of $5.8 million in 2015 for the expiration of unused charitable contribution carryforwards which expired due to the 5 year carryover limitation. NiSource also recorded a valuation allowance for an additional $12.0 million of charitable contribution carryforwards that are set to expire in 2016-2019 in the event that NiSource does not have sufficient taxable income to utilize the carryforward amounts. Deferred income taxes result from temporary differences between the financial statement carrying amounts and the tax basis of existing assets and liabilities. The principal components of NiSource’s net deferred tax liability were as follows: At December 31, (in millions) 2017 2016 Deferred tax liabilities Accelerated depreciation and other property-related differences $ 2,260.7 $ 3,323.5 Unrecovered gas and fuel costs — 25.9 Other regulatory assets 309.5 449.2 Total Deferred Tax Liabilities 2,570.2 3,798.6 Deferred tax assets Other regulatory liabilities including impact of TCJA 406.0 93.1 Pension and other postretirement/postemployment benefits 136.7 261.7 Net operating loss carryforward and Alternative Minimum Tax credit carryforward 576.0 646.2 Environmental liabilities 24.0 47.0 Other accrued liabilities 37.2 45.5 Other, net 97.4 177.1 Total Deferred Tax Assets 1,277.3 1,270.6 Net Deferred Tax Liabilities $ 1,292.9 $ 2,528.0 State income tax net operating loss benefits are recorded at their realizable value. NiSource anticipates it is more likely than not that it will realize $65.8 million and $43.6 million of these tax benefits as of December 31, 2017 and 2016 , respectively, prior to their expiration. These tax benefits are primarily related to Indiana and Pennsylvania. The carryforward periods for these tax benefits expire in various tax years from 2028 to 2037 . The remaining net operating loss carryforward tax benefit represents a Federal carryforward of $508.5 million that will expire in 2037 and an Alternative Minimum Tax credit of $1.7 million that will carry forward indefinitely. Unrecognized tax benefits for the periods reported are immaterial. NiSource recognizes accrued interest on unrecognized tax benefits, accrued interest on other income tax liabilities and tax penalties in income tax expense. Interest expense recorded on unrecognized tax benefits and other income tax liabilities was immaterial for all periods presented. There were no accruals for penalties recorded in the Statements of Consolidated Income for the years ended December 31, 2017 , 2016 and 2015 , and there were no balances for accrued penalties recorded on the Consolidated Balance Sheets as of December 31, 2017 and 2016 . NiSource is subject to income taxation in the United States and various state jurisdictions, primarily Indiana, Pennsylvania, Kentucky, Massachusetts, Maryland and Virginia. Because NiSource is part of the IRS’s Large and Mid-Size Business program, each year’s federal income tax return is typically audited by the IRS. As of December 31, 2017 , tax years through 2016 have been audited and are effectively closed to further assessment. The audit of tax year 2017 under the CAP program is expected to be completed in 2018. NiSource has been accepted into the CAP maintenance program for the audit of tax year 2018. The statute of limitations in each of the state jurisdictions in which NiSource operates remains open until the years are settled for federal income tax purposes, at which time amended state income tax returns reflecting all federal income tax adjustments are filed. As of December 31, 2017 , there were no state income tax audits in progress that would have a material impact on the consolidated financial statements.

Pension And Other Postretiremen

Pension And Other Postretirement Benefits	12 Months Ended
	Dec. 31, 2017
Pension and Other Postretirement Benefits Cost (Reversal of Cost) [Abstract]
Pension and Other Postretirement Benefits	Pension and Other Postretirement Benefits NiSource provides defined contribution plans and noncontributory defined benefit retirement plans that cover certain of its employees. Benefits under the defined benefit retirement plans reflect the employees’ compensation, years of service and age at retirement. Additionally, NiSource provides health care and life insurance benefits for certain retired employees. The majority of employees may become eligible for these benefits if they reach retirement age while working for NiSource. The expected cost of such benefits is accrued during the employees’ years of service. Current rates of rate-regulated companies include postretirement benefit costs, including amortization of the regulatory assets that arose prior to inclusion of these costs in rates. For most plans, cash contributions are remitted to grantor trusts. NiSource Pension and Other Postretirement Benefit Plans’ Asset Management . NiSource employs a liability-driven investing strategy for the pension plan, as noted below. While the majority of assets continue in a total return investment approach, a glide path has been implemented. A mix of equities and fixed income investments are used to maximize the long-term return of plan assets and hedge the liabilities at a prudent level of risk. NiSource utilizes a total return investment approach for the other postretirement benefit plans. Risk tolerance is established through careful consideration of plan liabilities, plan funded status, and asset class volatility. The investment portfolio contains a diversified blend of equity and fixed income investments. Furthermore, equity investments are diversified across U.S. and non-U.S. stocks, as well as growth, value, small and large capitalizations. Other assets such as private equity funds are used judiciously to enhance long-term returns while improving portfolio diversification. Derivatives may be used to gain market exposure in an efficient and timely manner; however, derivatives may not be used to leverage the portfolio beyond the market value of the underlying assets. Investment risk is measured and monitored on an ongoing basis through quarterly investment portfolio reviews, annual liability measurements, and periodic asset/liability studies. NiSource utilizes a building block approach with proper consideration of diversification and rebalancing in determining the long-term rate of return for plan assets. Historical markets are studied and long-term historical relationships between equities and fixed income are analyzed to ensure that they are consistent with the widely accepted capital market principle that assets with higher volatility generate greater return over the long run. Current market factors, such as inflation and interest rates, are evaluated before long-term capital market assumptions are determined. Peer data and historical returns are reviewed to check for reasonability and appropriateness. The most important component of an investment strategy is the portfolio asset mix, or the allocation between the various classes of securities available to the pension and other postretirement benefit plans for investment purposes. The asset mix and acceptable minimum and maximum ranges established for the NiSource plan assets represents a long-term view and are listed in the table below. In 2012, a dynamic asset allocation policy for the pension fund was approved. This policy calls for a gradual reduction in the allocation of return-seeking assets (equities, real estate and private equity) and a corresponding increase in the allocation of liability-hedging assets (fixed income) as the funded status of the plans increase above 90% (as measured by the market value of qualified pension plan assets divided by the projected benefit obligations of the qualified pension plans). In 2016, a study was conducted and approved resulting in the addition of new asset classes in the return-seeking portfolio allocation (core real estate, diversified credit) and a shift in the hedging allocation (fixed income). Planned implementation of the new asset classes began in 2017. During 2017, a $277 million discretionary contribution was made and further implementation of new asset classes is under review while a new asset-liability study is completed. As of December 31, 2017, the asset mix and acceptable minimum and maximum ranges established by the policy for the pension and other postretirement benefit plans are as follows: Asset Mix Policy of Funds: Defined Benefit Pension Plan Postretirement Benefit Plan Asset Category Minimum Maximum Minimum Maximum Domestic Equities 16% 36% 0% 55% International Equities 8% 18% 0% 25% Fixed Income 39% 51% 20% 100% Diversified Credit 0% 13% 0% 0% Real Estate 0% 13% 0% 0% Short-Term Investments 0% 10% 0% 10% As of December 31, 2016, the asset mix and acceptable minimum and maximum ranges established by the policy for the pension and other postretirement benefit plans were as follows: Asset Mix Policy of Funds: Defined Benefit Pension Plan Postretirement Benefit Plan Asset Category Minimum Maximum Minimum Maximum Domestic Equities 25% 45% 35% 55% International Equities 15% 25% 15% 25% Fixed Income 23% 37% 20% 50% Real Estate/Private Equity/Hedge Funds 0% 15% 0% 0% Short-Term Investments 0% 10% 0% 10% Pension Plan and Postretirement Plan Asset Mix at December 31, 2017 and December 31, 2016 : Defined Benefit Pension Assets December 31, Postretirement December 31, Asset Class (in millions) Asset Value % of Total Assets Asset Value % of Total Assets Domestic Equities $ 698.2 32.3 % $ 96.0 36.6 % International Equities 351.0 16.2 % 39.8 15.2 % Fixed Income 977.6 45.3 % 117.5 44.8 % Real Estate 49.9 2.3 % — — Cash/Other 83.3 3.9 % 9.2 3.4 % Total $ 2,160.0 100.0 % $ 262.5 100.0 % Defined Benefit Pension Assets December 31, Postretirement Benefit Plan Assets December 31, Asset Class (in millions) Asset Value % of Total Assets Asset Value % of Total Assets Domestic Equities $ 755.2 43.1 % $ 97.9 42.3 % International Equities 339.9 19.4 % 41.8 18.0 % Fixed Income 565.8 32.3 % 87.0 37.6 % Real Estate/Private Equity/Hedge Funds 74.8 4.3 % — — Cash/Other 15.2 0.9 % 4.7 2.1 % Total $ 1,750.9 100.0 % $ 231.4 100.0 % The categorization of investments into the asset classes in the table above are based on definitions established by the NiSource Benefits Committee. Fair Value Measurements. The following table sets forth, by level within the fair value hierarchy, the Master Trust and other postretirement benefits investment assets at fair value as of December 31, 2017 and 2016 . Assets and liabilities are classified in their entirety based on the lowest level of input that is significant to the fair value measurement. Total Master Trust and other postretirement benefits investment assets at fair value classified within Level 3 were $98.9 million and $73.1 million as of December 31, 2017 and December 31, 2016 , respectively. Such amounts were approximately 4% of the Master Trust and other postretirement benefits’ total investments as reported on the statement of net assets available for benefits at fair value as of December 31, 2017 and 2016 . Valuation Techniques Used to Determine Fair Value: Level 1 Measurements Most common and preferred stocks are traded in active markets on national and international securities exchanges and are valued at closing prices on the last business day of each period presented. Cash is stated at cost which approximates fair value, with the exception of cash held in foreign currencies which fluctuates with changes in the exchange rates. Short-term bills and notes are priced based on quoted market values. Level 2 Measurements Most U.S. Government Agency obligations, mortgage/asset-backed securities, and corporate fixed income securities are generally valued by benchmarking model-derived prices to quoted market prices and trade data for identical or comparable securities. To the extent that quoted prices are not available, fair value is determined based on a valuation model that includes inputs such as interest rate yield curves and credit spreads. Securities traded in markets that are not considered active are valued based on quoted market prices, broker or dealer quotations, or alternative pricing sources with reasonable levels of price transparency. Other fixed income includes futures and options which are priced on bid valuation or settlement pricing. Level 3 Measurements Private equity investment strategies include buy-out, venture capital, growth equity, distressed debt, and mezzanine debt. Private equity investments are held through limited partnerships. Limited partnerships are valued at estimated fair market value based on their proportionate share of the partnership's fair value as recorded in the partnerships' audited financial statements. Partnership interests represent ownership interests in private equity funds and real estate funds. Real estate partnerships invest in natural resources, commercial real estate and distressed real estate. The fair value of these investments is determined by reference to the funds' underlying assets, which are principally securities, private businesses, and real estate properties. The value of interests held in limited partnerships, other than securities, is determined by the general partner, based upon third-party appraisals of the underlying assets, which include inputs such as cost, operating results, discounted cash flows and market based comparable data. Private equity and real estate limited partnerships typically call capital over a three to five year period and pay out distributions as the underlying investments are liquidated. The typical expected life of these limited partnerships is 10-15 years and these investments typically cannot be redeemed prior to liquidation. Not Classified Commingled funds that hold underlying investments that have prices which are derived from the quoted prices in active markets are not classified within the fair value hierarchy. Instead, these assets are measured at estimated fair value using the net asset value per share of the investments. The funds' underlying assets are principally marketable equity and fixed income securities. Units held in commingled funds are valued at the unit value as reported by the investment managers. For the year ended December 31, 2017 , there were no significant changes to valuation techniques to determine the fair value of NiSource's pension and other postretirement benefits' assets. Fair Value Measurements at December 31, 2017 : (in millions) December 31, Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Significant (Level 3) Pension plan assets: Cash $ 9.7 $ 9.7 $ — $ — Equity securities U.S. equities 0.3 0.3 — — Fixed income securities Government 143.4 — 143.4 — Corporate 332.6 — 332.6 — Mutual Funds U.S. multi-strategy 231.5 231.5 — — International equities 85.8 85.8 — — Fixed income 242.3 242.3 — — Private equity limited partnerships U.S. multi-strategy (1) 26.7 — — 26.7 International multi-strategy (2) 19.1 — — 19.1 Distressed opportunities 3.2 — — 3.2 Real estate 49.9 — — 49.9 Commingled funds (3) Short-term money markets 34.1 U.S. equities 466.6 International equities 265.1 Fixed income 244.9 Pension plan assets subtotal 2,155.2 569.6 476.0 98.9 Other postretirement benefit plan assets: Mutual funds U.S. equities 83.8 83.8 — — International equities 39.8 39.8 — — Fixed income 117.3 117.3 — — Commingled funds (3) Short-term money markets 9.4 U.S. equities 12.2 Other postretirement benefit plan assets subtotal 262.5 240.9 — — Due to brokers, net (4) (2.5 ) Accrued income/dividends 7.3 Total pension and other postretirement benefit plan assets $ 2,422.5 $ 810.5 $ 476.0 $ 98.9 (1) This class includes limited partnerships/fund of funds that invest in a diverse portfolio of private equity strategies, including buy-outs, venture capital, growth capital, special situations and secondary markets, primarily inside the United States. (2) This class includes limited partnerships/fund of funds that invest in diverse portfolio of private equity strategies, including buy-outs, venture capital, growth capital, special situations and secondary markets, primarily outside the United States. (3) This class of investments is measured at fair value using the net asset value per share and has not been classified in the fair value hierarchy. (4) This class represents pending trades with brokers. The table below sets forth a summary of changes in the fair value of the Plan’s Level 3 assets for the year ended December 31, 2017 : Balance at January 1, 2017 Total gains or losses (unrealized / realized) Purchases (Sales) Balance at December 31, 2017 Fixed income securities Other fixed income $ 0.1 $ (0.1 ) $ — $ — $ — Private equity limited partnerships U.S. multi-strategy 34.8 2.1 0.9 (11.1 ) 26.7 International multi-strategy 24.9 1.1 0.1 (7.0 ) 19.1 Distressed opportunities 4.1 0.4 — (1.3 ) 3.2 Real estate 9.2 (0.6 ) 42.1 (0.8 ) 49.9 Total $ 73.1 $ 2.9 $ 43.1 $ (20.2 ) $ 98.9 The table below sets forth a summary of unfunded commitments, redemption frequency and redemption notice periods for certain investments that are measured at fair value using the net asset value per share for the year ended December 31, 2017 : (in millions) Fair Value Redemption Frequency Redemption Notice Period Commingled Funds Short-term money markets $ 43.5 Daily 1 day U.S. equities 478.8 Monthly 3 days International equities 265.1 Monthly 10-30 days Fixed income 244.9 Monthly 3 days Total $ 1,032.3 Fair Value Measurements at December 31, 2016 : (in millions) December 31, Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Significant (Level 3) Pension plan assets: Cash $ 1.9 $ 1.9 $ — $ — Fixed income securities Government 42.2 — 42.2 — Corporate 104.1 — 104.1 — Other fixed income 0.1 — — 0.1 Mutual Funds U.S. multi-strategy 283.2 283.2 — — International equities 116.6 116.6 — — Fixed income 135.6 135.6 — — Private equity limited partnerships U.S. multi-strategy (1) 34.8 — — 34.8 International multi-strategy (2) 24.9 — — 24.9 Distressed opportunities 4.1 — — 4.1 Real Estate 9.2 — — 9.2 Commingled funds (3) Short-term money markets 16.6 U.S. equities 472.0 International equities 223.2 Fixed income 280.7 Pension plan assets subtotal 1,749.2 537.3 146.3 73.1 Other postretirement benefit plan assets: Mutual funds U.S. equities 85.4 85.4 — — International equities 41.8 41.8 — — Fixed income 86.8 86.8 — — Commingled funds (3) Short-term money markets 9.5 U.S. equities 12.5 Other postretirement benefit plan assets subtotal 236.0 214.0 — — Due to brokers, net (4) (5.0 ) Receivables/payables 2.1 Total pension and other postretirement benefit plan assets $ 1,982.3 $ 751.3 $ 146.3 $ 73.1 (1) This class includes limited partnerships/fund of funds that invest in a diverse portfolio of private equity strategies, including buy-outs, venture capital, growth capital, special situations and secondary markets, primarily in the United States. (2) This class includes limited partnerships/fund of funds that invest in a diverse portfolio of private equity strategies, including buy-outs, venture capital, growth capital, special situations and secondary markets, primarily outside the United States. (3) This class of investments is measured at fair value using the net asset value per share and has not been classified in the fair value hierarchy. (4) This class represents pending trades with brokers. The table below sets forth a summary of changes in the fair value of the Plan’s Level 3 assets for the year ended December 31, 2016 : Balance at January 1, 2016 Total gains or losses (unrealized / realized) Purchases (Sales) Balance at December 31, 2016 Fixed income securities Other fixed income $ 0.1 $ — $ — $ — $ 0.1 Private equity limited partnerships U.S. multi-strategy 46.4 2.1 0.8 (14.5 ) 34.8 International multi-strategy 29.3 2.0 1.0 (7.4 ) 24.9 Distress opportunities 5.9 (0.4 ) 0.1 (1.5 ) 4.1 Real estate 13.6 0.1 0.1 (4.6 ) 9.2 Total $ 95.3 $ 3.8 $ 2.0 $ (28.0 ) $ 73.1 The table below sets forth a summary of unfunded commitments, redemption frequency and redemption notice periods for certain investments that are measured at fair value using the net asset value per share for the year ended December 31, 2016 : (in millions) Fair Value Redemption Frequency Redemption Notice Period Commingled Funds Short-term money markets $ 26.1 Daily 1 day U.S. equities 484.5 Monthly 3 days International equities 223.2 Monthly 14-30 days Fixed income 280.7 Monthly 3 days Total $ 1,014.5 NiSource Pension and Other Postretirement Benefit Plans’ Funded Status and Related Disclosure . The following table provides a reconciliation of the plans’ funded status and amounts reflected in NiSource’s Consolidated Balance Sheets at December 31 based on a December 31 measurement date: Pension Benefits Other Postretirement Benefits (in millions) 2017 2016 2017 2016 Change in projected benefit obligation (1) Benefit obligation at beginning of year $ 2,165.8 $ 2,206.7 $ 529.0 $ 525.8 Service cost 30.0 30.7 4.8 5.0 Interest cost 68.3 89.7 17.8 22.0 Plan participants’ contributions — — 5.7 5.9 Plan amendments 0.9 — 1.6 7.5 Actuarial (gain) loss 98.3 (2.7 ) 36.2 1.0 Settlement loss 1.6 — — — Benefits paid (172.3 ) (158.6 ) (39.3 ) (38.9 ) Estimated benefits paid by incurred subsidy — — 0.5 0.7 Projected benefit obligation at end of year $ 2,192.6 $ 2,165.8 $ 556.3 $ 529.0 Change in plan assets Fair value of plan assets at beginning of year $ 1,750.9 $ 1,747.1 $ 231.4 $ 225.9 Actual return on plan assets 299.1 159.1 33.1 13.0 Employer contributions 282.3 3.3 31.6 25.5 Plan participants’ contributions — — 5.7 5.9 Benefits paid (172.3 ) (158.6 ) (39.3 ) (38.9 ) Fair value of plan assets at end of year $ 2,160.0 $ 1,750.9 $ 262.5 $ 231.4 Funded Status at end of year $ (32.6 ) $ (414.9 ) $ (293.8 ) $ (297.6 ) Amounts recognized in the statement of financial position consist of: Noncurrent assets 9.8 — — — Current liabilities (2.8 ) (2.9 ) (0.7 ) (0.7 ) Noncurrent liabilities (39.6 ) (412.0 ) (293.1 ) (296.9 ) Net amount recognized at end of year (2) $ (32.6 ) $ (414.9 ) $ (293.8 ) $ (297.6 ) Amounts recognized in accumulated other comprehensive income or regulatory asset/liability (3) Unrecognized prior service credit $ 2.5 $ 1.0 $ (23.1 ) $ (29.2 ) Unrecognized actuarial loss 692.9 835.5 84.2 68.3 Net amount recognized at end of year $ 695.4 $ 836.5 $ 61.1 $ 39.1 (1) The change in benefit obligation for Pension Benefits represents the change in Projected Benefit Obligation while the change in benefit obligation for Other Postretirement Benefits represents the change in accumulated postretirement benefit obligation. (2) NiSource recognizes in its Consolidated Balance Sheets the underfunded and overfunded status of its various defined benefit postretirement plans, measured as the difference between the fair value of the plan assets and the benefit obligation. (3) NiSource determined that for certain rate-regulated subsidiaries the future recovery of pension and other postretirement benefits costs is probable. These rate-regulated subsidiaries recorded regulatory assets and liabilities of $733.5 million and $0.1 million , respectively, as of December 31, 2017 , and $847.5 million and $0.3 million , respectively, as of December 31, 2016 that would otherwise have been recorded to accumulated other comprehensive loss. NiSource’s accumulated benefit obligation for its pension plans was $2,170.4 million and $2,148.9 million as of December 31, 2017 and 2016 , respectively. The accumulated benefit obligation as of a date is the actuarial present value of benefits attributed by the pension benefit formula to employee service rendered prior to that date and based on current and past compensation levels. The accumulated benefit obligation differs from the projected benefit obligation disclosed in the table above in that it includes no assumptions about future compensation levels. NiSource is required to reflect the funded status of the pension and postretirement benefit plans on the Consolidated Balance Sheet. The funded status of the plans is measured as the difference between the plan assets' fair value and the projected benefit obligation. NiSource has presented the noncurrent aggregate of all underfunded plans within "Accrued liability for postretirement and postemployment benefits." The portion of the amount by which the actuarial present value of benefits included in the projected benefit obligation exceeds the fair value of plan assets, payable in the next 12 months, is reflected in "Accrued compensation and other benefits." NiSource has presented the aggregate of all overfunded plans within "Deferred charges and other." Information for pension plans with an accumulated benefit obligation in excess of plan assets: December 31, 2017 2016 Accumulated Benefit Obligation $ 1,502.5 $ 2,148.9 Funded Status Projected Benefit Obligation 1,524.7 2,165.8 Fair Value of Plan Assets 1,482.3 1,750.9 Funded Status of Underfunded Pension Plans at End of Year $ (42.4 ) $ (414.9 ) Information for pension plans with plan assets in excess of the accumulated benefit obligation: December 31, 2017 2016 Accumulated Benefit Obligation $ 667.9 $ — Funded Status Projected Benefit Obligation 667.9 — Fair Value of Plan Assets 677.7 — Funded Status of Overfunded Pension Plans at End of Year $ 9.8 $ — In aggregate, NiSource pension plans were underfunded by $32.6 million at December 31, 2017 compared to being underfunded at December 31, 2016 by $414.9 million . The improvement in the funded status was due primarily to employer contributions and favorable asset returns offset by a decrease in discount rates. NiSource contributed $282.3 million and $3.3 million to its pension plans in 2017 and 2016 , respectively. NiSource’s other postretirement benefit plans were underfunded by $293.8 million at December 31, 2017 compared to being underfunded at December 31, 2016 by $297.6 million . The improvement in funded status was primarily due to employer contributions and favorable asset returns slightly offset by a decrease in discount rates. NiSource contributed $31.6 million and $25.5 million to its other postretirement benefit plans in 2017 and 2016 , respectively. No amounts of NiSource’s pension or other postretirement benefit plans’ assets are expected to be returned to NiSource or any of its subsidiaries in 2017 . In 2017 , one of NiSource's qualified pension plans paid lump sum payouts in excess of the plan's 2017 service cost plus interest cost and, therefore, settlement accounting was required. A settlement charge of $13.7 million was recorded in 2017 . Net periodic pension benefit cost for 2017 was decreased by $3.2 million as a result of the interim remeasurement. The following table provides the key assumptions that were used to calculate the pension and other postretirement benefits obligations for NiSource’s various plans as of December 31: Pension Benefits Other Postretirement Benefits 2017 2016 2017 2016 Weighted-average assumptions to Determine Benefit Obligation Discount Rate 3.58 % 4.03 % 3.67 % 4.12 % Rate of Compensation Increases 4.00 % 4.00 % — — Health Care Trend Rates Trend for Next Year — — 8.52 % 8.43 % Ultimate Trend — — 4.50 % 4.50 % Year Ultimate Trend Reached — — 2025 2024 Assumed health care cost trend rates have a significant effect on the amounts reported for the health care plans. A one-percentage-point change in assumed health care cost trend rates would have the following effects: (in millions) 1% point increase 1% point decrease Effect on service and interest components of net periodic cost $ 1.1 $ (0.9 ) Effect on accumulated postretirement benefit obligation 29.7 (25.9 ) NiSource expects to make contributions of approximately $2.9 million to its pension plans and approximately $25.0 million to its postretirement medical and life plans in 2018. The following table provides benefits expected to be paid in each of the next five fiscal years, and in the aggregate for the five fiscal years thereafter. The expected benefits are estimated based on the same assumptions used to measure NiSource’s benefit obligation at the end of the year and includes benefits attributable to the estimated future service of employees: (in millions) Pension Benefits Other Federal Year(s) 2018 $ 176.2 $ 34.3 $ 0.5 2019 173.7 35.3 0.5 2020 172.1 36.3 0.5 2021 172.0 36.9 0.5 2022 171.3 36.9 0.5 2023-2027 784.7 178.9 1.9 The following table provides the components of the plans’ actuarially determined net periodic benefits cost for each of the three years ended December 31, 2017 , 2016 and 2015 : Pension Benefits Other Postretirement Benefits (in millions) 2017 2016 2015 2017 2016 2015 Components of Net Periodic Benefit Cost Service cost $ 30.0 $ 30.7 $ 34.8 $ 4.8 $ 5.0 $ 6.4 Interest cost 68.3 89.7 95.9 17.8 22.0 24.9 Expected return on assets (123.1 ) (132.9 ) (167.2 ) (15.9 ) (17.2 ) (28.2 ) Amortization of prior service cost (credit) (0.7 ) (0.2 ) 0.1 (4.4 ) (4.9 ) (5.2 ) Recognized actuarial loss 52.9 61.2 59.3 3.0 3.1 3.4 Net Periodic Benefit Costs 27.4 48.5 22.9 5.3 8.0 1.3 Additional loss recognized due to: Settlement loss 13.7 — 2.5 — — — Total Net Periodic Benefits Cost $ 41.1 $ 48.5 $ 25.4 $ 5.3 $ 8.0 $ 1.3 The following table provides the key assumptions that were used to calculate the net periodic benefits cost for NiSource’s various plans: Pension Benefits Other Postretirement Benefits 2017 2016 2015 2017 2016 2015 Weighted-average Assumptions to Determine Net Periodic Benefit Cost Discount rate - service cost (1) 4.40 % 4.24 % 3.81 % 4.58 % 4.33 % 3.94 % Discount rate - interest cost (1) 3.31 % 4.24 % 3.81 % 3.48 % 4.33 % 3.94 % Expected Long-Term Rate of Return on Plan Assets 7.25 % 8.00 % 8.30 % 6.99 % 7.85 % 8.15 % Rate of Compensation Increases 4.00 % 4.00 % 4.00 % — — — (1) In January 2017, NiSource changed the method used to estimate the service and interest components of net periodic benefit cost for pension and other postretirement benefits. This change, compared to the previous method, resulted in a decrease in the actuarially-determined service and interest cost components. Historically, NiSource estimated service and interest cost utilizing a single weighted-average discount rate derived from the yield curve used to measure the benefit obligation at the beginning of the period. For fiscal 2017 and beyond, NiSource now utilizes a full yield curve approach to estimate these components by applying the specific spot rates along the yield curve used in the determination of the benefit obligation to the relevant projected cash flows. NiSource believes it is appropriate to assume a 7.25% and 6.99% rate of return on pension and other postretirement plan assets, respectively, for its calculation of 2017 pension benefits cost. These rates are primarily based on asset mix and historical rates of return and were adjusted in the current year due to anticipated changes in asset allocation and projected market returns. The following table provides other changes in plan assets and projected benefit obligations recognized in other comprehensive income or regulatory asset or liability: Pension Benefits Other Postretirement Benefits (in millions) 2017 2016 2017 2016 Other Changes in Plan Assets and Projected Benefit Obligations Recognized in Other Comprehensive Income or Regulatory Asset or Liability Net prior service cost $ 0.9 $ — $ 1.6 $ 7.5 Net actuarial loss (gain) (76.1 ) (28.9 ) 18.9 5.3 Settlements (13.7 ) — — — Less: amortization of prior service cost 0.7 0.2 4.4 4.9 Less: amortization of net actuarial loss (52.9 ) (61.2 ) (3.0 ) (3.1 ) Total Recognized in Other Comprehensive Income or Regulatory Asset or Liability $ (141.1 ) $ (89.9 ) $ 21.9 $ 14.6 Amount Recognized in Net Periodic Benefits Cost and Other Comprehensive Income or Regulatory Asset or Liability $ (100.0 ) $ (41.4 ) $ 27.2 $ 22.6 Based on a December 31 measurement date, the net unrecognized actuarial loss, unrecognized prior service cost (credit), and unrecognized transition obligation that will be amortized into net periodic benefit cost during 2018 for the pension plans are $40.9 million , $(0.4) million and zero , respectively, and for other postretirement benefit plans are $3.8 million , $(4.0) million and zero , respectively.

Common Stock

Common Stock	12 Months Ended
	Dec. 31, 2017
Common Stock [Abstract]
Common Stock	Common Stock As of December 31, 2017 , NiSource had 400,000,000 authorized shares of common stock with a $0.01 par value. ATM Program and Forward Sale Agreement. On May 3, 2017, NiSource entered into four separate equity distribution agreements, pursuant to which NiSource may sell, from time to time, up to an aggregate of $500.0 million of its common stock. As of December 31, 2017, the ATM program (including the impacts of forward sales agreements discussed below) had approximately $10.0 million of equity available for issuance. The program expires on December 31, 2018. The following table summarizes NiSource's activity under the ATM program: Year Ending December 31, 2017 2016 2015 Number of shares issued 11,931,376 — — Average price per share $ 26.58 — — Proceeds, net of fees ( in millions) $ 314.7 — — On November 13, 2017, under the ATM program, NiSource executed a forward agreement, which allows NiSource to issue a fixed number of shares at a price to be settled in the future. From November 13, 2017 to December 8, 2017, 6,345,860 shares were borrowed from third parties and sold by the dealer at a weighted average price of $27.24 per share. NiSource may settle this agreement in shares, cash, or net shares by November 12, 2018. NiSource has classified the forward agreement as an equity transaction in accordance with relevant GAAP. As a result of this classification, no amounts have been recorded in the financial statements as of and for the period ended December 31, 2017. Delivery of shares will eventually result in dilution to basic EPS upon settlement. In periods prior to the settlement date, a dilutive effect of the forward agreement on NiSource's EPS could occur during periods when the average market price per share of NiSource common stock is above the share price adjusted forward sale price. See Note 4, "Earnings Per Share," for additional information. Had NiSource settled all 6,345,860 shares under the forward agreement at December 31, 2017, NiSource would have received approximately $171.2 million , based on a net price of $26.98 per share. Common Stock Dividend. Holders of shares of NiSource’s common stock are entitled to receive dividends when, as and if declared by the Board out of funds legally available. The policy of the Board has been to declare cash dividends on a quarterly basis payable on or about the 20th day of February, May, August and November. NiSource has paid quarterly common dividends totaling $0.70 , $0.64 and $0.83 per share for the years ended December 31, 2017 , 2016 and 2015 , respectively. At its January 26, 2018 meeting, the Board declared a quarterly common dividend of $0.195 per share, payable on February 20, 2018 to holders of record on February 9, 2018 . NiSource has certain debt covenants which could potentially limit the amount of dividends the Company could pay in order to maintain compliance with these covenants. Refer to Note 14 , "Long-Term Debt," for more information. As of December 31, 2017 , these covenants did not restrict the amount of dividends that were available to be paid. Dividend Reinvestment and Stock Purchase Plan. NiSource offered a Dividend Reinvestment and Stock Purchase Plan which allowed participants to reinvest dividends and make voluntary cash payments to purchase additional shares of common stock. This plan was terminated effective December 31, 2017 in favor of an independent plan sponsored by NiSource’s transfer agent, Computershare Trust Company, N.A.

Share-Based Compensation

Share-Based Compensation	12 Months Ended
	Dec. 31, 2017
Share-based Compensation [Abstract]
Share-Based Compensation	Share-Based Compensation The NiSource stockholders originally approved and adopted the NiSource Inc. 2010 Omnibus Incentive Plan (“Omnibus Plan”) at the Annual Meeting of Stockholders held on May 11, 2010. Stockholders re-approved the Omnibus Plan as amended at the Annual Meeting of Stockholders held on May 12, 2015. The Omnibus Plan provides for awards to employees and non-employee directors of incentive and nonqualified stock options, stock appreciation rights, restricted stock, restricted stock units, performance shares, performance units, cash-based awards and other stock-based awards and supersedes the long-term incentive plan approved by stockholders on April 13, 1994 (“1994 Plan”) and the Director Stock Incentive Plan (“Director Plan”). The Omnibus Plan provides that the number of shares of common stock of NiSource available for awards is 8,000,000 plus the number of shares subject to outstanding awards that expire or terminate for any reason that were granted under either the 1994 Plan or the Director Plan, plus the number of shares that were awarded as a result of the Separation-related adjustments (discussed below). At December 31, 2017 , there were 4,455,389 shares reserved for future awards under the Omnibus Plan. NiSource recognized stock-based employee compensation expense of $15.3 million , $15.1 million and $18.8 million , during 2017 , 2016 and 2015 , respectively, as well as related tax benefits of $5.9 million , $5.8 million and $7.2 million , respectively. Additionally, NiSource adopted ASU 2016-09 in the third quarter of 2016 and recognized excess tax benefits from the distribution of vested share-based employee compensation in 2017 and 2016. For the twelve months ended December 31, 2017 and December 31, 2016, $4.4 million and $7.2 million of such benefits were recorded, respectively. As of December 31, 2017 , the total remaining unrecognized compensation cost related to non-vested awards amounted to $19.4 million , which will be amortized over the weighted-average remaining requisite service period of 1.8 years. Separation-related Adjustments . In connection with the Separation, NiSource and CPG entered into an Employee Matters Agreement, effective July 1, 2015. Under the terms of the Employee Matters Agreement, and pursuant to the terms of the Omnibus Plan, the Compensation Committee of the Board of NiSource approved an adjustment to outstanding awards granted under the Omnibus Plan in order to preserve the intrinsic aggregate value of such awards before the Separation (the “Valuation Adjustment”). The Separation-related adjustments did not have a material impact on either compensation expense or the potentially dilutive securities to be considered in the calculation of diluted earnings per share of common stock. Former NiSource employees transferred to CPG as a result of the Separation surrendered their outstanding unvested NiSource awards effective July 1, 2015. Restricted Stock Units and Restricted Stock . Restricted stock units and shares of restricted stock granted to employees in 2017 and 2016 were immaterial. In 2015 , NiSource granted 660,230 restricted stock units and shares of restricted stock to employees, subject to service conditions. The total grant date fair value of the restricted stock units and shares of restricted stock was $23.9 million , based on the average market price of NiSource’s common stock at the date of each grant less the present value of any dividends not received during the vesting period, which will be expensed over the vesting period which is generally three years. Including the effect of the Valuation Adjustment, 635,795 non-vested restricted stock units and shares of restricted stock granted in 2015 were outstanding as of December 31, 2017 . If an employee terminates employment before the service conditions lapse under the 2015 , 2016 or 2017 awards due to (1) Retirement or Disability (as defined in the award agreement), or (2) death, the service conditions will lapse on the date of such termination with respect to a pro rata portion of the restricted stock units and shares of restricted stock based upon the percentage of the service period satisfied between the grant date and the date of the termination of employment. In the event of a change in control (as defined in the award agreement), all unvested shares of restricted stock and restricted stock units awarded prior to 2015 will immediately vest and all unvested shares of restricted stock and restricted stock units awarded in 2015, 2016 and 2017 will immediately vest upon termination of employment occurring in connection with a change in control. Termination due to any other reason will result in all unvested shares of restricted stock and restricted stock units awarded being forfeited effective on the employee’s date of termination. (shares) Restricted Stock Units Weighted Average Grant Date Fair Value Per Unit ($) Nonvested at December 31, 2016 1,642,030 12.05 Granted 10,983 22.87 Forfeited (85,436 ) 14.64 Vested (869,451 ) 9.33 Nonvested at December 31, 2017 698,126 15.09 Performance Shares . In 2017 , NiSource granted 660,750 performance shares subject to service, performance and market conditions. The grant date fair value of the awards was $12.9 million , based on the average market price of NiSource’s common stock at the date of each grant less the present value of dividends not received during the vesting period which will be expensed over the three year requisite service period. The performance conditions are based on achievement of certain non-GAAP financial measures: cumulative net operating earnings per share, a non-GAAP financial measure that NiSource defines as income from continuing operations adjusted for certain items, for the three-year period ending December 31, 2019; and relative total shareholder return, a market measure that NiSource defines as the annualized growth in dividends and share price of a share of NiSource's common stock (calculated using a 20 trading day average of NiSource's closing price beginning on December 31, 2016 and ending on December 31, 2019) compared to the total shareholder return performance of a predetermined peer group of companies. A Monte Carlo analysis was used to value the portion of these awards dependent on market conditions. As of December 31, 2017 , 604,944 non-vested performance shares granted were outstanding. The service conditions for these awards lapse on February 28, 2020. In 2016 , NiSource granted 647,305 performance shares subject to service, performance and market conditions. The grant date fair value of the awards was $12.6 million , based on the average market price of NiSource’s common stock at the date of each grant less the present value of dividends not received during the vesting period which will be expensed over the three year requisite service period. The performance conditions are based on achievement of certain non-GAAP financial measures: cumulative net operating earnings per share, a non-GAAP financial measure that NiSource defines as income from continuing operations adjusted for certain items, for the three-year period ending December 31, 2018; and relative total shareholder return, a market measure that NiSource defines as the annualized growth in dividends and share price of a share of NiSource's common stock (calculated using a 20 trading day average of NiSource's closing price beginning on December 31, 2015 and ending on December 31, 2018) compared to the total shareholder return performance of a predetermined peer group of companies. A Monte Carlo analysis was used to value the portion of these awards dependent on market conditions. As of December 31, 2017 , 579,829 non-vested performance shares granted were outstanding. The service conditions for these awards lapse on February 28, 2019. In 2015 , NiSource did not grant any performance shares subject to performance and service conditions. (shares) Performance Awards Weighted Average Grant Date Fair Value Per Unit ($) Nonvested at December 31, 2016 647,305 19.50 Granted 660,750 19.50 Forfeited (123,282 ) 19.45 Vested — — Nonvested at December 31, 2017 1,184,773 19.52 Non-employee Director Awards . As of May 11, 2010, awards to non-employee directors may be made only under the Omnibus Plan. Currently, restricted stock units are granted annually to non-employee directors, subject to a non-employee director’s election to defer receipt of such restricted stock unit award. The non-employee director’s annual award of restricted stock units vest on the last day of the non-employee director’s annual term corresponding to the year the restricted stock units were awarded subject to special pro-rata vesting rules in the event of Retirement or Disability (as defined in the award agreement), or death. The vested restricted stock units are payable as soon as practicable following vesting except as otherwise provided pursuant to the non-employee director’s election to defer. Certain restricted stock units remain outstanding from the Director Plan. All such awards are fully vested and shall be distributed to the directors upon their separation from the Board. As of December 31, 2017 , 225,613 restricted stock units are outstanding to non-employee directors under either the Omnibus Plan or the Director Plan. Of this amount, 54,964 restricted stock units are unvested and expected to vest. 401(k) Match, Profit Sharing and Company Contribution. NiSource has a voluntary 401(k) savings plan covering eligible employees that allows for periodic discretionary matches as a percentage of each participant’s contributions payable in cash for nonunion employees and generally payable in shares of NiSource common stock for union employees, subject to collective bargaining. NiSource also has a retirement savings plan that provides for discretionary profit sharing contributions similarly payable in cash or shares of NiSource common stock to eligible employees based on earnings results; and eligible employees hired after January 1, 2010 receive a non-elective company contribution of 3% of eligible pay similarly payable in cash or shares of NiSource common stock. For the years ended December 31, 2017 , 2016 and 2015 , NiSource recognized 401(k) match, profit sharing and non-elective contribution expense of $37.6 million , $32.3 million and $27.4 million , respectively.

Long-Term Debt

Long-Term Debt	12 Months Ended
	Dec. 31, 2017
Long-term Debt, Current and Noncurrent [Abstract]
Long-Term Debt	Long-Term Debt NiSource long-term debt as of December 31, 2017 and 2016 is as follows: Long-term debt type Maturity as of December 31, 2017 Weighted average interest rate (%) Outstanding balance as of December 31, (in millions) 2017 2016 Senior notes: NiSource September 2017 5.25 % $ — $ 210.4 NiSource March 2018 6.40 % 275.1 476.0 NiSource January 2019 6.80 % 255.1 500.0 NiSource March 2019 Variable (1) — 500.0 NiSource September 2020 5.45 % 325.1 550.0 NiSource December 2021 4.45 % 63.6 63.6 NiSource March 2022 6.13 % 180.0 500.0 NiSource November 2022 2.65 % 500.0 — NiSource February 2023 3.85 % 250.0 250.0 NiSource November 2025 5.89 % 265.0 265.0 NiSource May 2027 3.49 % 1,000.0 — NiSource December 2027 6.78 % 3.0 3.0 NiSource December 2040 6.25 % 250.0 250.0 NiSource June 2041 5.95 % 400.0 400.0 NiSource February 2042 5.80 % 250.0 250.0 NiSource February 2043 5.25 % 500.0 500.0 NiSource February 2044 4.80 % 750.0 750.0 NiSource February 2045 5.65 % 500.0 500.0 NiSource May 2047 4.38 % 1,000.0 — NiSource March 2048 3.95 % 750.0 — Total senior notes $ 7,516.9 $ 5,968.0 Medium term notes: NiSource April 2022 to May 2027 7.99 % $ 49.0 $ 106.0 NIPSCO August 2022 to August 2027 7.61 % 68.0 95.5 Columbia of Massachusetts December 2025 to February 2028 6.30 % 40.0 40.0 Total medium term notes $ 157.0 $ 241.5 Capital leases: NIPSCO May 2018 3.95 % $ 3.8 $ 12.7 NiSource Corporate Services October 2019 3.26 % 1.4 3.5 Columbia of Ohio October 2021 to June 2038 6.41 % 88.5 80.1 Columbia of Virginia August 2024 to July 2029 12.21 % 5.2 5.5 Columbia of Kentucky May 2027 3.79 % 0.4 — Columbia of Pennsylvania August 2027 to June 2036 5.45 % 31.0 31.9 Columbia of Massachusetts December 2033 to July 2036 4.37 % 22.8 23.7 Total capital leases 153.1 157.4 Pollution control bonds - NIPSCO April 2019 5.85 % 41.0 96.0 Unamortized issuance costs and discounts (71.5 ) $ (41.6 ) Total Long-Term Debt $ 7,796.5 $ 6,421.3 (1) Rate of one month Libor plus 95 basis points. On November 30, 2017, NiSource Finance and Capital Markets merged with and into NiSource and NiSource became the primary obligor of NiSource Finance's and Capital Market's outstanding obligations. The merger does not have any impact on NiSource's consolidated financial statements or the credit rating of outstanding debt securities. None of NiSource's subsidiaries guarantee any third party debt. Details of NiSource's other 2017 long-term debt related activity are summarized below: • On March 27, 2017, Capital Markets redeemed $30.0 million of 7.86% and $2.0 million of 7.85% medium-term notes at maturity. • On April 3, 2017, Capital Markets redeemed $12.0 million of 7.82% , $10.0 million of 7.92% , $2.0 million of 7.93% and $1.0 million of 7.94% medium-term notes at maturity. • On May 22, 2017, NiSource Finance closed its placement of $2.0 billion in aggregate principal amount of its senior notes, comprised of $1.0 billion of 3.49% senior notes due 2027 and $1.0 billion of 4.375% senior notes due 2047. Related to this placement, NiSource settled $950.0 million of aggregate notional value forward-starting interest rate swaps, originally entered into to mitigate interest risk associated with the planned issuance of these notes. Refer to Note 9 , "Risk Management Activities," for additional information. • During the second quarter of 2017, NiSource Finance executed a tender offer for $990.7 million of outstanding notes consisting of a combination of its 6.40% notes due 2018, 6.80% notes due 2019, 5.45% notes due 2020, and 6.125% notes due 2022. In conjunction with the debt retired, NiSource Finance recorded a $111.5 million loss on early extinguishment of long-term debt, primarily attributable to early redemption premiums. • On June 12, 2017, NIPSCO redeemed $22.5 million of 7.59% medium-term notes at maturity. • On July 1, 2017, NIPSCO redeemed $55.0 million of 5.70% pollution control bonds at maturity. • On August 4, 2017, NIPSCO redeemed $5.0 million of 7.02% medium-term notes at maturity. • On September 14, 2017, NiSource Finance closed its placement of $750.0 million of 3.95% senior notes due 2048. Related to this placement, NiSource settled $750.0 million of aggregate notional value treasury lock agreements, originally entered into to mitigate the interest risk associated with the planned issuance of these notes. Refer to Note 9 , "Risk Management Activities," for additional information. • On September 15, 2017, NiSource Finance redeemed $210.4 million of 5.25% senior unsecured notes at maturity. • On November 17, 2017, NiSource Finance closed its placement of $500.0 million of 2.65% senior notes due 2022 to repay a $500.0 million variable-rate term loan due March 29, 2019. Related to this placement, NiSource settled $250.0 million of aggregate notional value treasury lock agreements originally entered into to mitigate the interest risk associated with the planned issuance of these notes. Refer to Note 9 , “Risk Management Activities,” for additional information. Details of NiSource's 2016 long-term debt related activity are summarized below: • On March 15, 2016, NiSource Finance redeemed $201.5 million of 10.75% senior unsecured notes at maturity. • On March 31, 2016, NiSource Finance entered into a $500 million term loan agreement with a syndicate of banks. The term loan matures March 29, 2019, at which point any and all outstanding borrowings under the agreement are due. Interest charged on borrowings depends on the variable rate structure elected by NiSource Finance at the time of each borrowing. The available variable rate structures from which NiSource Finance may choose are defined in the term loan agreement. As of December 31, 2016, NiSource Finance had $500.0 million of outstanding borrowings under the term loan agreement. • In June 2016, NiSource Finance entered into forward-starting interest rate swaps with an aggregate notional amount of $500.0 million to hedge the variability in cash flows attributable to changes in the benchmark interest rate during the period from the effective date of the swaps to the anticipated date of forecasted debt issuances, expected to take place by the end of 2018. The forward-starting interest rate swaps were designated as cash flow hedges at the time the agreements were executed, whereby any gain or loss recognized from the effective date of the swaps to the date the associated debt is issued for the effective portion of the hedge is recorded net of tax in AOCI and amortized as a component of interest expense over the life of the designated debt. If some portion of the hedges becomes ineffective, the associated gain or loss will be recognized in earnings. • On November 1, 2016, NIPSCO redeemed $130.0 million of 5.60% pollution control bonds at maturity. • On November 28, 2016, NiSource Finance redeemed $90.0 million of 5.41% senior unsecured notes at maturity. See Note 18 -A, "Contractual Obligations," for the outstanding long-term debt maturities at December 31, 2017 . Unamortized debt expense, premium and discount on long-term debt applicable to outstanding bonds are being amortized over the life of such bonds. NiSource is subject to a financial covenant under its revolving credit facility which requires NiSource to maintain a debt to capitalization ratio that does not exceed 70% . A similar covenant in a 2005 private placement note purchase agreement requires NiSource to maintain a debt to capitalization ratio that does not exceed 75% . As of December 31, 2017 , the ratio was 67.6% . NiSource is also subject to certain other non-financial covenants under the revolving credit facility. Such covenants include a limitation on the creation or existence of new liens on NiSource’s assets, generally exempting liens on utility assets, purchase money security interests, preexisting security interests and an additional subset of assets equal to $150 million . An asset sale covenant generally restricts the sale, conveyance, lease, transfer or other disposition of NiSource’s assets to those dispositions that are for a price not materially less than fair market of such assets, that would not materially impair the ability of NiSource to perform obligations under the revolving credit facility, and that together with all other such dispositions, would not have a material adverse effect. The covenant also restricts dispositions to no more than 10% of NiSource's consolidated total assets on December 31, 2015. The revolving credit facility also includes a cross-default provision, which triggers an event of default under the credit facility in the event of an uncured payment default relating to any indebtedness of NiSource or any of its subsidiaries in a principal amount of $50.0 million or more. NiSource’s indentures generally do not contain any financial maintenance covenants. However, NiSource’s indentures are generally subject to cross-default provisions ranging from uncured payment defaults of $5 million to $50 million , and limitations on the incurrence of liens on NiSource’s assets, generally exempting liens on utility assets, purchase money security interests, preexisting security interests and an additional subset of assets capped at 10% of NiSource’s consolidated net tangible assets.

Short-Term Borrowings

Short-Term Borrowings	12 Months Ended
	Dec. 31, 2017
Short-term Debt [Abstract]
Short-Term Borrowings	Short-Term Borrowings NiSource generates short-term borrowings from its revolving credit facility, commercial paper program, letter of credit issuances and accounts receivable transfer programs. Each of these borrowing sources is described further below. NiSource maintains a revolving credit facility to fund ongoing working capital requirements, including the provision of liquidity support for its commercial paper program, provide for issuance of letters of credit and also for general corporate purposes. NiSource's revolving credit facility has a program limit of $1.85 billion and is comprised of a syndicate of banks led by Barclays. At December 31, 2017 and 2016, NiSource had no outstanding borrowings under this facility. NiSource's commercial paper program has a program limit of up to $1.5 billion with a dealer group comprised of Barclays, Citigroup, Credit Suisse and Wells Fargo. At December 31, 2017 and 2016 , NiSource had $869.0 million and $1,178.0 million , respectively, of commercial paper outstanding. As of December 31, 2017 and 2016 , NiSource had $11.1 million and $14.7 million , respectively, of stand-by letters of credit outstanding all of which were under the revolving credit facility. Transfers of accounts receivable are accounted for as secured borrowings resulting in the recognition of short-term debt on the Consolidated Balance Sheets in the amount of $336.7 million and $310.0 million as of December 31, 2017 and 2016 , respectively. Refer to Note 17 , "Transfers of Financial Assets," for additional information. Short-term borrowings were as follows: At December 31, (in millions) 2017 2016 Commercial Paper weighted average interest rate of 1.97 % and 1.24% at December 31, 2017 and 2016, respectively. $ 869.0 $ 1,178.0 Accounts receivable securitization facility borrowings 336.7 310.0 Total Short-Term Borrowings $ 1,205.7 $ 1,488.0 Given their maturities are less than 90 days, cash flows related to the borrowings and repayments of the items listed above are presented net in the Statements of Consolidated Cash Flows.

Fair Value

Fair Value	12 Months Ended
	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Fair Value	Fair Value A. Fair Value Measurements Recurring Fair Value Measurements . The following tables present financial assets and liabilities measured and recorded at fair value on NiSource’s Consolidated Balance Sheets on a recurring basis and their level within the fair value hierarchy as of December 31, 2017 and December 31, 2016 : Recurring Fair Value Measurements December 31, 2017 ( in millions ) Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Balance as of December 31, 2017 Assets Risk management assets $ — $ 21.1 $ — $ 21.1 Available-for-sale securities — 133.9 — 133.9 Total $ — $ 155.0 $ — $ 155.0 Liabilities Risk management liabilities $ — $ 71.4 $ 0.3 $ 71.7 Total $ — $ 71.4 $ 0.3 $ 71.7 Recurring Fair Value Measurements December 31, 2016 ( in millions ) Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Balance as of December 31, 2016 Assets Risk management assets $ 5.4 $ 43.6 $ — $ 49.0 Available-for-sale securities — 131.5 — 131.5 Total $ 5.4 $ 175.1 $ — $ 180.5 Liabilities Risk management liabilities $ 1.2 $ 58.9 $ 1.2 $ 61.3 Total $ 1.2 $ 58.9 $ 1.2 $ 61.3 Risk management assets and liabilities include interest rate swaps, exchange-traded NYMEX futures and NYMEX options and non-exchange-based forward purchase contracts. Exchange-traded derivative contracts are based on unadjusted quoted prices in active markets and are classified within Level 1. These financial assets and liabilities are secured with cash on deposit with the exchange; therefore, nonperformance risk has not been incorporated into these valuations. Certain non-exchange-traded derivatives are valued using broker or over-the-counter, on-line exchanges. In such cases, these non-exchange-traded derivatives are classified within Level 2. Non-exchange-based derivative instruments include swaps, forwards, options and treasury lock agreements. In certain instances, these instruments may utilize models to measure fair value. NiSource uses a similar model to value similar instruments. Valuation models utilize various inputs that include quoted prices for similar assets or liabilities in active markets, quoted prices for identical or similar assets or liabilities in markets that are not active, other observable inputs for the asset or liability and market-corroborated inputs, (i.e., inputs derived principally from or corroborated by observable market data by correlation or other means). Where observable inputs are available for substantially the full term of the asset or liability, the instrument is categorized within Level 2. Certain derivatives trade in less active markets with a lower availability of pricing information and models may be utilized in the valuation. When such inputs have a significant impact on the measurement of fair value, the instrument is categorized within Level 3. Credit risk is considered in the fair value calculation of derivative instruments that are not exchange-traded. Credit exposures are adjusted to reflect collateral agreements which reduce exposures. As of December 31, 2017 and 2016 , there were no material transfers between fair value hierarchies. Additionally, there were no changes in the method or significant assumptions used to estimate the fair value of NiSource’s financial instruments. NiSource has entered into forward-starting interest rate swaps to hedge the interest rate risk on coupon payments of forecasted issuances of long-term debt. These derivatives are designated as cash flow hedges. Credit risk is considered in the fair value calculation of each agreement. As they are based on observable data and valuations of similar instruments, the hedges are categorized within Level 2 of the fair value hierarchy. There was no exchange of premium at the initial date of the swaps and treasury lock agreements, and NiSource can settle the contracts at any time. For additional information see Note 9 , "Risk Management Activities." NIPSCO has entered into long-term forward natural gas purchase instruments that range from five to ten years to lock in a fixed price for its natural gas customers. NiSource values these contracts using a pricing model that incorporates market-based information when available, as these instruments trade less frequently and are classified within Level 2 of the fair value hierarchy. For additional information see Note 9 , “Risk Management Activities.” Available-for-sale securities are investments pledged as collateral for trust accounts related to NiSource’s wholly-owned insurance company. Available-for-sale securities are included within “Other investments” in the Consolidated Balance Sheets. NiSource values U.S. Treasury, corporate and mortgage-backed securities using a matrix pricing model that incorporates market-based information. These securities trade less frequently and are classified within Level 2. Total unrealized gains and losses from available-for-sale securities are included in other comprehensive income. The amortized cost, gross unrealized gains and losses and fair value of available-for-sale securities at December 31, 2017 and 2016 were: December 31, 2017 (in millions) Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Available-for-sale securities U.S. Treasury debt securities $ 26.9 $ — $ (0.1 ) $ 26.8 Corporate/Other debt securities 106.8 0.9 (0.6 ) 107.1 Total $ 133.7 $ 0.9 $ (0.7 ) $ 133.9 December 31, 2016 (in millions) Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Available-for-sale securities U.S. Treasury debt securities $ 35.0 $ 0.1 $ (0.6 ) $ 34.5 Corporate/Other debt securities 98.7 0.3 (2.0 ) 97.0 Total $ 133.7 $ 0.4 $ (2.6 ) $ 131.5 Realized gains and losses on available-for-sale securities were immaterial for the year-ended December 31, 2017 and 2016. The cost of maturities sold is based upon specific identification. At December 31, 2017 , approximately $13.7 million of U.S. Treasury debt securities and approximately $ 2.9 million of Corporate/Other debt securities have maturities of less than a year. There are no material items in the fair value reconciliation of Level 3 assets and liabilities measured at fair value on a recurring basis for the years ended December 31, 2017 and 2016 . Non-recurring Fair Value Measurements . There were no significant non-recurring fair value measurements recorded during the twelve months ended December 31, 2017 . B. Other Fair Value Disclosures for Financial Instruments . The carrying amount of cash and cash equivalents, restricted cash, notes receivable, customer deposits and short-term borrowings is a reasonable estimate of fair value due to their liquid or short-term nature. NiSource’s long-term borrowings are recorded at historical amounts. The following method and assumptions were used to estimate the fair value of each class of financial instruments. Long-term debt . The fair values of outstanding long-term debt is estimated based on the quoted market prices for the same or similar securities. Certain premium costs associated with the early settlement of long-term debt are not taken into consideration in determining fair value. These fair value measurements are classified within Level 2 of the fair value hierarchy. For the years ended December 31, 2017 and 2016 , there was no change in the method or significant assumptions used to estimate the fair value of long-term debt. The carrying amount and estimated fair values of these financial instruments were as follows: At December 31, (in millions) Carrying Amount 2017 Estimated Fair Value 2017 Carrying Amount 2016 Estimated Fair Value 2016 Long-term debt (including current portion) $ 7,796.5 $ 8,603.4 $ 6,421.3 $ 7,064.1

Transfers Of Financial Assets

Transfers Of Financial Assets	12 Months Ended
	Dec. 31, 2017
Transfers and Servicing [Abstract]
Transfers Of Financial Assets	Transfers of Financial Assets Columbia of Ohio, NIPSCO and Columbia of Pennsylvania each maintain a receivables agreement whereby they transfer their customer accounts receivables to third party financial institutions through wholly-owned and consolidated special purpose entities. The three agreements expire between March 2018 and October 2018 and may be further extended if mutually agreed to by the parties thereto. All receivables transferred to third parties are valued at face value, which approximates fair value due to their short-term nature. The amount of the undivided percentage ownership interest in the accounts receivables transferred is determined in part by required loss reserves under the agreements. Transfers of accounts receivable are accounted for as secured borrowings resulting in the recognition of short-term borrowings on the Consolidated Balance Sheets. As of December 31, 2017 , the maximum amount of debt that could be recognized related to NiSource’s accounts receivable programs is $375.0 million . The following table reflects the gross receivables balance and net receivables transferred as well as short-term borrowings related to the securitization transactions as of December 31, 2017 and 2016 : (in millions) December 31, December 31, Gross Receivables $ 635.3 $ 618.3 Less: Receivables not transferred 298.6 308.3 Net receivables transferred $ 336.7 $ 310.0 Short-term debt due to asset securitization $ 336.7 $ 310.0 During 2017 and 2016 , $26.7 million and $64.0 million , respectively, was recorded as cash flows from financing activities related to the change in short-term borrowings due to securitization transactions. Fees associated with the securitization transactions were $2.5 million , $2.3 million and $2.5 million for the years ended December 31, 2017 , 2016 and 2015, respectively. NiSource remains responsible for collecting on the receivables securitized and the receivables cannot be transferred to another party.

Other Commitments And Contingen

Other Commitments And Contingencies	12 Months Ended
	Dec. 31, 2017
Commitments and Contingencies Disclosure [Abstract]
Other Commitments And Contingencies	Other Commitments and Contingencies A. Contractual Obligations . NiSource has certain contractual obligations requiring payments at specified periods. The obligations include long-term debt, lease obligations, energy commodity contracts and obligations for various services including pipeline capacity and outsourcing of IT services. The total contractual obligations in existence at December 31, 2017 and their maturities were: (in millions) Total 2018 2019 2020 2021 2022 After Long-term debt (1) $ 7,714.9 $ 275.1 296.1 $ 296.1 $ 325.1 $ 63.6 $ 710.0 $ 6,045.0 Capital leases (2) 254.4 18.1 15.7 15.4 15.5 15.5 174.2 Interest payments on long-term debt 6,701.2 364.4 344.4 334.6 316.8 307.7 5,033.3 Operating leases (3) 57.2 13.8 10.2 7.3 6.2 4.4 15.3 Energy commodity contracts 216.7 102.5 57.3 56.9 — — — Service obligations: Pipeline service obligations 2,649.9 538.9 520.5 390.7 344.7 331.0 524.1 IT service obligations 311.5 88.3 71.5 63.5 50.7 37.5 — Other service obligations 178.2 48.3 43.3 43.3 43.3 — — Other liabilities 28.7 28.7 — — — — — Total contractual obligations $ 18,112.7 $ 1,478.1 $ 1,359.0 $ 1,236.8 $ 840.8 $ 1,406.1 $ 11,791.9 (1) Long-term debt balance excludes unamortized issuance costs and discounts of $71.5 million. (2) Capital lease payments shown above are inclusive of interest totaling $91.9 million. (3) Operating lease balances do not include amounts for fleet leases that can be renewed beyond the initial lease term. The Company anticipates renewing the leases beyond the initial term, but the anticipated payments associated with the renewals do not meet the definition of expected minimum lease payments and therefore are not included above. Expected payments are $29.3 million in 2018, $27.5 million in 2019, $19.7 million in 2020, $13.9 million in 2021, $9.6 million in 2022 and $7.4 million thereafter. Operating and Capital Lease Commitments. NiSource leases assets in several areas of its operations including fleet vehicles and equipment, rail cars for coal delivery and certain operations centers. Payments made in connection with operating leases were $ 49.5 million in 2017 , $ 52.0 million in 2016 and $ 47.5 million in 2015 , and are primarily charged to operation and maintenance expense as incurred. Capital lease assets and related accumulated depreciation included in the Consolidated Balance Sheets were $ 171.2 million and $ 32.4 million at December 31, 2017 , and $ 167.0 million and $ 20.6 million at December 31, 2016 , respectively. Included in capital leases are the adjusted payments for the NIPSCO service agreement with Pure Air. Refer to section E, "Other Matters," below for additional information. Purchase and Service Obligations. NiSource has entered into various purchase and service agreements whereby NiSource is contractually obligated to make certain minimum payments in future periods. NiSource’s purchase obligations are for the purchase of physical quantities of natural gas, electricity and coal. NiSource’s service agreements encompass a broad range of business support and maintenance functions which are generally described below. NiSource’s subsidiaries have entered into various energy commodity contracts to purchase physical quantities of natural gas, electricity and coal. These amounts represent minimum quantities of these commodities NiSource is obligated to purchase at both fixed and variable prices. To the extent contractual purchase prices are variable, obligations disclosed in the table above are valued at market prices as of December 31, 2017. In July 2008, the IURC issued an order approving NIPSCO’s purchase power agreements with subsidiaries of Iberdrola Renewables, Buffalo Ridge I LLC and Barton Windpower LLC. These agreements provide NIPSCO the opportunity and obligation to purchase up to 100 mw of wind power generated commencing in early 2009. The contracts extend 15 and 20 years, representing 50 mw of wind power each. No minimum quantities are specified within these agreements due to the variability of electricity generation from wind, so no amounts related to these contracts are included in the table above. Upon any termination of the agreements by NIPSCO for any reason (other than material breach by Buffalo Ridge I LLC or Barton Windpower LLC), NIPSCO may be required to pay a termination charge that could be material depending on the events giving rise to termination and the timing of the termination. NIPSCO began purchasing wind power in April 2009. NiSource has pipeline service agreements that provide for pipeline capacity, transportation and storage services. These agreements, which have expiration dates ranging from 2018 to 2045 , require NiSource to pay fixed monthly charges. NIPSCO has contracts with three major rail operators providing for coal transportation services for which there are certain minimum payments. These service contracts extend for various periods through 2021 . On December 31, 2013, NiSource Corporate Services Company signed a seven-year agreement with IBM to continue to provide business process and support functions to NiSource under a combination of fixed and variable charges, with the variable charges fluctuating based on the actual need for such services. The agreement was effective January 1, 2014 with a commencement date of April 1, 2014. In April 2017, NiSource initiated a process to terminate its agreement with IBM and began negotiating contracts with IT service providers other than IBM. NiSource reached an agreement with IBM resolving all termination issues under the service agreement in the fourth quarter of 2017. Liabilities recorded related to termination charges as of December 31, 2017 are not material to the Consolidated Financial Statements. In May and June 2017, NiSource executed agreements with new IT service providers. The new agreements have terms ending at various dates throughout 2022. Transition of responsibilities from IBM to the new service providers was substantially complete as of the end of 2017. Costs associated with transition activities, including legal and consulting fees, were expensed as incurred. B. Guarantees and Indemnities . As a part of normal business, NiSource and certain subsidiaries enter into various agreements providing financial or performance assurance to third parties on behalf of certain subsidiaries. Such agreements include guarantees and stand-by letters of credit. These agreements are entered into primarily to support or enhance the creditworthiness otherwise attributed to a subsidiary on a stand-alone basis, thereby facilitating the extension of sufficient credit to accomplish the subsidiaries’ intended commercial purposes. At December 31, 2017 and 2016, NiSource had issued stand-by letters of credit of $11.1 million and $14.7 million , respectively, for the benefit of third parties. C. Legal Proceedings . The Company is party to certain claims and legal proceedings arising in the ordinary course of business, none of which is deemed to be individually material at this time. Due to the inherent uncertainty of litigation, there can be no assurance that the resolution of any particular claim or proceeding would not have a material adverse effect on the Company’s results of operations, financial position or liquidity. If one or more of such matters were decided against the Company, the effects could be material to the Company’s results of operations in the period in which the Company would be required to record or adjust the related liability and could also be material to the Company’s cash flows in the periods the Company would be required to pay such liability. D. Environmental Matters . NiSource operations are subject to environmental statutes and regulations related to air quality, water quality, hazardous waste and solid waste. NiSource believes that it is in substantial compliance with the environmental regulations currently applicable to its operations. It is management's continued intent to address environmental issues in cooperation with regulatory authorities in such a manner as to achieve mutually acceptable compliance plans. However, there can be no assurance that fines and penalties will not be incurred. Management expects a significant portion of environmental assessment and remediation costs to be recoverable through rates for certain NiSource companies. As of December 31, 2017 and 2016 , NiSource had recorded a liability of $111.4 million to cover environmental remediation at various sites. The current portion of this liability is included in "Legal and environmental" in the Consolidated Balance Sheets. The noncurrent portion is included in "Other noncurrent liabilities" in the Consolidated Balance Sheets. NiSource recognizes costs associated with environmental remediation obligations when the incurrence of such costs is probable and the amounts can be reasonably estimated. The original estimates for remediation activities may differ materially from the amount ultimately expended. The actual future expenditures depend on many factors, including currently enacted laws and regulations, the nature and extent of impact, the method of remediation and the availability of cost recovery. These expenditures are not currently estimable at some sites. NiSource periodically adjusts its liability as information is collected and estimates become more refined. Electric Operations' compliance estimates disclosed below are reflective of NIPSCO's Integrated Resource Plan submitted to the IURC on November 1, 2016. See section E, "Other Matters," below for additional information. Air The actions listed below could require further reductions in emissions from various emission sources. NiSource will continue to closely monitor developments in these matters. Future legislative and regulatory programs could significantly limit allowed GHG emissions or impose a cost or tax on GHG emissions. Additionally, rules that increase methane leak detection, require emission reductions or impose additional requirements for natural gas facilities could restrict GHG emissions and impose additional costs. NiSource will carefully monitor all GHG reduction proposals and regulations. Clean Power Plan. On October 23, 2015, the EPA issued a final rule to regulate CO 2 emissions from existing fossil-fuel EGUs under section 111(d) of the CAA. The final rule establishes national CO 2 emission-rate standards that are applied to each state’s mix of affected EGUs to establish state-specific emission-rate and mass-emission limits. The final rule requires each state to submit a plan indicating how the state will meet the EPA's emission-rate or mass-emission limit, including possibly imposing reduction obligations on specific units. If a state does not submit a satisfactory plan, the EPA will impose a federal plan on that state. On February 9, 2016, the U.S. Supreme Court stayed implementation of the CPP until litigation is decided on its merits. On October 16, 2017, the EPA published in the Federal Register a Notice of Proposed Rulemaking that would repeal the CPP. The public will have until April 26, 2018 to comment on this proposal, after which time the proposal may become final. On December 28, 2017, in a separate but related action, the EPA published an Advanced Notice of Proposed Rulemaking in the Federal Register to solicit information from the public about a potential future rulemaking to limit greenhouse gas emissions from existing fossil-fuel EGUs. The public will have until February 26, 2018 to comment on the proposal. NIPSCO will continue to monitor this matter and cannot estimate its impact at this time. Should costs be incurred to comply with the CPP, NIPSCO believes such costs will be eligible for recovery through customer rates. Waste CERCLA. NiSource subsidiaries are potentially responsible parties at waste disposal sites under the CERCLA (commonly known as Superfund) and similar state laws. Under CERCLA, each potentially responsible party can be held jointly, severally and strictly liable for the remediation costs as the EPA, or state, can allow the parties to pay for remedial action or perform remedial action themselves and request reimbursement from the potentially responsible parties. NiSource’s affiliates have retained CERCLA environmental liabilities, including remediation liabilities, associated with certain current and former operations. These liabilities are not material to the Consolidated Financial Statements. MGP. A program has been instituted to identify and investigate former MGP sites where Gas Distribution Operations subsidiaries or predecessors may have liability. The program has identified sixty-four such sites where liability is probable. Remedial actions at many of these sites are being overseen by state or federal environmental agencies through consent agreements or voluntary remediation agreements. NiSource utilizes a probabilistic model to estimate its future remediation costs related to its MGP sites. The model was prepared with the assistance of a third party and incorporates NiSource and general industry experience with remediating MGP sites. NiSource completes an annual refresh of the model in the second quarter of each fiscal year. No material changes to the estimated future remediation costs were noted as a result of the refresh completed as of June 30, 2017. The total estimated liability at NiSource related to the facilities subject to remediation was $106.9 million and $105.5 million at December 31, 2017 and 2016 , respectively. The liability represents NiSource’s best estimate of the probable cost to remediate the facilities. NiSource believes that it is reasonably possible that remediation costs could vary by as much as $25 million in addition to the costs noted above. Remediation costs are estimated based on the best available information, applicable remediation standards at the balance sheet date, and experience with similar facilities. CCRs. On April 17, 2015, the EPA issued a final rule for regulation of CCRs. The rule regulates CCRs under the RCRA Subtitle D, which determines them to be nonhazardous. The rule is implemented in phases and requires increased groundwater monitoring, reporting, recordkeeping and posting of related information to the Internet. The rule also establishes requirements related to CCR management and disposal. The rule will allow NIPSCO to continue its byproduct beneficial use program. The publication of the CCR rule resulted in revisions to previously recorded legal obligations associated with the retirement of certain NIPSCO facilities. The actual asset retirement costs related to the CCR rule may vary substantially from the estimates used to record the increased asset retirement obligation due to the uncertainty about the compliance strategies that will be used and the preliminary nature of available data used to estimate costs. In addition, to comply with the rule, NIPSCO will be required to incur future capital expenditures to modify its infrastructure and manage CCRs. Capital compliance costs are currently expected to total approximately $193 million . As allowed by the EPA, NIPSCO will continue to collect data over time to determine the specific compliance solutions and associated costs and, as a result, the actual costs may vary. NIPSCO filed a petition on November 1, 2016 with the IURC seeking approval of the projects and recovery of the costs associated with CCR compliance. On June 9, 2017, NIPSCO filed with the IURC a settlement reached with certain parties regarding the CCR projects and treatment of associated costs. The IURC approved the settlement in an order on December 13, 2017. Water ELG. On November 3, 2015, the EPA issued a final rule to amend the ELG and standards for the Steam Electric Power Generating category. The final rule became effective January 4, 2016. The rule imposes new water treatment and discharge requirements on NIPSCO's electric generating facilities to be applied between 2018 and 2023. On April 25, 2017, the EPA published notice in the Federal Register that the EPA is reconsidering the ELG in response to several petitions for reconsideration. On September 18, 2017, the EPA published notice in the Federal Register their intention to postpone the earliest compliance dates for flue gas desulfurization wastewater and bottom ash transport water requirements to potentially consider revisions to technology and numeric limits achievable. NIPSCO is unable to estimate the impact of the postponement of these compliance dates at this time. Based upon a preliminary engineering study, capital compliance costs are currently expected to cost approximately $170 million . On November 1, 2016, NIPSCO filed a petition with the IURC seeking approval of the projects and recovery of the costs associated with ELG compliance. Given the current postponement of certain compliance dates under the ELG rule, NIPSCO has agreed with the settling parties as part of the settlement agreement discussed in the "CCRs" subsection above, that these ELG projects and related costs would be addressed in a later proceeding. E. Other Matters. NIPSCO 2016 Integrated Resource Plan. Environmental, regulatory and economic factors, including low natural gas prices and aging coal-fired units, have led NIPSCO to pursue modification of its current electric generation supply mix to include less coal-fired generation. Due to enacted CCR and ELG (subsequently postponed) regulations, NIPSCO would expect to have incurred over $1 billion in operating, maintenance, environmental and other costs if the current fleet of coal-fired generating units were to remain operational. On November 1, 2016, NIPSCO submitted its 2016 Integrated Resource Plan with the IURC. The plan evaluated demand-side and supply-side resource alternatives to reliably and cost effectively meet NIPSCO customers' future energy requirements over the ensuing 20 years. The 2016 Integrated Resource Plan indicates that the most viable option for customers and NIPSCO involves the retirement of Bailly Generating Station (Units 7 and 8) as soon as mid-2018 and two units (Units 17 and 18) at the R.M. Schahfer Generating Station by the end of 2023. It is projected over the long term that the cost to customers to retire these units at these dates will be lower than maintaining and upgrading them for continuing generation. NiSource and NIPSCO committed to the retirement of the Bailly Generating Station units in connection with the filing of the 2016 Integrated Resource Plan, pending approval by the MISO. In the fourth quarter of 2016, the MISO approved NIPSCO's plan to retire the Bailly Generating Station units by May 31, 2018. In accordance with ASC 980-360, the remaining net book value of the Bailly Generating Station units was reclassified from "Net utility plant" to "Other property, at cost, less accumulated depreciation" on the Consolidated Balance Sheets. In connection with the MISO's approval of NIPSCO's planned retirement of the Bailly Generating Station units, NiSource recorded $22.1 million of plant retirement-related charges in the fourth quarter of 2016. These charges were comprised of contract termination charges related to NIPSCO's capital lease with Pure Air (discussed further below), voluntary employee severance benefits, and write downs of certain materials and supplies inventory balances. These charges are presented within "Operation and maintenance" on the Statements of Consolidated Income. On February 1, 2018, as previously approved by the MISO, NIPSCO commenced a four-month outage of Bailly Generating Station Unit 8 in order to begin work on converting the unit to a synchronous condenser (a piece of equipment designed to maintain voltage to ensure continued reliability on the transmission system). Refer to Note 25, "Subsequent Event," for additional information. NIPSCO Pure Air. NIPSCO has a service agreement with Pure Air, a general partnership between Air Products and Chemicals, Inc. and First Air Partners LP, under which Pure Air provides scrubber services to reduce sulfur dioxide emissions for Units 7 and 8 at the Bailly Generating Station. Services under this contract commenced on July 1, 1992 and expired on June 30, 2012. The agreement was renewed effective July 1, 2012 for ten years requiring NIPSCO to pay for the services under a combination of fixed and variable charges. NiSource has made an exhaustive effort to obtain information needed from Pure Air to determine the status of Pure Air as a VIE. However, NIPSCO has not been able to obtain this information and, as a result, it is unclear whether Pure Air is a VIE and if NIPSCO is the primary beneficiary. NIPSCO will continue to request the information required to determine whether Pure Air is a VIE. NIPSCO has no exposure to loss related to the service agreement with Pure Air and payments under this agreement were $ 22.0 million and $ 21.7 million for the years ended December 31, 2017 and 2016 , respectively. In accordance with GAAP, the renewed agreement was evaluated to determine whether the arrangement qualifies as a lease. Based on the terms of the agreement, the arrangement qualified for capital lease accounting. As the effective date of the new agreement was July 1, 2012, NiSource capitalized this lease beginning in the third quarter of 2012. As further discussed above in this Note 18 under the heading "NIPSCO 2016 Integrated Resource Plan," NIPSCO plans to retire the generation station units serviced by Pure Air by May 31, 2018. In December 2016, as allowed by the provisions of the service agreement, NIPSCO provided Pure Air formal notice of intent to terminate the service agreement, effective May 31, 2018. Providing this notice to Pure Air triggered a contract termination liability of $16 million which was recorded in fourth quarter of 2016. This expense was included as part of the plant retirement-related charges discussed above. Payment of this liability is not due until NIPSCO ceases use of the scrubber services. The liability is presented in "Other accruals" on the Consolidated Balance Sheets. In addition, NIPSCO remeasured the remaining capital lease asset and obligation to reflect the change in estimated remaining minimum lease payments. This remeasurement was a non-cash transaction that had no impact on the Statements of Consolidated Income.

Accumulated Other Comprehensive

Accumulated Other Comprehensive Loss	12 Months Ended
	Dec. 31, 2017
Components of Accumulated Other Comprehensive Income (Loss) [Abstract]
Accumulated Other Comprehensive Loss	Accumulated Other Comprehensive Loss The following table displays the activity of Accumulated Other Comprehensive Loss, net of tax: (in millions) Gains and Losses on Securities (1) Gains and Losses on Cash Flow Hedges (1) Pension and OPEB Items (1) Accumulated Other Comprehensive Loss (1) Balance as of January 1, 2015 $ 0.3 $ (23.6 ) $ (27.3 ) $ (50.6 ) Other comprehensive loss before reclassifications (0.5 ) (11.0 ) (5.0 ) (16.5 ) Amounts reclassified from accumulated other comprehensive loss (0.3 ) 3.2 2.6 5.5 Net current-period other comprehensive loss (0.8 ) (7.8 ) (2.4 ) (11.0 ) Allocation of AOCI to noncontrolling interest — 2.0 — 2.0 Distribution of CPG to shareholders (Refer to Note 3, "Discontinued Operations") — 13.9 10.6 24.5 Balance as of December 31, 2015 $ (0.5 ) $ (15.5 ) $ (19.1 ) $ (35.1 ) Other comprehensive income before reclassifications — 7.1 0.5 7.6 Amounts reclassified from accumulated other comprehensive loss (0.1 ) 1.5 1.0 2.4 Net current-period other comprehensive income (loss) (0.1 ) 8.6 1.5 10.0 Balance as of December 31, 2016 $ (0.6 ) $ (6.9 ) $ (17.6 ) $ (25.1 ) Other comprehensive income (loss) before reclassifications 0.6 (24.2 ) 1.9 (21.7 ) Amounts reclassified from accumulated other comprehensive loss 0.2 1.7 1.5 3.4 Net current-period other comprehensive income (loss) 0.8 (22.5 ) 3.4 (18.3 ) Balance as of December 31, 2017 $ 0.2 $ (29.4 ) $ (14.2 ) $ (43.4 ) (1) All amounts are net of tax. Amounts in parentheses indicate debits.

Other, Net

Other, Net	12 Months Ended
	Dec. 31, 2017
Other Nonoperating Income (Expense) [Abstract]
Other, Net	Other, Net Year Ended December 31, (in millions) 2017 2016 2015 Interest Income $ 4.6 $ 3.4 $ 0.8 AFUDC Equity 12.6 11.6 11.5 Charitable Contributions (19.9 ) (4.5 ) (4.8 ) Miscellaneous (1) (0.1 ) (9.0 ) 9.9 Total Other, net $ (2.8 ) $ 1.5 $ 17.4 (1) Miscellaneous in 2016 primarily consists of a TUA-related charge of $8.6 million to reflect the estimated amount owed to the upgrade sponsors for the portion of the multiplier previously collected for taxes. In 2015, Miscellaneous primarily consisted of TUA income.

Interest Expense, Net

Interest Expense, Net	12 Months Ended
	Dec. 31, 2017
Interest Expense [Abstract]
Interest Expense, Net	Interest Expense, Net Year Ended December 31, (in millions) 2017 2016 2015 Interest on long-term debt $ 354.8 $ 352.3 $ 377.5 Interest on short-term borrowings 14.9 9.2 2.2 Debt discount/cost amortization 7.2 7.6 8.7 Accounts receivable securitization fees 2.5 2.3 2.5 Allowance for borrowed funds used and interest capitalized during construction (6.2 ) (5.6 ) (5.4 ) Debt-based post-in-service carrying charges (36.4 ) (35.1 ) (21.4 ) Other 16.4 18.8 16.1 Total Interest Expense, net $ 353.2 $ 349.5 $ 380.2

Segments Of Business

Segments Of Business	12 Months Ended
	Dec. 31, 2017
Segment Reporting [Abstract]
Segments Of Business	Segments of Business At December 31, 2017 , NiSource’s operations are divided into two primary reportable segments. The Gas Distribution Operations segment provides natural gas service and transportation for residential, commercial and industrial customers in Ohio, Pennsylvania, Virginia, Kentucky, Maryland, Indiana and Massachusetts. The Electric Operations segment provides electric service in 20 counties in the northern part of Indiana. The following table provides information about business segments. NiSource uses operating income as its primary measurement for each of the reported segments and makes decisions on finance, dividends and taxes at the corporate level on a consolidated basis. Segment revenues include intersegment sales to affiliated subsidiaries, which are eliminated in consolidation. Affiliated sales are recognized on the basis of prevailing market, regulated prices or at levels provided for under contractual agreements. Operating income is derived from revenues and expenses directly associated with each segment. Year Ended December 31, (in millions) 2017 2016 2015 Operating Revenues Gas Distribution Operations Unaffiliated $ 3,087.9 $ 2,818.2 $ 3,068.7 Intersegment 14.2 12.4 0.4 Total 3,102.1 2,830.6 3,069.1 Electric Operations Unaffiliated 1,785.7 1,660.8 1,573.6 Intersegment 0.8 0.8 0.8 Total 1,786.5 1,661.6 1,574.4 Corporate and Other Unaffiliated 1.0 13.5 9.5 Intersegment 510.8 413.3 396.4 Total 511.8 426.8 405.9 Eliminations (525.8 ) (426.5 ) (397.6 ) Consolidated Operating Revenues $ 4,874.6 $ 4,492.5 $ 4,651.8 Year Ended December 31, (in millions) 2017 2016 2015 Operating Income (Loss) Gas Distribution Operations $ 545.6 $ 574.0 $ 555.8 Electric Operations 364.8 291.4 264.4 Corporate and Other 0.2 (7.2 ) (20.3 ) Consolidated Operating Income $ 910.6 $ 858.2 $ 799.9 Depreciation and Amortization Gas Distribution Operations $ 269.3 $ 252.9 $ 232.6 Electric Operations 277.8 274.5 267.7 Corporate and Other 23.2 19.7 24.1 Consolidated Depreciation and Amortization $ 570.3 $ 547.1 $ 524.4 Assets Gas Distribution Operations $ 12,048.8 $ 11,096.4 $ 10,094.5 Electric Operations 5,478.6 5,233.3 5,265.3 Corporate and Other 2,434.3 2,362.2 2,132.7 Consolidated Assets $ 19,961.7 $ 18,691.9 $ 17,492.5 Capital Expenditures (1) Gas Distribution Operations $ 1,125.6 $ 1,054.4 $ 917.0 Electric Operations 592.4 420.6 400.3 Corporate and Other 35.8 15.4 50.2 Consolidated Capital Expenditures $ 1,753.8 $ 1,490.4 $ 1,367.5 (1 Amounts differ from those presented on the Statements of Consolidated Cash Flows primarily due to the inclusion of capital expenditures included in current liabilities and AFUDC Equity.

Quarterly Financial Data

Quarterly Financial Data	12 Months Ended
	Dec. 31, 2017
Quarterly Financial Data [Abstract]
Quarterly Financial Data (Unaudited)	Quarterly Financial Data (Unaudited) Quarterly financial data does not always reveal the trend of NiSource’s business operations due to nonrecurring items and seasonal weather patterns, which affect earnings and related components of revenue and operating income. (in millions, except per share data) First Quarter Second Quarter (1) Third Quarter Fourth Quarter (2) 2017 Operating Revenues $ 1,598.6 $ 990.7 $ 917.0 $ 1,368.3 Operating Income 416.5 124.5 99.6 270.0 Income (Loss) from Continuing Operations 211.3 (44.3 ) 14.0 (52.4 ) Loss from Discontinued Operations - net of taxes — (0.1 ) — — Net Income (Loss) 211.3 (44.4 ) 14.0 (52.4 ) Basic Earnings (Loss) Per Share Continuing Operations 0.65 (0.14 ) 0.04 (0.16 ) Discontinued Operations — — — — Basic Earnings (Loss) Per Share $ 0.65 $ (0.14 ) $ 0.04 $ (0.16 ) Diluted Earnings (Loss) Per Share Continuing Operations 0.65 (0.14 ) 0.04 (0.16 ) Discontinued Operations — — — — Diluted Earnings (Loss) Per Share $ 0.65 $ (0.14 ) $ 0.04 $ (0.16 ) 2016 Operating Revenues $ 1,436.6 $ 897.6 $ 861.3 $ 1,297.0 Operating Income 381.4 138.2 113.7 224.9 Income from Continuing Operations 186.6 29.0 23.7 88.8 Results from Discontinued Operations - net of taxes — (0.1 ) 3.5 — Net Income 186.6 28.9 27.2 88.8 Basic Earnings Per Share Continuing Operations 0.58 0.09 0.07 0.28 Discontinued Operations — — 0.01 — Basic Earnings Per Share $ 0.58 $ 0.09 $ 0.08 $ 0.28 Diluted Earnings Per Share Continuing Operations 0.58 0.09 0.07 0.27 Discontinued Operations — — 0.01 — Diluted Earnings Per Share $ 0.58 $ 0.09 $ 0.08 $ 0.27 (1) The decrease in income from continuing operations during the second quarter of 2017 relates primarily to a $111.5 million loss on early extinguishment of long-term debt, primarily attributable to early redemption premiums. See Note 14, "Long-Term Debt," for additional information. (2) The decrease in income from continuing operations during the fourth quarter of 2017 was due primarily to increased tax expense as a result of the impact of implementing the provisions of the TCJA. See Note 10, "Income Taxes," for additional information.

Supplemental Cash Flow Informat

Supplemental Cash Flow Information	12 Months Ended
	Dec. 31, 2017
Supplemental Cash Flow Information [Abstract]
Supplemental Cash Flow Information	Supplemental Cash Flow Information The following table provides additional information regarding NiSource’s Consolidated Statements of Cash Flows for the years ended December 31, 2017 , 2016 and 2015 : Year Ended December 31, (in millions) 2017 2016 2015 Supplemental Disclosures of Cash Flow Information Non-cash transactions: Capital expenditures included in current liabilities $ 173.0 $ 125.3 $ 121.6 Assets acquired under a capital lease 11.5 4.0 47.5 Schedule of interest and income taxes paid: Cash paid for interest, net of interest capitalized amounts $ 339.9 $ 337.8 $ 390.4 Cash paid for income taxes, net of refunds 5.5 8.0 21.3

Subsequent Event

Subsequent Event	12 Months Ended
	Dec. 31, 2017
Subsequent Events [Abstract]
Subsequent Events	Subsequent Event Unit 8 Outage at Bailly Generating Station. On February 1, 2018, as previously approved by the MISO, NIPSCO commenced a four-month outage of Bailly Generating Station Unit 8 in order to begin work on converting the unit to a synchronous condenser (a piece of equipment designed to maintain voltage to ensure continued reliability on the transmission system). Approximately $15 million of net book value of Unit 8 remained in “Net Utility Plant” as it is expected to remain used and useful upon completion of the synchronous condenser, while the remaining net book value of approximately $143 million was reclassified to “Regulatory assets (noncurrent)” on the Consolidated Balance Sheets. These amounts continue to be amortized at a rate consistent with their inclusion in customer rates. NIPSCO expects to complete the retirement of Units 7 and 8 by May 31, 2018. Refer to Note 18-E, “Other Matters,” for additional information on the planned retirement of Units 7 and 8 at Bailly Generation Station.

Valuation And Qualifying Accoun

Valuation And Qualifying Accounts	12 Months Ended
	Dec. 31, 2017
Valuation and Qualifying Accounts [Abstract]
Schedule of Valuation and Qualifying Accounts Disclosure	NISOURCE INC. SCHEDULE II – VALUATION AND QUALIFYING ACCOUNTS Twelve months ended December 31, 2017 Additions ($ in millions) Balance Jan. 1, 2017 Charged to Costs and Expenses Charged to Other Account (1) Deductions for Purposes for which Reserves were Created Balance Dec. 31, 2017 Reserves Deducted in Consolidated Balance Sheet from Assets to Which They Apply: Reserve for accounts receivable $ 23.3 $ 14.8 $ 39.1 $ 58.9 $ 18.3 Reserve for other investments 3.0 — — — 3.0 Twelve months ended December 31, 2016 Additions ($ in millions) Balance Charged to Costs and Expenses Charged to Other Account (1) Deductions for Purposes for which Reserves were Created Balance Reserves Deducted in Consolidated Balance Sheet from Assets to Which They Apply: Reserve for accounts receivable $ 20.3 $ 19.7 $ 48.5 $ 65.2 $ 23.3 Reserve for other investments 3.0 — — — 3.0 Twelve months ended December 31, 2015 Additions ($ in millions) Balance Charged to Costs and Expenses Charged to Other Account (1) Deductions for Purposes for which Reserves were Created Balance Reserves Deducted in Consolidated Balance Sheet from Assets to Which They Apply: Reserve for accounts receivable $ 24.9 $ 22.5 $ 56.7 $ 83.8 $ 20.3 Reserve for other investments 3.0 — — — 3.0 (1) Charged to Other Accounts reflects the deferral of bad debt expense to a regulatory asset.

Nature Of Operations And Summ36

Nature Of Operations And Summary Of Significant Accounting Policies (Policy)	12 Months Ended
	Dec. 31, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Company Structure And Principles Of Consolidation	Company Structure and Principles of Consolidation. NiSource, a Delaware corporation headquartered in Merrillville, Indiana, is an energy holding company whose subsidiaries are fully regulated natural gas and electric utility companies serving approximately 3.9 million customers in seven states. NiSource generates substantially all of its operating income through these rate-regulated businesses. The consolidated financial statements include the accounts of NiSource and its majority-owned subsidiaries after the elimination of all intercompany accounts and transactions.
Use Of Estimates	Use of Estimates. The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements, and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates.
Cash, Cash Equivalents, And Restricted Cash	Cash, Cash Equivalents and Restricted Cash. NiSource considers all highly liquid investments with original maturities of three months or less to be cash equivalents. NiSource reports amounts deposited in brokerage accounts for margin requirements as restricted cash. In addition, NiSource has amounts deposited in trust to satisfy requirements for the provision of various property, liability, workers compensation, and long-term disability insurance, which is classified as restricted cash and disclosed as cash and cash equivalents on the Statements of Consolidated Cash Flows.
Accounts Receivable And Unbilled Revenue	Accounts Receivable and Unbilled Revenue. Accounts receivable on the Consolidated Balance Sheets includes both billed and unbilled amounts. Unbilled amounts of accounts receivable relate to a portion of a customer’s consumption of gas or electricity from the date of the last cycle billing date through the last day of the month (balance sheet date). Factors taken into consideration when estimating unbilled revenue include historical usage, customer rates and weather. Accounts receivable fluctuates from year to year depending in large part on weather impacts and price volatility. NiSource's accounts receivable on the Consolidated Balance Sheets include unbilled revenue, less reserves, in the amounts of $359.4 million and $329.7 million as of December 31, 2017 and 2016 , respectively. The reserve for uncollectible receivables is NiSource’s best estimate of the amount of probable credit losses in the existing accounts receivable. NiSource determined the reserve based on historical experience and in consideration of current market conditions. Account balances are charged against the allowance when it is anticipated the receivable will not be recovered.
Investments In Debt And Equity Securities	Investments in Debt Securities. NiSource’s investments in debt securities are carried at fair value and are designated as available-for-sale. These investments are included within “Other investments” on the Consolidated Balance Sheets. Unrealized gains and losses, net of deferred income taxes, are reflected as accumulated other comprehensive income. These investments are monitored for other than temporary declines in market value. Realized gains and losses and permanent impairments are reflected in the Statements of Consolidated Income. No material impairment charges were recorded for the years ended December 31, 2017 , 2016 or 2015 . Refer to Note 16 , "Fair Value," for additional information.
Basis Of Accounting For Rate-Regulated Subsidiaries	Basis of Accounting for Rate-Regulated Subsidiaries. Rate-regulated subsidiaries account for and report assets and liabilities consistent with the economic effect of the way in which regulators establish rates, if the rates established are designed to recover the costs of providing the regulated service and it is probable that such rates can be charged and collected. Certain expenses and credits subject to utility regulation or rate determination normally reflected in income are deferred on the Consolidated Balance Sheets and are later recognized in income as the related amounts are included in customer rates and recovered from or refunded to customers. In the event that regulation significantly changes the opportunity for NiSource to recover its costs in the future, all or a portion of NiSource’s regulated operations may no longer meet the criteria for regulatory accounting. In such an event, a write-down of all or a portion of NiSource’s existing regulatory assets and liabilities could result. If transition cost recovery was approved by the appropriate regulatory bodies that would meet the requirements under GAAP for continued accounting as regulatory assets and liabilities during such recovery period, the regulatory assets and liabilities would be reported at the recoverable amounts. If unable to continue to apply the provisions of regulatory accounting, NiSource would be required to apply the provisions of ASC 980-20, Discontinuation of Rate-Regulated Accounting . In management’s opinion, NiSource’s regulated subsidiaries will be subject to regulatory accounting for the foreseeable future. Refer to Note 8 , "Regulatory Matters," for additional information.
Utility Plant And Other Property And Related Depreciation And Maintenance	Plant and Other Property and Related Depreciation and Maintenance. Property, plant and equipment (principally utility plant) is stated at cost. The rate-regulated subsidiaries record depreciation using composite rates on a straight-line basis over the remaining service lives of the electric, gas and common properties as approved by the appropriate regulators. Non-utility property is generally depreciated on a straight-line basis over the life of the associated asset. Refer to Note 5 , "Property, Plant and Equipment," for additional information related to depreciation expense at Units 7 and 8 at Bailly Generating Station. For rate-regulated companies, AFUDC is capitalized on all classes of property except organization costs, land, autos, office equipment, tools and other general property purchases. The allowance is applied to construction costs for that period of time between the date of the expenditure and the date on which such project is placed in service. The pre-tax rate for AFUDC was 4.0% in 2017 , 4.5% in 2016 and 4.7% in 2015 . Generally, NiSource’s subsidiaries follow the practice of charging maintenance and repairs, including the cost of removal of minor items of property, to expense as incurred. When NiSource’s subsidiaries retire regulated property, plant and equipment, original cost plus the cost of retirement, less salvage value, is charged to accumulated depreciation. However, when it becomes probable a regulated asset will be retired substantially in advance of its original expected useful life or is abandoned, the cost of the asset and the corresponding accumulated depreciation is recognized as a separate asset. If the asset is still in operation, the net amount is classified as "Other property, at cost, less accumulated depreciation" on the Consolidated Balance Sheets. If the asset is no longer operating, the net amount is classified in "Regulatory assets" on the Consolidated Balance Sheets. If NiSource is able to recover a full return of and on investment, the carrying value of the asset is based on historical cost. If NiSource is not able to recover a full return on investment, a loss on impairment is recognized to the extent the net book value of the asset exceeds the present value of future revenues discounted at the incremental borrowing rate. When NiSource’s subsidiaries sell entire regulated operating units, or retire or sell nonregulated properties, the original cost and accumulated depreciation and amortization balances are removed from "Property, Plant and Equipment" on the Consolidated Balance Sheets. Any gain or loss is recorded in earnings, unless otherwise required by the applicable regulatory body. Refer to Note 5 , "Property, Plant and Equipment," for further information. External and internal costs associated with computer software developed for internal use are capitalized. Capitalization of such costs commences upon the completion of the preliminary stage of each project. Once the installed software is ready for its intended use, such capitalized costs are amortized on a straight-line basis generally over a period of five years, except for certain significant enterprise-wide technology investments which are amortized over a ten-year period.
Goodwill And Other Intangible Assets	Goodwill and Other Intangible Assets. Substantially all of NiSource's goodwill relates to the excess of cost over the fair value of the net assets acquired in the Columbia acquisition on November 1, 2000. NiSource tests its goodwill for impairment annually as of May 1, or more frequently if events and circumstances indicate that goodwill might be impaired. Fair value of NiSource's reporting units is determined using a combination of income and market approaches. NiSource has other intangible assets consisting primarily of franchise rights apart from goodwill that were identified as part of the purchase price allocations associated with the acquisition of Columbia of Massachusetts which is being amortized on a straight-line basis over forty years from the date of acquisition. See Note 6 , "Goodwill and Other Intangible Assets," for additional information
Revenue Recognition	Revenue Recognition. Revenue is recorded as products and services are delivered. Utility revenues are billed to customers monthly on a cycle basis. Revenues are recorded on the accrual basis and also include estimates for electricity and gas delivered but not billed. The accruals for unbilled revenues are reversed in the subsequent accounting period when meters are actually read and customers are billed. On occasion, NiSource's regulated subsidiaries are permitted to implement new rates that have not been formally approved by their state regulatory commissions, which are subject to refund. As permitted by accounting principles generally accepted in the United States, each regulated subsidiary recognizes this revenue and establishes a reserve for amounts that could be refunded based on its experience for the jurisdiction in which the rates were implemented. In connection with such revenues, estimated rate refund liabilities are recorded which reflect management’s current judgment of the ultimate outcomes of the proceedings. No provisions are made when, in the opinion of management, the facts and circumstances preclude a reasonable estimate of the outcome.
Accounts Receivable Transfer Program	Accounts Receivable Transfer Program. Certain of NiSource’s subsidiaries have agreements with third parties to sell certain accounts receivable without recourse. These transfers of accounts receivable are accounted for as secured borrowings. The entire gross receivables balance remains on the December 31, 2017 and 2016 Consolidated Balance Sheets and short-term debt is recorded in the amount of proceeds received from the purchasers involved in the transactions. Refer to Note 17 , "Transfers of Financial Assets," for further information.
Fuel Adjustment Clause	Gas Cost and Fuel Adjustment Clause. NiSource’s regulated subsidiaries defer most differences between gas and fuel purchase costs and the recovery of such costs in revenues, and adjust future billings for such deferrals on a basis consistent with applicable state-approved tariff provisions. These deferred balances are recorded as "Regulatory assets" or "Regulatory liabilities," as appropriate, on the Consolidated Balance Sheets. Refer to Note 8 , "Regulatory Matters," for additional information.
Gas Inventory	Inventory. Both the LIFO inventory methodology and the weighted average cost methodology are used to value natural gas in storage, as approved by regulators for all of NiSource’s regulated subsidiaries. Inventory valued using LIFO was $45.5 million and $46.1 million at December 31, 2017 and 2016 , respectively. Based on the average cost of gas using the LIFO method, the estimated replacement cost of gas in storage was less than the stated LIFO cost by $17.4 million and $9.4 million at December 31, 2017 and 2016 , respectively. Gas inventory valued using the weighted average cost methodology was $239.6 million at December 31, 2017 and $233.8 million at December 31, 2016 . Electric production fuel is valued using the weighted average cost inventory methodology, as approved by NIPSCO's regulator. Materials and supplies are valued using the weighted average cost inventory methodology.
Accounting For Exchange And Balancing Arrangements Of Natural Gas	Accounting for Exchange and Balancing Arrangements of Natural Gas. NiSource’s Gas Distribution Operations segment enters into balancing and exchange arrangements of natural gas as part of its operations and off-system sales programs. NiSource records a receivable or payable for any of its respective cumulative gas imbalances, as well as for any gas inventory borrowed or lent under a Gas Distribution Operations exchange agreement. Exchange gas is valued based on individual regulatory jurisdiction requirements (for example, historical spot rate, spot at the beginning of the month). These receivables and payables are recorded as “Exchange gas receivable” or “Exchange gas payable” on NiSource’s Consolidated Balance Sheets, as appropriate.
Accounting For Risk Management Activities	Accounting for Risk Management Activities. NiSource accounts for its derivatives and hedging activities in accordance with ASC 815. NiSource recognizes all derivatives as either assets or liabilities on the Consolidated Balance Sheets at fair value, unless such contracts are exempted as a normal purchase normal sale under the provisions of the standard. The accounting for changes in the fair value of a derivative depends on the intended use of the derivative and resulting designation. NiSource has elected not to net fair value amounts for any of its derivative instruments or the fair value amounts recognized for its right to receive cash collateral or obligation to pay cash collateral arising from those derivative instruments recognized at fair value, which are executed with the same counterparty under a master netting arrangement. See Note 9 , "Risk Management Activities," for additional information.
Income Taxes And Investment Tax Credits	Income Taxes and Investment Tax Credits. NiSource records income taxes to recognize full interperiod tax allocations. Under the asset and liability method, deferred income taxes are provided for the tax consequences of temporary differences by applying enacted statutory tax rates applicable to future years to differences between the financial statement carrying amount and the tax basis of existing assets and liabilities. Previously recorded investment tax credits of the regulated subsidiaries were deferred on the balance sheet and are being amortized to book income over the regulatory life of the related properties to conform to regulatory policy. To the extent certain deferred income taxes of the regulated companies are recoverable or payable through future rates, regulatory assets and liabilities have been established. Regulatory assets for income taxes are primarily attributable to property-related tax timing differences for which deferred taxes had not been provided in the past, when regulators did not recognize such taxes as costs in the ratemaking process. Regulatory liabilities for income taxes are primarily attributable to the regulated companies’ obligation to refund to ratepayers deferred income taxes provided at rates higher than the current Federal income tax rate. Such property-related amounts are credited to ratepayers using either the average rate assumption method or the reverse South Georgia method. Non property-related amounts are credited to ratepayers consistent with state utility commission direction. Pursuant to the U.S. Internal Revenue Code and relevant state taxing authorities, NiSource and its subsidiaries file consolidated income tax returns for Federal and certain state jurisdictions. NiSource and its subsidiaries are parties to an agreement (the “Intercompany Income Tax Allocation Agreement”) that provides for the allocation of consolidated tax liabilities. The Intercompany Income Tax Allocation Agreement generally provides that each party is allocated an amount of tax similar to that which would be owed had the party been separately subject to tax.
Environmental Expenditures	Environmental Expenditures. NiSource accrues for costs associated with environmental remediation obligations when the incurrence of such costs is probable and the amounts can be reasonably estimated, regardless of when the expenditures are actually made. The undiscounted estimated future expenditures are based on currently enacted laws and regulations, existing technology and estimated site-specific costs where assumptions may be made about the nature and extent of site contamination, the extent of cleanup efforts, costs of alternative cleanup methods and other variables. The liability is adjusted as further information is discovered or circumstances change. The reserves for estimated environmental expenditures are recorded on the Consolidated Balance Sheets in “Legal and environmental” for short-term portions of these liabilities and “Other noncurrent liabilities” for the respective long-term portions of these liabilities. Rate-regulated subsidiaries applying regulatory accounting establish regulatory assets on the Consolidated Balance Sheets to the extent that future recovery of environmental remediation costs is probable through the regulatory process. Refer to Note 18 , "Other Commitments and Contingencies," for further information.
Excise Taxes	Excise Taxes. NiSource accounts for excise taxes that are customer liabilities by separately stating on its invoices the tax to its customers and recording amounts invoiced as liabilities payable to the applicable taxing jurisdiction. Such balances are presented within "Other accruals" on the Consolidated Balance Sheets. These types of taxes collected from customers, comprised largely of sales taxes, are presented on a net basis affecting neither revenues nor cost of sales. NiSource accounts for excise taxes for which it is liable by recording a liability for the expected tax with a corresponding charge to “Other taxes” expense on the Statements of Consolidated Income.
Accrued Insurance Liabilities	Accrued Insurance Liabilities. NiSource accrues for insurance costs related to workers compensation, automobile, property, general and employment practices liabilities based on the most probable value of each claim. Claim values are determined by professional, licensed loss adjusters who consider the facts of the claim, anticipated indemnification and legal expenses, and respective state rules. Claims are reviewed by NiSource at least quarterly and an adjustment is made to the accrual based on the most current information. NiSource’s actual exposure to liability is minimal due to coverage from its wholly-owned captive insurer who then transfers risk to third party insurance providers for the majority of costs paid to claimants above NiSource's deductible.

Recent Accounting Pronounceme37

Recent Accounting Pronouncements (Tables)	12 Months Ended
	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Description of New Accounting Pronouncements Not yet Adopted	Recently Issued Accounting Pronouncements NiSource is currently evaluating the impact of certain ASUs on its Consolidated Financial Statements or Notes to Consolidated Financial Statements, which are described below: Standard Description Effective Date Effect on the financial statements or other significant matters ASU 2016-13, Financial Instruments-Credit Losses (Topic 326) The pronouncement changes the impairment model for most financial assets, replacing the current "incurred loss" model. ASU 2016-13 will require the use of an "expected loss" model for instruments measured at amortized cost and will also require entities to record allowances for available-for-sale debt securities rather than reduce the carrying amount. Annual periods beginning after December 15, 2019, including interim periods therein. Early adoption is permitted for annual or interim periods beginning after December 15, 2018. NiSource is currently evaluating the impact of adoption, if any, on the Consolidated Financial Statements and Notes to Consolidated Financial Statements. Standard Description Effective Date Effect on the financial statements or other significant matters ASU 2018-01, Leases (Topic 842): Land Easement Practical Expedient for Transition to Topic 842 The pronouncement offers a practical expedient for accounting for land easements under ASU 2016-02. This practical expedient allows an entity the option of not evaluating existing land easements under ASC 842. New or modified land easements will still require evaluation under ASC 842 on a prospective basis beginning on the date of adoption. Annual periods beginning after December 15, 2018, including interim periods therein. Early adoption is permitted. NiSource has formed an internal stakeholder group that meets periodically to share information and gather data related to leasing activity at NiSource. This includes compiling a list of all contracts that could meet the definition of a lease under the new standard and evaluating the accounting for these contracts under the new standard to determine the ultimate impact the new standard will have on NiSource’s financial statements. Also, this procedure has identified process improvements to ensure data from newly initiated leases is captured to comply with the new standard. This work included the assistance of a third-party advisory firm. NiSource maintains a substantial number of easements and expects ASU 2018-01 will ease the process of implementation of ASC 842. NiSource plans to adopt these standards effective January 1, 2019. ASU 2016-02, Leases (Topic 842) The pronouncement introduces a lessee model that brings most leases on the balance sheet. The standard requires that lessees recognize the following for all leases (with the exception of short-term leases, as that term is defined in the standard) at the lease commencement date: (1) a lease liability, which is a lessee’s obligation to make lease payments arising from a lease, measured on a discounted basis; and (2) a right-of-use asset, which is an asset that represents the lessee’s right to use, or control the use of, a specified asset for the lease term. ASU 2018-02, Income Statement-Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income The pronouncement permits entities the option to reclassify tax effects that are stranded in accumulated other comprehensive income as a result of the implementation of the TCJA to retained earnings. Annual periods beginning after December 15, 2018, including interim periods therein. Early adoption is permitted for interim periods beginning after December 15, 2017. NiSource is currently evaluating the impact of adoption on the Consolidated Financial Statements and Notes to Consolidated Financial Statements.
Schedule of New Accounting Pronouncements and Changes in Accounting Principles	Recently Adopted Accounting Pronouncements Standard Adoption ASU 2017-12, Derivatives and Hedging (Topic 815): Targeted Improvements to accounting for Hedging Activities NiSource elected to adopt this ASU effective September 30, 2017. As a result, NiSource is no longer required to separately measure and report hedge ineffectiveness. The guidance also eases the requirements related to ongoing hedge effectiveness assessments at NiSource. The adoption of this standard did not have a material impact on the Consolidated Financial Statements or Notes to Consolidated Financial Statements. Standard Adoption ASU 2017-09, Compensation-Stock Compensation (Topic 718): Scope of Modification Accounting NiSource elected to adopt this ASU effective July 1, 2017. The adoption of this standard did not have a material impact on the Consolidated Financial Statements or Notes to Consolidated Financial Statements. ASU 2017-07, Compensation - Retirement Benefits (Topic 715): Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost NiSource adopted this ASU effective January 1, 2018. Beginning with NiSource's Form 10-Q for the first quarter of 2018, NiSource will continue to present the service cost component of net periodic benefit cost within "Operation and maintenance"; however, other components of the net periodic benefit cost (including regulatory deferrals) will be presented separately within "Other, net" in the Statement of Consolidated Income. This change in income statement presentation will be implemented on a retrospective basis. Beginning prospectively on the date of adoption, only the service cost component of NiSource's net periodic benefit cost is eligible for capitalization as "Property, Plant and Equipment" on the Consolidated Balance Sheets. NiSource's regulated subsidiaries have adopted this ASU for regulatory reporting purposes. ASU 2017-04, Intangibles - Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment NiSource elected to adopt this ASU effective January 1, 2017. The adoption of this standard did not have a material impact on the Consolidated Financial Statements or Notes to Consolidated Financial Statements. ASU 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash (a consensus of the FASB Emerging Issues Task Force) NiSource elected to adopt this ASU effective October 1, 2017. Restricted cash on the Statements of Consolidated Cash Flows is no longer presented as an investing activity and is instead included as a component of beginning and ending cash balances. The adoption of this standard is reflected in the Statements of Consolidated Cash Flows beginning with NiSource's Annual Report on Form 10-K for the year ended December 31, 2017 (including all prior periods presented). ASU 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments (a consensus of the Emerging Issues Task Force) NiSource adopted this ASU effective January 1, 2018. The adoption of this standard did not have a material impact on the Consolidated Financial Statements or Notes to Consolidated Financial Statements. ASU 2016-12, Revenue from Contracts with Customers (Topic 606): Narrow-Scope Improvements and Practical Expedients NiSource adopted the provisions of ASC 606 beginning on January 1, 2018 using a modified retrospective method, which was applied to all contracts. No material adjustments were made to January 1, 2018 opening balances as a result of the adoption. During the process of implementation, NiSource first separated its various revenue streams into high-level categories, which served as the basis for accounting analysis and documentation as it related to the pronouncement's impact on NiSource's revenues. Substantially all of NiSource’s revenues are tariff based, which NiSource concluded are in the scope of ASC 606. NiSource has identified its performance obligations created under tariff-based sales as the commodity (natural gas or electric, which includes generation and capacity) and delivery. Under ASC 606, NiSource's revenue from such tariff based sales continues to be equivalent to the natural gas or electricity supplied and billed each period (including unbilled revenues), and the adoption of the standards did not result in a material shift in the amount or timing of revenue recognition for such sales. In addition, the pattern and amount of revenue recognized for the remaining NiSource revenue streams were not materially affected as a result of the adoption of ASC 606. NiSource has outlined footnote disclosures intended to satisfy ASC 606's disclosure requirements, which will enhance its disclosures on revenue recognition policies and elections. Beginning prospectively upon date of adoption, NiSource will include revenue disaggregated by customer class and by operating segment in its footnote disclosures. In addition, NiSource will separately disclose those revenues that are not in scope of ASC 606, such as revenue earned under ASC 980 Alternative Revenue Programs. As required under the modified retrospective method of adoption, results for reporting periods beginning after January 1, 2018 will be presented under ASC 606, while prior period amounts will not be adjusted and will continue to be reported in accordance with historic accounting guidance. ASU 2016-08, Revenue from Contracts with Customers (Topic 606): Principal versus Agent Considerations ASU 2014-09, Revenue from Contracts with Customers (Topic 606)

Discontinued Operations (Tables

Discontinued Operations (Tables)	12 Months Ended
	Dec. 31, 2017
Discontinued Operations and Disposal Groups [Abstract]
Schedule of Discontinued Operations	Results from discontinued operations for 2015 are provided in the following table. These results are primarily from NiSource's former Columbia Pipeline Group Operations segment. Year Ended December 31, 2015 (in millions) Columbia Pipeline Group Operations Corporate and Other Total Operating Revenues Transportation and storage $ 561.4 $ — $ 561.4 Other 94.3 — 94.3 Total Operating Revenues 655.7 — 655.7 Operating Expenses Cost of sales (excluding depreciation and amortization) 0.2 — 0.2 Operation and maintenance 375.8 (1) — 375.8 Depreciation and amortization 66.4 — 66.4 Gain on sale of assets (13.6 ) — (13.6 ) Other taxes 38.0 — 38.0 Total Operating Expenses 466.8 — 466.8 Equity Earnings in Unconsolidated Affiliates 29.1 — 29.1 Operating Income from Discontinued Operations 218.0 — 218.0 Other Income (Deductions) Interest expense, net (37.1 ) — (37.1 ) Other, net 7.8 0.4 8.2 Total Other Income (Deductions) (29.3 ) 0.4 (28.9 ) Income from Discontinued Operations before Income Taxes 188.7 0.4 189.1 Income Taxes 84.7 0.9 85.6 Income (Loss) from Discontinued Operations - net of taxes $ 104.0 $ (0.5 ) $ 103.5 (1) Includes approximately $55.4 million of transaction costs related to the Separation.

Earnings Per Share (Tables)

Earnings Per Share (Tables)	12 Months Ended
	Dec. 31, 2017
Earnings Per Share [Abstract]
Schedule of Weighted Average Number of Shares	The computation of diluted average common shares is as follows: Year Ended December 31, (in thousands) 2017 2016 2015 Denominator Basic average common shares outstanding 329,388 321,805 317,746 Dilutive potential common shares: Shares contingently issuable under employee stock plans 547 165 — Shares restricted under stock plans 821 1,554 2,090 Diluted Average Common Shares 330,756 323,524 319,836

Property, Plant And Equipment (

Property, Plant And Equipment (Tables)	12 Months Ended
	Dec. 31, 2017
Property, Plant and Equipment [Abstract]
Schedule Of Property, Plant And Equipment	NiSource’s property, plant and equipment on the Consolidated Balance Sheets are classified as follows: At December 31, (in millions) 2017 2016 Property, Plant and Equipment Gas Distribution Utility (1) $ 12,531.0 $ 11,556.6 Electric Utility (1) 7,403.8 7,043.3 Corporate 141.3 105.0 Construction Work in Process 950.5 663.1 Non-Utility and Other (2) 623.3 681.7 Total Property, Plant and Equipment $ 21,649.9 $ 20,049.7 Accumulated Depreciation and Amortization Gas Distribution Utility (1) $ (3,227.8 ) $ (3,119.2 ) Electric Utility (1) (3,673.2 ) (3,442.0 ) Corporate (52.6 ) (52.5 ) Non-Utility and Other (2) (336.8 ) (368.0 ) Total Accumulated Depreciation and Amortization $ (7,290.4 ) $ (6,981.7 ) Net Property, Plant and Equipment $ 14,359.5 $ 13,068.0 (1) NIPSCO’s common utility plant and associated accumulated depreciation and amortization are allocated between Gas Distribution Utility and Electric Utility Property, Plant and Equipment. (2) Non-Utility and Other as of December 31, 2017 includes net book value of $247.8 million related to Bailly Generating Station (Units 7 and 8) which was reclassified from Electric Utility in the fourth quarter of 2016. Depreciation expense for the remaining net book value will continue to be recorded at the composite depreciation rate most recently approved by the IURC. See Note 18 -E, "Other Matters," and Note 25, "Subsequent Event," for additional information.
Schedule Of Depreciation Provisions For Utility Plant As A Percentage Of The Original Cost	The weighted average depreciation provisions for utility plant, as a percentage of the original cost, for the periods ended December 31, 2017 , 2016 and 2015 were as follows: 2017 2016 2015 Electric Operations 3.4 % 3.3 % 3.1 % Gas Distribution Operations 2.1 % 2.1 % 2.0 %

Goodwill and Other Intangible41

Goodwill and Other Intangible Assets (Tables)	12 Months Ended
	Dec. 31, 2017
Goodwill and Intangible Assets Disclosure [Abstract]
Schedule of Goodwill	The following presents NiSource's goodwill balance allocated by segment as of December 31, 2017 : (in millions) Gas Distribution Operations Electric Operations Corporate and Other Total Goodwill $ 1,690.7 $ — $ — $ 1,690.7

Asset Retirement Obligations (T

Asset Retirement Obligations (Tables)	12 Months Ended
	Dec. 31, 2017
Asset Retirement Obligation [Abstract]
Changes In Liability For Asset Retirement Obligations	Changes in NiSource’s liability for asset retirement obligations for the years 2017 and 2016 are presented in the table below: (in millions) 2017 2016 Beginning Balance $ 262.6 $ 254.0 Accretion recorded as a regulatory asset/liability 10.3 9.2 Additions 2.4 — Settlements (15.6 ) (7.5 ) Change in estimated cash flows 9.0 (1) 6.9 (2) Ending Balance $ 268.7 $ 262.6 (1) The change in estimated cash flows for 2017 is primarily attributed to changes in estimated costs and settlement timing for electric generating stations and the changes in estimated costs for retirement of gas mains. (2) The change in estimated cash flows for 2016 is primarily attributed to the changes in estimated costs for retirement of gas mains partially offset by revisions to estimated costs associated with the EPA's final rule for regulation of CCRs and changes to cost estimates for certain solid waste management units. See Note 18-D, "Environmental Matters," for additional information on CCRs.

Regulatory Matters (Tables)

Regulatory Matters (Tables)	12 Months Ended
	Dec. 31, 2017
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory Assets	Regulatory assets were comprised of the following items: At December 31, (in millions) 2017 2016 Regulatory Assets Unrecognized pension and other postretirement benefit costs (see Note 11) $ 733.5 $ 847.5 Deferred pension and other postretirement benefit costs (see Note 11) 70.7 59.6 Environmental costs (see Note 18-D) 63.4 62.6 Regulatory effects of accounting for income taxes (see Note 1-O and Note 10) 238.8 238.4 Underrecovered gas and fuel costs (see Note 1-K) 25.5 73.5 Depreciation 181.0 187.1 Post-in-service carrying charges 173.3 142.0 Safety activity costs 66.5 41.5 DSM programs 40.0 48.4 Other 208.5 184.8 Total Regulatory Assets $ 1,801.2 $ 1,885.4
Regulatory Liabilities	Regulatory liabilities were comprised of the following items: At December 31, (in millions) 2017 2016 Regulatory Liabilities Overrecovered gas and fuel costs (see Note 1-K) $ 27.6 $ 54.8 Cost of removal (see Note 7) 1,096.8 1,174.5 Regulatory effects of accounting for income taxes (see Note 1-O and Note 10) 1,563.4 30.0 Deferred pension and other postretirement benefit costs (see Note 11) 59.0 41.2 Other 48.8 81.3 Total Regulatory Liabilities $ 2,795.6 $ 1,381.8

Risk Management Activities (Tab

Risk Management Activities (Tables)	12 Months Ended
	Dec. 31, 2017
Risk Management Activities [Abstract]
Schedule of Derivative Instruments in Statement of Financial Position, Fair Value	Risk management assets and liabilities on NiSource’s derivatives are presented on the Consolidated Balance Sheets as shown below: December 31, (in millions) 2017 2016 Risk Management Assets - Current (1) Interest rate risk programs $ 14.0 $ 17.0 Commodity price risk programs 0.5 7.4 Total $ 14.5 $ 24.4 Risk Management Assets - Noncurrent (2) Interest rate risk programs $ 5.6 $ 17.1 Commodity price risk programs 1.0 7.5 Total $ 6.6 $ 24.6 Risk Management Liabilities - Current Interest rate risk programs $ 38.6 $ 15.3 Commodity price risk programs 4.6 1.5 Total $ 43.2 $ 16.8 Risk Management Liabilities - Noncurrent Interest rate risk programs $ — $ 24.5 Commodity price risk programs 28.5 20.0 Total $ 28.5 $ 44.5 (1) Presented in "Prepayments and other" on the Consolidated Balance Sheets. (2) Presented in "Deferred charges and other" on the Consolidated Balance Sheets.

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Schedule Of Components Of Income Tax Expense	The components of income tax expense (benefit) were as follows: Year Ended December 31, (in millions) 2017 2016 2015 Income Taxes Current Federal $ — $ — $ — State 7.8 (0.1 ) 6.0 Total Current 7.8 (0.1 ) 6.0 Deferred Federal 302.7 165.6 124.1 State 5.0 18.0 13.6 Total Deferred 307.7 183.6 137.7 Deferred Investment Credits (1.0 ) (1.4 ) (2.4 ) Income Taxes from Continuing Operations $ 314.5 $ 182.1 $ 141.3
Schedule Of Reasons Behind Differences In Computation Of Total Income Taxes	Total income taxes from continuing operations were different from the amount that would be computed by applying the statutory federal income tax rate to book income before income tax. The major reasons for this difference were as follows: Year Ended December 31, (in millions) 2017 2016 2015 Book income from Continuing Operations before income taxes $ 443.1 $ 510.2 $ 339.9 Tax expense at statutory Federal income tax rate 155.0 35.0 % 178.6 35.0 % 118.9 35.0 % Increases (reductions) in taxes resulting from: State income taxes, net of Federal income tax benefit 6.9 1.5 11.3 2.2 14.8 4.4 Property and plant (including accelerated depreciation) (2.4 ) (0.5 ) (1.5 ) (0.3 ) (1.6 ) (0.4 ) Charitable contribution carryover (1.2 ) (0.3 ) 2.8 0.5 17.8 5.2 Remeasurement due to TCJA 161.1 36.4 — — — — Employee stock ownership plan dividends and other compensation (6.5 ) (1.5 ) (9.5 ) (1.9 ) (2.9 ) (0.9 ) Tax accrual adjustments and other, net 1.6 0.4 0.4 0.2 (5.7 ) (1.7 ) Income Taxes from Continuing Operations $ 314.5 71.0 % $ 182.1 35.7 % $ 141.3 41.6 %
Schedule Of Principal Components Of Net Deferred Tax Liability	Deferred income taxes result from temporary differences between the financial statement carrying amounts and the tax basis of existing assets and liabilities. The principal components of NiSource’s net deferred tax liability were as follows: At December 31, (in millions) 2017 2016 Deferred tax liabilities Accelerated depreciation and other property-related differences $ 2,260.7 $ 3,323.5 Unrecovered gas and fuel costs — 25.9 Other regulatory assets 309.5 449.2 Total Deferred Tax Liabilities 2,570.2 3,798.6 Deferred tax assets Other regulatory liabilities including impact of TCJA 406.0 93.1 Pension and other postretirement/postemployment benefits 136.7 261.7 Net operating loss carryforward and Alternative Minimum Tax credit carryforward 576.0 646.2 Environmental liabilities 24.0 47.0 Other accrued liabilities 37.2 45.5 Other, net 97.4 177.1 Total Deferred Tax Assets 1,277.3 1,270.6 Net Deferred Tax Liabilities $ 1,292.9 $ 2,528.0

Pension And Other Postretirem46

Pension And Other Postretirement Benefits (Tables)	12 Months Ended
	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Schedule Of Allocation of Plan Assets	Asset Mix Policy of Funds: Defined Benefit Pension Plan Postretirement Benefit Plan Asset Category Minimum Maximum Minimum Maximum Domestic Equities 16% 36% 0% 55% International Equities 8% 18% 0% 25% Fixed Income 39% 51% 20% 100% Diversified Credit 0% 13% 0% 0% Real Estate 0% 13% 0% 0% Short-Term Investments 0% 10% 0% 10% As of December 31, 2016, the asset mix and acceptable minimum and maximum ranges established by the policy for the pension and other postretirement benefit plans were as follows: Asset Mix Policy of Funds: Defined Benefit Pension Plan Postretirement Benefit Plan Asset Category Minimum Maximum Minimum Maximum Domestic Equities 25% 45% 35% 55% International Equities 15% 25% 15% 25% Fixed Income 23% 37% 20% 50% Real Estate/Private Equity/Hedge Funds 0% 15% 0% 0% Short-Term Investments 0% 10% 0% 10% Pension Plan and Postretirement Plan Asset Mix at December 31, 2017 and December 31, 2016 : Defined Benefit Pension Assets December 31, Postretirement December 31, Asset Class (in millions) Asset Value % of Total Assets Asset Value % of Total Assets Domestic Equities $ 698.2 32.3 % $ 96.0 36.6 % International Equities 351.0 16.2 % 39.8 15.2 % Fixed Income 977.6 45.3 % 117.5 44.8 % Real Estate 49.9 2.3 % — — Cash/Other 83.3 3.9 % 9.2 3.4 % Total $ 2,160.0 100.0 % $ 262.5 100.0 % Defined Benefit Pension Assets December 31, Postretirement Benefit Plan Assets December 31, Asset Class (in millions) Asset Value % of Total Assets Asset Value % of Total Assets Domestic Equities $ 755.2 43.1 % $ 97.9 42.3 % International Equities 339.9 19.4 % 41.8 18.0 % Fixed Income 565.8 32.3 % 87.0 37.6 % Real Estate/Private Equity/Hedge Funds 74.8 4.3 % — — Cash/Other 15.2 0.9 % 4.7 2.1 % Total $ 1,750.9 100.0 % $ 231.4 100.0 %
Schedule Of Fair Value and Changes In The Fair Value Of The Plan Assets	Fair Value Measurements at December 31, 2017 : (in millions) December 31, Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Significant (Level 3) Pension plan assets: Cash $ 9.7 $ 9.7 $ — $ — Equity securities U.S. equities 0.3 0.3 — — Fixed income securities Government 143.4 — 143.4 — Corporate 332.6 — 332.6 — Mutual Funds U.S. multi-strategy 231.5 231.5 — — International equities 85.8 85.8 — — Fixed income 242.3 242.3 — — Private equity limited partnerships U.S. multi-strategy (1) 26.7 — — 26.7 International multi-strategy (2) 19.1 — — 19.1 Distressed opportunities 3.2 — — 3.2 Real estate 49.9 — — 49.9 Commingled funds (3) Short-term money markets 34.1 U.S. equities 466.6 International equities 265.1 Fixed income 244.9 Pension plan assets subtotal 2,155.2 569.6 476.0 98.9 Other postretirement benefit plan assets: Mutual funds U.S. equities 83.8 83.8 — — International equities 39.8 39.8 — — Fixed income 117.3 117.3 — — Commingled funds (3) Short-term money markets 9.4 U.S. equities 12.2 Other postretirement benefit plan assets subtotal 262.5 240.9 — — Due to brokers, net (4) (2.5 ) Accrued income/dividends 7.3 Total pension and other postretirement benefit plan assets $ 2,422.5 $ 810.5 $ 476.0 $ 98.9 (1) This class includes limited partnerships/fund of funds that invest in a diverse portfolio of private equity strategies, including buy-outs, venture capital, growth capital, special situations and secondary markets, primarily inside the United States. (2) This class includes limited partnerships/fund of funds that invest in diverse portfolio of private equity strategies, including buy-outs, venture capital, growth capital, special situations and secondary markets, primarily outside the United States. (3) This class of investments is measured at fair value using the net asset value per share and has not been classified in the fair value hierarchy. (4) This class represents pending trades with brokers. The table below sets forth a summary of changes in the fair value of the Plan’s Level 3 assets for the year ended December 31, 2017 : Balance at January 1, 2017 Total gains or losses (unrealized / realized) Purchases (Sales) Balance at December 31, 2017 Fixed income securities Other fixed income $ 0.1 $ (0.1 ) $ — $ — $ — Private equity limited partnerships U.S. multi-strategy 34.8 2.1 0.9 (11.1 ) 26.7 International multi-strategy 24.9 1.1 0.1 (7.0 ) 19.1 Distressed opportunities 4.1 0.4 — (1.3 ) 3.2 Real estate 9.2 (0.6 ) 42.1 (0.8 ) 49.9 Total $ 73.1 $ 2.9 $ 43.1 $ (20.2 ) $ 98.9 The table below sets forth a summary of unfunded commitments, redemption frequency and redemption notice periods for certain investments that are measured at fair value using the net asset value per share for the year ended December 31, 2017 : (in millions) Fair Value Redemption Frequency Redemption Notice Period Commingled Funds Short-term money markets $ 43.5 Daily 1 day U.S. equities 478.8 Monthly 3 days International equities 265.1 Monthly 10-30 days Fixed income 244.9 Monthly 3 days Total $ 1,032.3 Fair Value Measurements at December 31, 2016 : (in millions) December 31, Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Significant (Level 3) Pension plan assets: Cash $ 1.9 $ 1.9 $ — $ — Fixed income securities Government 42.2 — 42.2 — Corporate 104.1 — 104.1 — Other fixed income 0.1 — — 0.1 Mutual Funds U.S. multi-strategy 283.2 283.2 — — International equities 116.6 116.6 — — Fixed income 135.6 135.6 — — Private equity limited partnerships U.S. multi-strategy (1) 34.8 — — 34.8 International multi-strategy (2) 24.9 — — 24.9 Distressed opportunities 4.1 — — 4.1 Real Estate 9.2 — — 9.2 Commingled funds (3) Short-term money markets 16.6 U.S. equities 472.0 International equities 223.2 Fixed income 280.7 Pension plan assets subtotal 1,749.2 537.3 146.3 73.1 Other postretirement benefit plan assets: Mutual funds U.S. equities 85.4 85.4 — — International equities 41.8 41.8 — — Fixed income 86.8 86.8 — — Commingled funds (3) Short-term money markets 9.5 U.S. equities 12.5 Other postretirement benefit plan assets subtotal 236.0 214.0 — — Due to brokers, net (4) (5.0 ) Receivables/payables 2.1 Total pension and other postretirement benefit plan assets $ 1,982.3 $ 751.3 $ 146.3 $ 73.1 (1) This class includes limited partnerships/fund of funds that invest in a diverse portfolio of private equity strategies, including buy-outs, venture capital, growth capital, special situations and secondary markets, primarily in the United States. (2) This class includes limited partnerships/fund of funds that invest in a diverse portfolio of private equity strategies, including buy-outs, venture capital, growth capital, special situations and secondary markets, primarily outside the United States. (3) This class of investments is measured at fair value using the net asset value per share and has not been classified in the fair value hierarchy. (4) This class represents pending trades with brokers. The table below sets forth a summary of changes in the fair value of the Plan’s Level 3 assets for the year ended December 31, 2016 : Balance at January 1, 2016 Total gains or losses (unrealized / realized) Purchases (Sales) Balance at December 31, 2016 Fixed income securities Other fixed income $ 0.1 $ — $ — $ — $ 0.1 Private equity limited partnerships U.S. multi-strategy 46.4 2.1 0.8 (14.5 ) 34.8 International multi-strategy 29.3 2.0 1.0 (7.4 ) 24.9 Distress opportunities 5.9 (0.4 ) 0.1 (1.5 ) 4.1 Real estate 13.6 0.1 0.1 (4.6 ) 9.2 Total $ 95.3 $ 3.8 $ 2.0 $ (28.0 ) $ 73.1 The table below sets forth a summary of unfunded commitments, redemption frequency and redemption notice periods for certain investments that are measured at fair value using the net asset value per share for the year ended December 31, 2016 : (in millions) Fair Value Redemption Frequency Redemption Notice Period Commingled Funds Short-term money markets $ 26.1 Daily 1 day U.S. equities 484.5 Monthly 3 days International equities 223.2 Monthly 14-30 days Fixed income 280.7 Monthly 3 days Total $ 1,014.5
Schedule Of Reconciliation Of The Plan Funded Status	The following table provides a reconciliation of the plans’ funded status and amounts reflected in NiSource’s Consolidated Balance Sheets at December 31 based on a December 31 measurement date: Pension Benefits Other Postretirement Benefits (in millions) 2017 2016 2017 2016 Change in projected benefit obligation (1) Benefit obligation at beginning of year $ 2,165.8 $ 2,206.7 $ 529.0 $ 525.8 Service cost 30.0 30.7 4.8 5.0 Interest cost 68.3 89.7 17.8 22.0 Plan participants’ contributions — — 5.7 5.9 Plan amendments 0.9 — 1.6 7.5 Actuarial (gain) loss 98.3 (2.7 ) 36.2 1.0 Settlement loss 1.6 — — — Benefits paid (172.3 ) (158.6 ) (39.3 ) (38.9 ) Estimated benefits paid by incurred subsidy — — 0.5 0.7 Projected benefit obligation at end of year $ 2,192.6 $ 2,165.8 $ 556.3 $ 529.0 Change in plan assets Fair value of plan assets at beginning of year $ 1,750.9 $ 1,747.1 $ 231.4 $ 225.9 Actual return on plan assets 299.1 159.1 33.1 13.0 Employer contributions 282.3 3.3 31.6 25.5 Plan participants’ contributions — — 5.7 5.9 Benefits paid (172.3 ) (158.6 ) (39.3 ) (38.9 ) Fair value of plan assets at end of year $ 2,160.0 $ 1,750.9 $ 262.5 $ 231.4 Funded Status at end of year $ (32.6 ) $ (414.9 ) $ (293.8 ) $ (297.6 ) Amounts recognized in the statement of financial position consist of: Noncurrent assets 9.8 — — — Current liabilities (2.8 ) (2.9 ) (0.7 ) (0.7 ) Noncurrent liabilities (39.6 ) (412.0 ) (293.1 ) (296.9 ) Net amount recognized at end of year (2) $ (32.6 ) $ (414.9 ) $ (293.8 ) $ (297.6 ) Amounts recognized in accumulated other comprehensive income or regulatory asset/liability (3) Unrecognized prior service credit $ 2.5 $ 1.0 $ (23.1 ) $ (29.2 ) Unrecognized actuarial loss 692.9 835.5 84.2 68.3 Net amount recognized at end of year $ 695.4 $ 836.5 $ 61.1 $ 39.1 (1) The change in benefit obligation for Pension Benefits represents the change in Projected Benefit Obligation while the change in benefit obligation for Other Postretirement Benefits represents the change in accumulated postretirement benefit obligation. (2) NiSource recognizes in its Consolidated Balance Sheets the underfunded and overfunded status of its various defined benefit postretirement plans, measured as the difference between the fair value of the plan assets and the benefit obligation. (3) NiSource determined that for certain rate-regulated subsidiaries the future recovery of pension and other postretirement benefits costs is probable. These rate-regulated subsidiaries recorded regulatory assets and liabilities of $733.5 million and $0.1 million , respectively, as of December 31, 2017 , and $847.5 million and $0.3 million , respectively, as of December 31, 2016 that would otherwise have been recorded to accumulated other comprehensive loss.
Schedule of Benefit Obligations in Excess of Fair Value of Plan Assets	Information for pension plans with an accumulated benefit obligation in excess of plan assets: December 31, 2017 2016 Accumulated Benefit Obligation $ 1,502.5 $ 2,148.9 Funded Status Projected Benefit Obligation 1,524.7 2,165.8 Fair Value of Plan Assets 1,482.3 1,750.9 Funded Status of Underfunded Pension Plans at End of Year $ (42.4 ) $ (414.9 ) Information for pension plans with plan assets in excess of the accumulated benefit obligation: December 31, 2017 2016 Accumulated Benefit Obligation $ 667.9 $ — Funded Status Projected Benefit Obligation 667.9 — Fair Value of Plan Assets 677.7 — Funded Status of Overfunded Pension Plans at End of Year $ 9.8 $ —
Schedule Of Significant Actuarial Assumptions In Determining Funded Status Plan	The following table provides the key assumptions that were used to calculate the pension and other postretirement benefits obligations for NiSource’s various plans as of December 31: Pension Benefits Other Postretirement Benefits 2017 2016 2017 2016 Weighted-average assumptions to Determine Benefit Obligation Discount Rate 3.58 % 4.03 % 3.67 % 4.12 % Rate of Compensation Increases 4.00 % 4.00 % — — Health Care Trend Rates Trend for Next Year — — 8.52 % 8.43 % Ultimate Trend — — 4.50 % 4.50 % Year Ultimate Trend Reached — — 2025 2024 The following table provides the key assumptions that were used to calculate the net periodic benefits cost for NiSource’s various plans: Pension Benefits Other Postretirement Benefits 2017 2016 2015 2017 2016 2015 Weighted-average Assumptions to Determine Net Periodic Benefit Cost Discount rate - service cost (1) 4.40 % 4.24 % 3.81 % 4.58 % 4.33 % 3.94 % Discount rate - interest cost (1) 3.31 % 4.24 % 3.81 % 3.48 % 4.33 % 3.94 % Expected Long-Term Rate of Return on Plan Assets 7.25 % 8.00 % 8.30 % 6.99 % 7.85 % 8.15 % Rate of Compensation Increases 4.00 % 4.00 % 4.00 % — — —
Schedule Of One-Percentage-Point Change In Assumed Health Care Cost Trend Rates	Assumed health care cost trend rates have a significant effect on the amounts reported for the health care plans. A one-percentage-point change in assumed health care cost trend rates would have the following effects: (in millions) 1% point increase 1% point decrease Effect on service and interest components of net periodic cost $ 1.1 $ (0.9 ) Effect on accumulated postretirement benefit obligation 29.7 (25.9 )
Schedule Of Expected Payments To Participants In Pension Plan	The expected benefits are estimated based on the same assumptions used to measure NiSource’s benefit obligation at the end of the year and includes benefits attributable to the estimated future service of employees: (in millions) Pension Benefits Other Federal Year(s) 2018 $ 176.2 $ 34.3 $ 0.5 2019 173.7 35.3 0.5 2020 172.1 36.3 0.5 2021 172.0 36.9 0.5 2022 171.3 36.9 0.5 2023-2027 784.7 178.9 1.9
Components Of The Plans' Net Periodic Benefits Cost	The following table provides the components of the plans’ actuarially determined net periodic benefits cost for each of the three years ended December 31, 2017 , 2016 and 2015 : Pension Benefits Other Postretirement Benefits (in millions) 2017 2016 2015 2017 2016 2015 Components of Net Periodic Benefit Cost Service cost $ 30.0 $ 30.7 $ 34.8 $ 4.8 $ 5.0 $ 6.4 Interest cost 68.3 89.7 95.9 17.8 22.0 24.9 Expected return on assets (123.1 ) (132.9 ) (167.2 ) (15.9 ) (17.2 ) (28.2 ) Amortization of prior service cost (credit) (0.7 ) (0.2 ) 0.1 (4.4 ) (4.9 ) (5.2 ) Recognized actuarial loss 52.9 61.2 59.3 3.0 3.1 3.4 Net Periodic Benefit Costs 27.4 48.5 22.9 5.3 8.0 1.3 Additional loss recognized due to: Settlement loss 13.7 — 2.5 — — — Total Net Periodic Benefits Cost $ 41.1 $ 48.5 $ 25.4 $ 5.3 $ 8.0 $ 1.3
Schedule Of Changes In Plan Assets And Projected Benefit Obligations Recognized In Other Comprehensive Income	The following table provides other changes in plan assets and projected benefit obligations recognized in other comprehensive income or regulatory asset or liability: Pension Benefits Other Postretirement Benefits (in millions) 2017 2016 2017 2016 Other Changes in Plan Assets and Projected Benefit Obligations Recognized in Other Comprehensive Income or Regulatory Asset or Liability Net prior service cost $ 0.9 $ — $ 1.6 $ 7.5 Net actuarial loss (gain) (76.1 ) (28.9 ) 18.9 5.3 Settlements (13.7 ) — — — Less: amortization of prior service cost 0.7 0.2 4.4 4.9 Less: amortization of net actuarial loss (52.9 ) (61.2 ) (3.0 ) (3.1 ) Total Recognized in Other Comprehensive Income or Regulatory Asset or Liability $ (141.1 ) $ (89.9 ) $ 21.9 $ 14.6 Amount Recognized in Net Periodic Benefits Cost and Other Comprehensive Income or Regulatory Asset or Liability $ (100.0 ) $ (41.4 ) $ 27.2 $ 22.6

Common Stock (Tables)

Common Stock (Tables)	12 Months Ended
	Dec. 31, 2017
Common Stock [Abstract]
Schedule Of Stock Offering Program	The following table summarizes NiSource's activity under the ATM program: Year Ending December 31, 2017 2016 2015 Number of shares issued 11,931,376 — — Average price per share $ 26.58 — — Proceeds, net of fees ( in millions) $ 314.7 — —

Share-Based Compensation (Table

Share-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2017
Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Schedule Of Transactions Of Share Based Compensation Other Than Stock Options	(shares) Restricted Stock Units Weighted Average Grant Date Fair Value Per Unit ($) Nonvested at December 31, 2016 1,642,030 12.05 Granted 10,983 22.87 Forfeited (85,436 ) 14.64 Vested (869,451 ) 9.33 Nonvested at December 31, 2017 698,126 15.09
Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Schedule Of Transactions Of Share Based Compensation Other Than Stock Options	(shares) Performance Awards Weighted Average Grant Date Fair Value Per Unit ($) Nonvested at December 31, 2016 647,305 19.50 Granted 660,750 19.50 Forfeited (123,282 ) 19.45 Vested — — Nonvested at December 31, 2017 1,184,773 19.52

Long-Term Debt (Tables)

Long-Term Debt (Tables)	12 Months Ended
	Dec. 31, 2017
Long-term Debt, Current and Noncurrent [Abstract]
Schedule of Long-Term Debt	NiSource long-term debt as of December 31, 2017 and 2016 is as follows: Long-term debt type Maturity as of December 31, 2017 Weighted average interest rate (%) Outstanding balance as of December 31, (in millions) 2017 2016 Senior notes: NiSource September 2017 5.25 % $ — $ 210.4 NiSource March 2018 6.40 % 275.1 476.0 NiSource January 2019 6.80 % 255.1 500.0 NiSource March 2019 Variable (1) — 500.0 NiSource September 2020 5.45 % 325.1 550.0 NiSource December 2021 4.45 % 63.6 63.6 NiSource March 2022 6.13 % 180.0 500.0 NiSource November 2022 2.65 % 500.0 — NiSource February 2023 3.85 % 250.0 250.0 NiSource November 2025 5.89 % 265.0 265.0 NiSource May 2027 3.49 % 1,000.0 — NiSource December 2027 6.78 % 3.0 3.0 NiSource December 2040 6.25 % 250.0 250.0 NiSource June 2041 5.95 % 400.0 400.0 NiSource February 2042 5.80 % 250.0 250.0 NiSource February 2043 5.25 % 500.0 500.0 NiSource February 2044 4.80 % 750.0 750.0 NiSource February 2045 5.65 % 500.0 500.0 NiSource May 2047 4.38 % 1,000.0 — NiSource March 2048 3.95 % 750.0 — Total senior notes $ 7,516.9 $ 5,968.0 Medium term notes: NiSource April 2022 to May 2027 7.99 % $ 49.0 $ 106.0 NIPSCO August 2022 to August 2027 7.61 % 68.0 95.5 Columbia of Massachusetts December 2025 to February 2028 6.30 % 40.0 40.0 Total medium term notes $ 157.0 $ 241.5 Capital leases: NIPSCO May 2018 3.95 % $ 3.8 $ 12.7 NiSource Corporate Services October 2019 3.26 % 1.4 3.5 Columbia of Ohio October 2021 to June 2038 6.41 % 88.5 80.1 Columbia of Virginia August 2024 to July 2029 12.21 % 5.2 5.5 Columbia of Kentucky May 2027 3.79 % 0.4 — Columbia of Pennsylvania August 2027 to June 2036 5.45 % 31.0 31.9 Columbia of Massachusetts December 2033 to July 2036 4.37 % 22.8 23.7 Total capital leases 153.1 157.4 Pollution control bonds - NIPSCO April 2019 5.85 % 41.0 96.0 Unamortized issuance costs and discounts (71.5 ) $ (41.6 ) Total Long-Term Debt $ 7,796.5 $ 6,421.3 (1) Rate of one month Libor plus 95 basis points.

Short-Term Borrowings (Tables)

Short-Term Borrowings (Tables)	12 Months Ended
	Dec. 31, 2017
Short-term Debt [Abstract]
Schedule Of Short-Term Borrowings	Short-term borrowings were as follows: At December 31, (in millions) 2017 2016 Commercial Paper weighted average interest rate of 1.97 % and 1.24% at December 31, 2017 and 2016, respectively. $ 869.0 $ 1,178.0 Accounts receivable securitization facility borrowings 336.7 310.0 Total Short-Term Borrowings $ 1,205.7 $ 1,488.0

Fair Value (Tables)

Fair Value (Tables)	12 Months Ended
	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis	The following tables present financial assets and liabilities measured and recorded at fair value on NiSource’s Consolidated Balance Sheets on a recurring basis and their level within the fair value hierarchy as of December 31, 2017 and December 31, 2016 : Recurring Fair Value Measurements December 31, 2017 ( in millions ) Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Balance as of December 31, 2017 Assets Risk management assets $ — $ 21.1 $ — $ 21.1 Available-for-sale securities — 133.9 — 133.9 Total $ — $ 155.0 $ — $ 155.0 Liabilities Risk management liabilities $ — $ 71.4 $ 0.3 $ 71.7 Total $ — $ 71.4 $ 0.3 $ 71.7 Recurring Fair Value Measurements December 31, 2016 ( in millions ) Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Balance as of December 31, 2016 Assets Risk management assets $ 5.4 $ 43.6 $ — $ 49.0 Available-for-sale securities — 131.5 — 131.5 Total $ 5.4 $ 175.1 $ — $ 180.5 Liabilities Risk management liabilities $ 1.2 $ 58.9 $ 1.2 $ 61.3 Total $ 1.2 $ 58.9 $ 1.2 $ 61.3
Available-For-Sale Debt Securities	The amortized cost, gross unrealized gains and losses and fair value of available-for-sale securities at December 31, 2017 and 2016 were: December 31, 2017 (in millions) Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Available-for-sale securities U.S. Treasury debt securities $ 26.9 $ — $ (0.1 ) $ 26.8 Corporate/Other debt securities 106.8 0.9 (0.6 ) 107.1 Total $ 133.7 $ 0.9 $ (0.7 ) $ 133.9 December 31, 2016 (in millions) Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Available-for-sale securities U.S. Treasury debt securities $ 35.0 $ 0.1 $ (0.6 ) $ 34.5 Corporate/Other debt securities 98.7 0.3 (2.0 ) 97.0 Total $ 133.7 $ 0.4 $ (2.6 ) $ 131.5
Carrying Amount And Estimated Fair Values Of Financial Instruments	The carrying amount and estimated fair values of these financial instruments were as follows: At December 31, (in millions) Carrying Amount 2017 Estimated Fair Value 2017 Carrying Amount 2016 Estimated Fair Value 2016 Long-term debt (including current portion) $ 7,796.5 $ 8,603.4 $ 6,421.3 $ 7,064.1

Transfers Of Financial Assets (

Transfers Of Financial Assets (Tables)	12 Months Ended
	Dec. 31, 2017
Transfers and Servicing [Abstract]
Schedule of Assets and Associated Liabilities Accounted for as Secured Borrowings	December 31, 2017 and 2016 : (in millions) December 31, December 31, Gross Receivables $ 635.3 $ 618.3 Less: Receivables not transferred 298.6 308.3 Net receivables transferred $ 336.7 $ 310.0 Short-term debt due to asset securitization $ 336.7 $ 310.0

Other Commitments And Conting53

Other Commitments And Contingencies (Tables)	12 Months Ended
	Dec. 31, 2017
Commitments and Contingencies Disclosure [Abstract]
Contractual Obligation, Fiscal Year Maturity Schedule [Table Text Block]	Contractual Obligations . NiSource has certain contractual obligations requiring payments at specified periods. The obligations include long-term debt, lease obligations, energy commodity contracts and obligations for various services including pipeline capacity and outsourcing of IT services. The total contractual obligations in existence at December 31, 2017 and their maturities were: (in millions) Total 2018 2019 2020 2021 2022 After Long-term debt (1) $ 7,714.9 $ 275.1 296.1 $ 296.1 $ 325.1 $ 63.6 $ 710.0 $ 6,045.0 Capital leases (2) 254.4 18.1 15.7 15.4 15.5 15.5 174.2 Interest payments on long-term debt 6,701.2 364.4 344.4 334.6 316.8 307.7 5,033.3 Operating leases (3) 57.2 13.8 10.2 7.3 6.2 4.4 15.3 Energy commodity contracts 216.7 102.5 57.3 56.9 — — — Service obligations: Pipeline service obligations 2,649.9 538.9 520.5 390.7 344.7 331.0 524.1 IT service obligations 311.5 88.3 71.5 63.5 50.7 37.5 — Other service obligations 178.2 48.3 43.3 43.3 43.3 — — Other liabilities 28.7 28.7 — — — — — Total contractual obligations $ 18,112.7 $ 1,478.1 $ 1,359.0 $ 1,236.8 $ 840.8 $ 1,406.1 $ 11,791.9 (1) Long-term debt balance excludes unamortized issuance costs and discounts of $71.5 million. (2) Capital lease payments shown above are inclusive of interest totaling $91.9 million. (3) Operating lease balances do not include amounts for fleet leases that can be renewed beyond the initial lease term. The Company anticipates renewing the leases beyond the initial term, but the anticipated payments associated with the renewals do not meet the definition of expected minimum lease payments and therefore are not included above. Expected payments are $29.3 million in 2018, $27.5 million in 2019, $19.7 million in 2020, $13.9 million in 2021, $9.6 million in 2022 and $7.4 million thereafter.

Accumulated Other Comprehensi54

Accumulated Other Comprehensive Loss (Tables)	12 Months Ended
	Dec. 31, 2017
Components of Accumulated Other Comprehensive Income (Loss) [Abstract]
Schedule of Accumulated Other Comprehensive Income (Loss)	The following table displays the activity of Accumulated Other Comprehensive Loss, net of tax: (in millions) Gains and Losses on Securities (1) Gains and Losses on Cash Flow Hedges (1) Pension and OPEB Items (1) Accumulated Other Comprehensive Loss (1) Balance as of January 1, 2015 $ 0.3 $ (23.6 ) $ (27.3 ) $ (50.6 ) Other comprehensive loss before reclassifications (0.5 ) (11.0 ) (5.0 ) (16.5 ) Amounts reclassified from accumulated other comprehensive loss (0.3 ) 3.2 2.6 5.5 Net current-period other comprehensive loss (0.8 ) (7.8 ) (2.4 ) (11.0 ) Allocation of AOCI to noncontrolling interest — 2.0 — 2.0 Distribution of CPG to shareholders (Refer to Note 3, "Discontinued Operations") — 13.9 10.6 24.5 Balance as of December 31, 2015 $ (0.5 ) $ (15.5 ) $ (19.1 ) $ (35.1 ) Other comprehensive income before reclassifications — 7.1 0.5 7.6 Amounts reclassified from accumulated other comprehensive loss (0.1 ) 1.5 1.0 2.4 Net current-period other comprehensive income (loss) (0.1 ) 8.6 1.5 10.0 Balance as of December 31, 2016 $ (0.6 ) $ (6.9 ) $ (17.6 ) $ (25.1 ) Other comprehensive income (loss) before reclassifications 0.6 (24.2 ) 1.9 (21.7 ) Amounts reclassified from accumulated other comprehensive loss 0.2 1.7 1.5 3.4 Net current-period other comprehensive income (loss) 0.8 (22.5 ) 3.4 (18.3 ) Balance as of December 31, 2017 $ 0.2 $ (29.4 ) $ (14.2 ) $ (43.4 ) (1) All amounts are net of tax. Amounts in parentheses indicate debits.

Other, Net (Tables)

Other, Net (Tables)	12 Months Ended
	Dec. 31, 2017
Other Nonoperating Income (Expense) [Abstract]
Schedule Of Other, Net	Year Ended December 31, (in millions) 2017 2016 2015 Interest Income $ 4.6 $ 3.4 $ 0.8 AFUDC Equity 12.6 11.6 11.5 Charitable Contributions (19.9 ) (4.5 ) (4.8 ) Miscellaneous (1) (0.1 ) (9.0 ) 9.9 Total Other, net $ (2.8 ) $ 1.5 $ 17.4 (1) Miscellaneous in 2016 primarily consists of a TUA-related charge of $8.6 million to reflect the estimated amount owed to the upgrade sponsors for the portion of the multiplier previously collected for taxes. In 2015, Miscellaneous primarily consisted of TUA income.

Interest Expense, Net (Tables)

Interest Expense, Net (Tables)	12 Months Ended
	Dec. 31, 2017
Interest Expense [Abstract]
Schedule Of Interest Expense, Net	Year Ended December 31, (in millions) 2017 2016 2015 Interest on long-term debt $ 354.8 $ 352.3 $ 377.5 Interest on short-term borrowings 14.9 9.2 2.2 Debt discount/cost amortization 7.2 7.6 8.7 Accounts receivable securitization fees 2.5 2.3 2.5 Allowance for borrowed funds used and interest capitalized during construction (6.2 ) (5.6 ) (5.4 ) Debt-based post-in-service carrying charges (36.4 ) (35.1 ) (21.4 ) Other 16.4 18.8 16.1 Total Interest Expense, net $ 353.2 $ 349.5 $ 380.2

Segments Of Business (Tables)

Segments Of Business (Tables)	12 Months Ended
	Dec. 31, 2017
Segment Reporting [Abstract]
Schedule Of Operating Income Derived From Revenues And Expenses By Segment	Year Ended December 31, (in millions) 2017 2016 2015 Operating Revenues Gas Distribution Operations Unaffiliated $ 3,087.9 $ 2,818.2 $ 3,068.7 Intersegment 14.2 12.4 0.4 Total 3,102.1 2,830.6 3,069.1 Electric Operations Unaffiliated 1,785.7 1,660.8 1,573.6 Intersegment 0.8 0.8 0.8 Total 1,786.5 1,661.6 1,574.4 Corporate and Other Unaffiliated 1.0 13.5 9.5 Intersegment 510.8 413.3 396.4 Total 511.8 426.8 405.9 Eliminations (525.8 ) (426.5 ) (397.6 ) Consolidated Operating Revenues $ 4,874.6 $ 4,492.5 $ 4,651.8 Year Ended December 31, (in millions) 2017 2016 2015 Operating Income (Loss) Gas Distribution Operations $ 545.6 $ 574.0 $ 555.8 Electric Operations 364.8 291.4 264.4 Corporate and Other 0.2 (7.2 ) (20.3 ) Consolidated Operating Income $ 910.6 $ 858.2 $ 799.9 Depreciation and Amortization Gas Distribution Operations $ 269.3 $ 252.9 $ 232.6 Electric Operations 277.8 274.5 267.7 Corporate and Other 23.2 19.7 24.1 Consolidated Depreciation and Amortization $ 570.3 $ 547.1 $ 524.4 Assets Gas Distribution Operations $ 12,048.8 $ 11,096.4 $ 10,094.5 Electric Operations 5,478.6 5,233.3 5,265.3 Corporate and Other 2,434.3 2,362.2 2,132.7 Consolidated Assets $ 19,961.7 $ 18,691.9 $ 17,492.5 Capital Expenditures (1) Gas Distribution Operations $ 1,125.6 $ 1,054.4 $ 917.0 Electric Operations 592.4 420.6 400.3 Corporate and Other 35.8 15.4 50.2 Consolidated Capital Expenditures $ 1,753.8 $ 1,490.4 $ 1,367.5 (1 Amounts differ from those presented on the Statements of Consolidated Cash Flows primarily due to the inclusion of capital expenditures included in current liabilities and AFUDC Equity.

Quarterly Financial Data (Table

Quarterly Financial Data (Tables)	12 Months Ended
	Dec. 31, 2017
Quarterly Financial Data [Abstract]
Schedule Of Quarterly Financial Data	(in millions, except per share data) First Quarter Second Quarter (1) Third Quarter Fourth Quarter (2) 2017 Operating Revenues $ 1,598.6 $ 990.7 $ 917.0 $ 1,368.3 Operating Income 416.5 124.5 99.6 270.0 Income (Loss) from Continuing Operations 211.3 (44.3 ) 14.0 (52.4 ) Loss from Discontinued Operations - net of taxes — (0.1 ) — — Net Income (Loss) 211.3 (44.4 ) 14.0 (52.4 ) Basic Earnings (Loss) Per Share Continuing Operations 0.65 (0.14 ) 0.04 (0.16 ) Discontinued Operations — — — — Basic Earnings (Loss) Per Share $ 0.65 $ (0.14 ) $ 0.04 $ (0.16 ) Diluted Earnings (Loss) Per Share Continuing Operations 0.65 (0.14 ) 0.04 (0.16 ) Discontinued Operations — — — — Diluted Earnings (Loss) Per Share $ 0.65 $ (0.14 ) $ 0.04 $ (0.16 ) 2016 Operating Revenues $ 1,436.6 $ 897.6 $ 861.3 $ 1,297.0 Operating Income 381.4 138.2 113.7 224.9 Income from Continuing Operations 186.6 29.0 23.7 88.8 Results from Discontinued Operations - net of taxes — (0.1 ) 3.5 — Net Income 186.6 28.9 27.2 88.8 Basic Earnings Per Share Continuing Operations 0.58 0.09 0.07 0.28 Discontinued Operations — — 0.01 — Basic Earnings Per Share $ 0.58 $ 0.09 $ 0.08 $ 0.28 Diluted Earnings Per Share Continuing Operations 0.58 0.09 0.07 0.27 Discontinued Operations — — 0.01 — Diluted Earnings Per Share $ 0.58 $ 0.09 $ 0.08 $ 0.27 (1) The decrease in income from continuing operations during the second quarter of 2017 relates primarily to a $111.5 million loss on early extinguishment of long-term debt, primarily attributable to early redemption premiums. See Note 14, "Long-Term Debt," for additional information. (2) The decrease in income from continuing operations during the fourth quarter of 2017 was due primarily to increased tax expense as a result of the impact of implementing the provisions of the TCJA. See Note 10, "Income Taxes," for additional information.

Supplemental Cash Flow Inform59

Supplemental Cash Flow Information (Tables)	12 Months Ended
	Dec. 31, 2017
Supplemental Cash Flow Information [Abstract]
Schedule of Cash Flow, Supplemental Disclosures	The following table provides additional information regarding NiSource’s Consolidated Statements of Cash Flows for the years ended December 31, 2017 , 2016 and 2015 : Year Ended December 31, (in millions) 2017 2016 2015 Supplemental Disclosures of Cash Flow Information Non-cash transactions: Capital expenditures included in current liabilities $ 173.0 $ 125.3 $ 121.6 Assets acquired under a capital lease 11.5 4.0 47.5 Schedule of interest and income taxes paid: Cash paid for interest, net of interest capitalized amounts $ 339.9 $ 337.8 $ 390.4 Cash paid for income taxes, net of refunds 5.5 8.0 21.3

Valuation and Qualifying Acco60

Valuation and Qualifying Accounts (Tables)	12 Months Ended
	Dec. 31, 2017
Valuation and Qualifying Accounts [Abstract]
Schedule of Valuation and Qualifying Accounts Disclosure	NISOURCE INC. SCHEDULE II – VALUATION AND QUALIFYING ACCOUNTS Twelve months ended December 31, 2017 Additions ($ in millions) Balance Jan. 1, 2017 Charged to Costs and Expenses Charged to Other Account (1) Deductions for Purposes for which Reserves were Created Balance Dec. 31, 2017 Reserves Deducted in Consolidated Balance Sheet from Assets to Which They Apply: Reserve for accounts receivable $ 23.3 $ 14.8 $ 39.1 $ 58.9 $ 18.3 Reserve for other investments 3.0 — — — 3.0 Twelve months ended December 31, 2016 Additions ($ in millions) Balance Charged to Costs and Expenses Charged to Other Account (1) Deductions for Purposes for which Reserves were Created Balance Reserves Deducted in Consolidated Balance Sheet from Assets to Which They Apply: Reserve for accounts receivable $ 20.3 $ 19.7 $ 48.5 $ 65.2 $ 23.3 Reserve for other investments 3.0 — — — 3.0 Twelve months ended December 31, 2015 Additions ($ in millions) Balance Charged to Costs and Expenses Charged to Other Account (1) Deductions for Purposes for which Reserves were Created Balance Reserves Deducted in Consolidated Balance Sheet from Assets to Which They Apply: Reserve for accounts receivable $ 24.9 $ 22.5 $ 56.7 $ 83.8 $ 20.3 Reserve for other investments 3.0 — — — 3.0 (1) Charged to Other Accounts reflects the deferral of bad debt expense to a regulatory asset.

Nature Of Operations And Summ61

Nature Of Operations And Summary Of Significant Accounting Policies (Narrative) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017USD ($)shares	Dec. 31, 2016USD ($)shares	Dec. 31, 2015USD ($)shares
Basis Of Accounting Presentation [Line Items]
Number of customers	3,900,000
Unbilled revenue, less reserves	$ 359.4	$ 329.7
Other than Temporary Impairment Losses, Investments	$ 0	$ 0	$ 0
Pre-tax rate for allowance for funds used during construction	4.00%	4.50%	4.70%
Anti-dilutive shares | shares	0	0	0
Inventory valued using LIFO	$ 45.5	$ 46.1
Excess of replacement over LIFO value	(17.4)	(9.4)
Inventory valued using the weighted average cost methodology	$ 239.6	$ 233.8

Discontinued Operations (Narrat

Discontinued Operations (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Nov. 17, 2017	Jul. 01, 2015	May 22, 2015
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Assets	$ 19,961.7	$ 18,691.9	$ 17,492.5
Tax benefit ffrom favorable estimate-to-actual adjustments	3.6
Transaction Costs Related to Separation			55.4
Debt Instrument, Face Amount				$ 500
Related Party Transaction, Other Revenues from Transactions with Related Party	151.6	150.5	$ 147.6
Columbia Pipeline Group Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Assets	0	0			$ 9,200
Liabilities	$ 0	$ 0			5,600
Equity					$ 3,600
Columbia Pipeline Group
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Debt Instrument, Face Amount						$ 2,750

Discontinued Operations (Income

Discontinued Operations (Income Statement Disclosures) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Transportation and storage				$ 561.4
Other				94.3
Total Operating Revenues				655.7
Cost of sales (excluding depreciation and amortization)				0.2
Operation and maintenance				375.8
Depreciation and amortization				66.4
Gain on sale of assets				(13.6)
Other taxes				38
Total Operating Expenses				466.8
Equity Earnings in Unconsolidated Affiliates				29.1
Operating Income from Discontinued Operations				218
Interest expense, net				(37.1)
Other, net				8.2
Total Other Income (Deductions)				(28.9)
Income from Discontinued Operations before Income Taxes				189.1
Income Taxes				85.6
Income (Loss) from Discontinued Operations - net of taxes		$ (0.1)	$ 3.4	103.5
Columbia Pipeline Group Operations | Discontinued Operations, Disposed of by Means Other than Sale, Spinoff [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Transportation and storage				561.4
Other				94.3
Total Operating Revenues				655.7
Cost of sales (excluding depreciation and amortization)				0.2
Operation and maintenance	[1]			375.8
Depreciation and amortization				66.4
Gain on sale of assets				(13.6)
Other taxes				38
Total Operating Expenses				466.8
Equity Earnings in Unconsolidated Affiliates				29.1
Operating Income from Discontinued Operations				218
Interest expense, net				(37.1)
Other, net				7.8
Total Other Income (Deductions)				(29.3)
Income from Discontinued Operations before Income Taxes				188.7
Income Taxes				84.7
Income (Loss) from Discontinued Operations - net of taxes				104
Corporate and Other | Discontinued Operations, Disposed of by Means Other than Sale, Spinoff [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Transportation and storage				0
Other				0
Total Operating Revenues				0
Cost of sales (excluding depreciation and amortization)				0
Operation and maintenance				0
Depreciation and amortization				0
Gain on sale of assets				0
Other taxes				0
Total Operating Expenses				0
Equity Earnings in Unconsolidated Affiliates				0
Operating Income from Discontinued Operations				0
Interest expense, net				0
Other, net				0.4
Total Other Income (Deductions)				0.4
Income from Discontinued Operations before Income Taxes				0.4
Income Taxes				0.9
Income (Loss) from Discontinued Operations - net of taxes				$ (0.5)
[1]	Includes approximately $55.4 million of transaction costs related to the Separation.

Earnings Per Share (Details)

Earnings Per Share (Details) - shares shares in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share [Abstract]
Basic Average Common Shares Outstanding	329,388	321,805	317,746
Shares contingently issuable under employee stock plans	547	165	0
Shares restricted under stock plans	821	1,554	2,090
Weighted Average Number of Shares Outstanding, Diluted	330,756	323,524	319,836

Property, Plant And Equipment65

Property, Plant And Equipment (Schedule Of Property, Plant And Equipment) (Details) - USD ($) $ in Millions		Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Property Plant and Equipment		$ 21,649.9	$ 20,049.7
Accumulated Depreciation and Amortization		(7,290.4)	(6,981.7)
Net Property, Plant and Equipment		14,359.5	13,068
Gas Distribution Utility
Property, Plant and Equipment [Line Items]
Property Plant and Equipment	[1]	12,531	11,556.6
Accumulated Depreciation and Amortization	[1]	(3,227.8)	(3,119.2)
Electric Utility
Property, Plant and Equipment [Line Items]
Property Plant and Equipment	[1],[2]	7,403.8	7,043.3
Accumulated Depreciation and Amortization	[1],[2]	(3,673.2)	(3,442)
Common Utility
Property, Plant and Equipment [Line Items]
Property Plant and Equipment		141.3	105
Accumulated Depreciation and Amortization		(52.6)	(52.5)
Construction Work In Process
Property, Plant and Equipment [Line Items]
Property Plant and Equipment		950.5	663.1
Non-Utility And Other
Property, Plant and Equipment [Line Items]
Property Plant and Equipment	[2]	623.3	681.7
Accumulated Depreciation and Amortization	[2]	$ (336.8)	$ (368)
[1]	NIPSCO’s common utility plant and associated accumulated depreciation and amortization are allocated between Gas Distribution Utility and Electric Utility Property, Plant and Equipment.
[2]	Non-Utility and Other as of December 31, 2017 includes net book value of $247.8 million related to Bailly Generating Station (Units 7 and 8) which was reclassified from Electric Utility in the fourth quarter of 2016. Depreciation expense for the remaining net book value will continue to be recorded at the composite depreciation rate most recently approved by the IURC. See Note 18-E, "Other Matters," and Note 25, "Subsequent Event," for additional information.

Property, Plant And Equipment P

Property, Plant And Equipment Property, Plant and Equipment (Schedule of Depreciation Provisions For Utility Plant as a Percentage of the Original Cost) (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Electric Operations
Property, Plant and Equipment [Line Items]
Depreciation Provisions For Utility Plant Percentage	3.40%	3.30%	3.10%
Gas Distribution Operations
Property, Plant and Equipment [Line Items]
Depreciation Provisions For Utility Plant Percentage	2.10%	2.10%	2.00%

Property, Plant and Equipment67

Property, Plant and Equipment (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Non Utility Plant Net Book Value	$ 247.8
Depreciation	501,500,000	$ 475,100,000	$ 449,000,000
Capitalized Computer Software, Gross	189,000,000	156,400,000
Capitalized Computer Software, Amortization	$ 44,000,000	$ 41,400,000	$ 41,100,000

Goodwill And Other Intangible68

Goodwill And Other Intangible Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Goodwill And Other Intangible Assets [Line Items]
Goodwill	$ 1,690.7	$ 1,690.7
Intangible assets excluding goodwill	231.7	242.7
Amortization	210.5	199.5
Amortization of Intangible Assets	$ 11	$ 11	$ 11
Finite-Lived Intangible Asset, Useful Life	40 years
Finite-Lived Intangible Assets, Amortization End Date	2,039
Gas Distribution Operations
Goodwill And Other Intangible Assets [Line Items]
Goodwill	$ 1,690.7
Electric Operations
Goodwill And Other Intangible Assets [Line Items]
Goodwill	0
Corporate and Other
Goodwill And Other Intangible Assets [Line Items]
Goodwill	$ 0

Asset Retirement Obligations (C

Asset Retirement Obligations (Changes In Company's Liability For Asset Retirement Obligations) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017		Dec. 31, 2016
Asset Retirement Obligation [Abstract]
Beginning Balance	$ 262.6		$ 254
Accretion recorded as a regulatory asset	10.3		9.2
Additions	2.4		0
Settlements	(15.6)		(7.5)
Change in estimated cash flows	9	[1]	6.9	[2]
Ending Balance	$ 268.7		$ 262.6
[1]	The change in estimated cash flows for 2017 is primarily attributed to changes in estimated costs and settlement timing for electric generating stations and the changes in estimated costs for retirement of gas mains.
[2]	The change in estimated cash flows for 2016 is primarily attributed to the changes in estimated costs for retirement of gas mains partially offset by revisions to estimated costs associated with the EPA's final rule for regulation of CCRs and changes to cost estimates for certain solid waste management units. See Note 18-D, "Environmental Matters," for additional information on CCRs.

Regulatory Matters (Narrative)

Regulatory Matters (Narrative) (Details) - USD ($) $ in Millions

Jan. 30, 2018

Jan. 26, 2018

Dec. 28, 2017

Dec. 22, 2017

Dec. 01, 2017

Oct. 31, 2017

Sep. 27, 2017

Sep. 19, 2017

Apr. 28, 2017

Apr. 26, 2017

Mar. 17, 2017

Dec. 27, 2016

Oct. 31, 2016

Apr. 29, 2016

Jul. 19, 2013

Dec. 31, 2017

Dec. 31, 2017

Dec. 31, 2016

Oct. 26, 2016

Regulatory Matters [Line Items]

Remaining Recovery Period of Regulatory Assets for which No Return on Investment During Recovery Period is Provided

41 years

Regulatory asset not earning return on investment

$ 1,558.4

$ 1,558.4

Expenses recovered as components of cost of service and regulatory orders

1,402.8

1,402.8

Regulatory assets Requiring Specific Rate Action

398.4

398.4

Seven Year Plan of Eligible Investments, Length of Plan

seven-year

Columbia Of Ohio

Regulatory Matters [Line Items]

Public Utilities, Requested Rate Increase (Decrease), Amount

$ 29

Public Utilities, Requested Biennial Rate Increase (Decrease), Amount increases

$ 98

Public Utilities, Approved Rate Increase (Decrease), Amount

$ 31.5

Regulatory Net Capital Expenditures Included In Filing

$ 235.9

Depreciation Before Rate Regulation

719.7

Decrease In Depreciation And Amortization Over That Reflected In Rates

82.3

Depreciation Regulatory Asset

49.3

49.3

$ 57.6

Requested IRP Extension

5 years

Columbia Of Virginia

Regulatory Matters [Line Items]

Public Utilities, Requested Rate Increase (Decrease), Amount

$ 37

Public Utilities, Approved Rate Increase (Decrease), Amount

$ 28.5

Columbia Of Kentucky

Regulatory Matters [Line Items]

Public Utilities, Approved Rate Increase (Decrease), Amount

$ 4.5

Regulatory Net Capital Expenditures Included In Filing

$ 24

NIPSCO

Regulatory Matters [Line Items]

Public Utilities, Requested Rate Increase (Decrease), Amount

$ 143.5

Columbia Of Maryland

Regulatory Matters [Line Items]

Public Utilities, Approved Rate Increase (Decrease), Amount

$ 2.4

Subsequent Event | NIPSCO

Regulatory Matters [Line Items]

Public Utilities, Requested Rate Increase (Decrease) Amount, Revised

$ 117.9

IRP | Columbia Of Ohio

Regulatory Matters [Line Items]

Deferred Depreciation

26.5

26.5

23.4

Capital Expenditure Program | Columbia Of Ohio

Regulatory Matters [Line Items]

Deferred Depreciation

49.8

49.8

31.8

EERM [Domain] | NIPSCO

Regulatory Matters [Line Items]

Deferred Depreciation

13.9

13.9

40.7

TDSIC [Domain] | NIPSCO

Regulatory Matters [Line Items]

Deferred Depreciation

10.3

10.3

5.5

Deferred Post-in-Service Carrying Charges

8.7

8.7

7.1

TDSC-7 [Domain] | NIPSCO

Regulatory Matters [Line Items]

Public Utilities, Approved Rate Increase (Decrease), Amount

$ 3.5

Regulatory Net Capital Expenditures Included In Filing

$ 59

2017 GSEP [Member] [Domain] | Columbia Of Massachusetts

Regulatory Matters [Line Items]

Public Utilities, Approved Rate Increase (Decrease), Amount

$ 8.1

IRP and CEP [Domain] | Columbia Of Ohio

Regulatory Matters [Line Items]

Deferred Post-in-Service Carrying Charges

$ 164.6

$ 164.6

$ 134.9

2017 GSEP | Columbia Of Massachusetts

Regulatory Matters [Line Items]

Regulatory Net Capital Expenditures Included In Filing

$ 72.9

2018 GSEP [Domain] | Columbia Of Massachusetts

Regulatory Matters [Line Items]

Public Utilities, Requested Rate Increase (Decrease), Amount

$ 9.7

Public Utilities, Requested Rate Increase (Decrease), Waiver Amount in excess of Cap

$ 3.1

Public Utilities, Requested Rate Increase (Decrease), Cumulative Amount, If Waiver is not Approved

6.6

Regulatory Net Capital Expenditures Included In Filing

83.9

TDSIC 2 [Domain] | NIPSCO

Regulatory Matters [Line Items]

Public Utilities, Approved Rate Increase (Decrease), Amount

$ 12.8

Regulatory Net Capital Expenditures Included In Filing

$ 133.6

TDSIC-3 [Domain] | Subsequent Event | NIPSCO

Regulatory Matters [Line Items]

Public Utilities, Requested Rate Increase (Decrease), Amount

$ 2

Regulatory Net Capital Expenditures Included In Filing

$ 75

Regulatory Matters (Regulatory

Regulatory Matters (Regulatory Assets) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Regulatory Assets [Line Items]
Total Assets	$ 1,801.2	$ 1,885.4
Unrecognized Pension Benefit And Other Postretirement Benefit Costs
Regulatory Assets [Line Items]
Total Assets	733.5	847.5
Other Postretirement Costs
Regulatory Assets [Line Items]
Total Assets	70.7	59.6
Environmental Costs
Regulatory Assets [Line Items]
Total Assets	63.4	62.6
Regulatory Effects Of Accounting For Income Taxes
Regulatory Assets [Line Items]
Total Assets	238.8	238.4
Underrecovered Gas And Fuel Costs
Regulatory Assets [Line Items]
Total Assets	25.5	73.5
Depreciation
Regulatory Assets [Line Items]
Total Assets	181	187.1
Post-In Service Carrying Charges
Regulatory Assets [Line Items]
Total Assets	173.3	142
Safety Activity Costs
Regulatory Assets [Line Items]
Total Assets	66.5	41.5
DSM Program
Regulatory Assets [Line Items]
Total Assets	40	48.4
Other Assets
Regulatory Assets [Line Items]
Total Assets	$ 208.5	$ 184.8

Regulatory Matters (Regulator72

Regulatory Matters (Regulatory Liabilities) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Regulatory Liabilities [Line Items]
Regulatory Liabilities	$ 2,795.6	$ 1,381.8
Overrecovered Gas And Fuel Costs
Regulatory Liabilities [Line Items]
Regulatory Liabilities	27.6	54.8
Cost Of Removal
Regulatory Liabilities [Line Items]
Regulatory Liabilities	1,096.8	1,174.5
Regulatory Effects Of Accounting For Income Taxes
Regulatory Liabilities [Line Items]
Regulatory Liabilities	1,563.4	30
Other Postretirement Costs
Regulatory Liabilities [Line Items]
Regulatory Liabilities	59	41.2
Other Liabilities
Regulatory Liabilities [Line Items]
Regulatory Liabilities	$ 48.8	$ 81.3

Risk Management Activities (Nar

Risk Management Activities (Narrative) (Details) - USD ($) $ in Millions	Sep. 05, 2017	May 11, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Nov. 17, 2017	Sep. 14, 2017	May 22, 2017
Derivative [Line Items]
Gain (Loss) on Cash Flow Hedge Ineffectiveness, Net			$ 0	$ 0	$ 0
Derivative Instruments, Gain (Loss) Recognized in Income, Ineffective Portion and Amount Excluded from Effectiveness Testing, Net			0	$ 0	$ 0
Debt Instrument, Face Amount						$ 500
Interest Rate Swap
Derivative [Line Items]
Derivative, Notional Amount			1,000
NiSource Finance | Interest Rate Swap
Derivative [Line Items]
Derivative, Notional Amount		$ 950						$ 950
Derivative, Gain (Loss) on Derivative, Net		$ 6.9
NiSource Finance | Treasury Lock
Derivative [Line Items]
Derivative, Notional Amount	$ 750					250	$ 750
Senior Notes | NiSource Finance
Derivative [Line Items]
Debt Instrument, Face Amount			2,000
Derivative, Gain (Loss) on Derivative, Net	$ 19
3.49% Notes due 2027 | Senior Notes | NiSource Finance
Derivative [Line Items]
Debt Instrument, Face Amount			$ 1,000
Debt Instrument, Interest Rate, Stated Percentage			3.49%
4.375% Notes due 2047 | Senior Notes | NiSource Finance
Derivative [Line Items]
Debt Instrument, Face Amount			$ 1,000
Debt Instrument, Interest Rate, Stated Percentage			4.375%
3.95% Notes Due 2048
Derivative [Line Items]
Debt Instrument, Interest Rate, Stated Percentage							3.95%
3.95% Notes Due 2048 | Senior Notes | NiSource Finance
Derivative [Line Items]
Debt Instrument, Face Amount							$ 750
Debt Instrument, Interest Rate, Stated Percentage							3.95%
2.65% Notes Due 2022 | NiSource Finance
Derivative [Line Items]
Debt Instrument, Face Amount						$ 500
Debt Instrument, Interest Rate, Stated Percentage						2.65%

Risk Management Activities (Sch

Risk Management Activities (Schedule of Derivative Instruments in Statement of Financial Position, Fair Value) (Details) - USD ($) $ in Millions		Dec. 31, 2017	Dec. 31, 2016
Derivatives, Fair Value [Line Items]
Derivative Asset		$ 21.1	$ 49
Derivative Liability		71.7	61.3
Risk Management Assets Current
Derivatives, Fair Value [Line Items]
Derivative Asset	[1]	14.5	24.4
Risk Management Assets Noncurrent
Derivatives, Fair Value [Line Items]
Derivative Asset	[2]	6.6	24.6
Risk Management Liabilities Current
Derivatives, Fair Value [Line Items]
Derivative Liability		43.2	16.8
Risk Management Liabilities Noncurrent
Derivatives, Fair Value [Line Items]
Derivative Liability		28.5	44.5
Interest rate risk programs | Risk Management Assets Current
Derivatives, Fair Value [Line Items]
Derivative Asset	[1]	14	17
Interest rate risk programs | Risk Management Assets Noncurrent
Derivatives, Fair Value [Line Items]
Derivative Asset	[2]	5.6	17.1
Interest rate risk programs | Risk Management Liabilities Current
Derivatives, Fair Value [Line Items]
Derivative Liability		38.6	15.3
Interest rate risk programs | Risk Management Liabilities Noncurrent
Derivatives, Fair Value [Line Items]
Derivative Liability		0	24.5
Commodity price risk programs | Risk Management Assets Current
Derivatives, Fair Value [Line Items]
Derivative Asset	[1]	0.5	7.4
Commodity price risk programs | Risk Management Assets Noncurrent
Derivatives, Fair Value [Line Items]
Derivative Asset	[2]	1	7.5
Commodity price risk programs | Risk Management Liabilities Current
Derivatives, Fair Value [Line Items]
Derivative Liability		4.6	1.5
Commodity price risk programs | Risk Management Liabilities Noncurrent
Derivatives, Fair Value [Line Items]
Derivative Liability		$ 28.5	$ 20
[1]	Presented in "Prepayments and other" on the Consolidated Balance Sheets.
[2]	Presented in "Deferred charges and other" on the Consolidated Balance Sheets.

Income Taxes (Narrative) (Detai

Income Taxes (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Taxes [Line Items]
Tax expense at statutory federal income tax rate, rate		35.00%	35.00%	35.00%
Effective income tax rates		71.00%	35.70%	41.60%
Increase (Decrease) in Effective Tax Rate		35.30%	5.90%
Expected benefits to be realized		$ 65.8	$ 43.6
Income Tax Rate Change Period			6 years
Tax Credit Carryforward, Expiration Date		Dec. 31, 2037
Operating Loss Carryforwards, Valuation Allowance		$ 508.5
Deferred Tax Assets, Tax Credit Carryforwards, Alternative Minimum Tax		1.7
Unrecognized Tax Benefits, Income Tax Penalties Accrued		0
Reduction in Deferred Tax Liability Due To Tax Law Change		1,300
Reduction in Regulatory Deferred tax Liabilities		400
Increase in Regulatory Liability Due to Tax Law Change		1,500
Increase in Tax Expense Due to Tax Law Change		200
Federal tax on excess stock compensation [Domain]
Income Taxes [Line Items]
Change in tax expense		161.1
Write Down of Charitable Contributions [Domain]
Income Taxes [Line Items]
Change in tax expense		15	$ 7.2
Unamortized tax benefits		12
PATH and 50% Bonus Depreciation [Domain]
Income Taxes [Line Items]
Change in tax expense		$ 5.8
Minimum
Income Taxes [Line Items]
Tax Credit Carryforward, Expiration Date		Dec. 31, 2028
Maximum
Income Taxes [Line Items]
Tax Credit Carryforward, Expiration Date		Dec. 31, 2037
Subsequent Event
Income Taxes [Line Items]
Tax expense at statutory federal income tax rate, rate	21.00%

Income Taxes (Schedule Of Compo

Income Taxes (Schedule Of Components Of Income Tax Expense) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Income Tax Expense/Benefit [Line Items]
Current federal income taxes	$ 0	$ 0	$ 0
Current state income taxes	7.8	(0.1)	6
Total current income taxes	7.8	(0.1)	6
Deferred federal income taxes	302.7	165.6	124.1
Deferred state income taxes	5	18	13.6
Total deferred income taxes	307.7	183.6	137.7
Deferred Investment Credits	(1)	(1.4)	(2.4)
Income Taxes from Continuing Operations	$ 314.5	$ 182.1	$ 141.3

Income Taxes (Schedule Of Reaso

Income Taxes (Schedule Of Reasons Behind Differences In Computation Of Total Income Taxes) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Reasons Behind Differences in Computation of Total Income Taxes [Line Items]
Book income from Continuing Operations before income taxes	$ 443.1	$ 510.2	$ 339.9
Tax expense at statutory federal income tax rate, value	$ 155	$ 178.6	$ 118.9
Tax expense at statutory federal income tax rate, rate	35.00%	35.00%	35.00%
State income taxes, net of federal income tax benefit, value	$ 6.9	$ 11.3	$ 14.8
State income taxes, net of federal income tax benefit, rate	1.50%	2.20%	4.40%
Regulatory treatment of depreciation differences, value	$ (2.4)	$ (1.5)	$ (1.6)
Regulatory treatment of depreciation differences, rate	(0.50%)	(0.30%)	(0.40%)
Employee Stock Ownership Plan Dividends, value	$ (6.5)	$ (9.5)	$ (2.9)
Employee Stock Ownership Plan Dividends, rate	(1.50%)	(1.90%)	(0.90%)
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Charitable Contributions, Amount	$ (1.2)	$ 2.8	$ 17.8
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Charitable Contributions, Percent	(0.30%)	0.50%	5.20%
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount	$ 161.1	$ 0	$ 0
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Percent	36.40%	0.00%	0.00%
Effective Income Tax Rate Reconciliation, Other Adjustments, Percent	0.40%	0.20%	(1.70%)
Tax accrual adjustments and other, net, value	$ 1.6	$ 0.4	$ (5.7)
Income Taxes from Continuing Operations	$ 314.5	$ 182.1	$ 141.3
Income Taxes from Continuing Operations, rate	71.00%	35.70%	41.60%

Income Taxes (Schedule Of Princ

Income Taxes (Schedule Of Principal Components Of Net Deferred Tax Liability) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Schedule of Principal Components of Net Tax Expense [Line Items]
Accelerated depreciation and other property differences	$ 2,260.7	$ 3,323.5
Unrecovered gas and fuel costs	0	25.9
Other regulatory assets	309.5	449.2
Total Deferred Tax Liabilities	2,570.2	3,798.6
Deferred investment tax credits and other regulatory liabilities	406	93.1
Pension and other postretirement/postemployment benefits	136.7	261.7
Net operating loss carryforward	576	646.2
Other accrued liabilities	24	47
Deferred Tax Assets, Other Accrued Liabilities	37.2	45.5
Other, net	97.4	177.1
Total Deferred Tax Assets	1,277.3	1,270.6
Total Deferred Tax Liabilities	$ 1,292.9	$ 2,528

Pension And Other Postretirem79

Pension And Other Postretirement Benefits (Narrative) (Details) - USD ($) $ in Millions		9 Months Ended	12 Months Ended
		Sep. 30, 2017	Dec. 31, 2018	Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets				$ 2,422.5		$ 1,982.3
Regulatory assets				1,801.2		1,885.4
Regulatory Liabilities				$ 2,795.6		1,381.8
Expected return on plan assets				7.25%
Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets				$ 2,160		1,750.9		$ 1,747.1
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss)				76.1		28.9
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)				(0.7)		(0.2)		0.1
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)				41.1		48.5		$ 25.4
Employer contributions		$ 277		282.3		3.3
Expected contribution				2.9
Defined Benefit Plan, Accumulated Benefit Obligation				2,170.4		2,148.9
Funded status of plan				32.6		414.9
Liability, Defined Benefit Plan	[1]			$ 32.6		$ 414.9
Expected return on plan assets				7.25%		8.00%		8.30%
Settlement loss				$ 13.7		$ 0		$ 2.5
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Remeasurement due to Settlement				3.2
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets				262.5		231.4		225.9
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss)				(18.9)		(5.3)
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)				(4.4)		(4.9)		(5.2)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)				5.3		8		$ 1.3
Employer contributions				31.6		25.5
Expected contribution				25
Funded status of plan				293.8		297.6
Liability, Defined Benefit Plan	[1]			$ 293.8		$ 297.6
Expected return on plan assets				6.99%		7.85%		8.15%
Settlement loss				$ 0		$ 0		$ 0
Significant Unobservable Inputs (Level 3) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets				$ 98.9		73.1		$ 95.3
Percentage of investments				4.00%
Significant Unobservable Inputs (Level 3) [Member] | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets						0
Scenario, Forecast | Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss)			$ 40.9
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)			0.4
Amortization of transition obligation			0
Scenario, Forecast | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss)			3.8
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)			4
Amortization of transition obligation			$ 0
Unrecognized Pension Benefit And Other Postretirement Benefit Costs
Defined Benefit Plan Disclosure [Line Items]
Regulatory assets				$ 733.5		847.5
Regulatory Liabilities				0.1		0.3
International Equities | Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets				351		339.9
International Equities | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets				39.8		41.8
Commingled Funds | Short-Term Money Markets | Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets				34.1	[2]	16.6	[3]
Commingled Funds | Short-Term Money Markets | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets				9.4	[2]	9.5	[3]
Commingled Funds | United States Equities | Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets				466.6	[2]	472	[3]
Commingled Funds | United States Equities | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets				12.2	[2]	12.5	[3]
Commingled Funds | International Equities | Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets				$ 265.1	[2]	$ 223.2	[3]
[1]	NiSource recognizes in its Consolidated Balance Sheets the underfunded and overfunded status of its various defined benefit postretirement plans, measured as the difference between the fair value of the plan assets and the benefit obligation.
[2]	This class of investments is measured at fair value using the net asset value per share and has not been classified in the fair value hierarchy.
[3]	This class of investments is measured at fair value using the net asset value per share and has not been classified in the fair value hierarchy.

Pension And Other Postretirem80

Pension And Other Postretirement Benefits (Schedule Of Portfolio Asset Mix) (Details)	Dec. 31, 2017	Dec. 31, 2016
Minimum | Domestic Equities | Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	16.00%	25.00%
Minimum | Domestic Equities | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	0.00%	35.00%
Minimum | International Equities | Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	8.00%	15.00%
Minimum | International Equities | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	0.00%	15.00%
Minimum | Fixed Income | Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	39.00%
Minimum | Fixed Income | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	20.00%
Minimum | Diversified Credit [Member] | Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	0.00%
Minimum | Diversified Credit [Member] | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	0.00%
Minimum | Real Estate [Member] | Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	0.00%
Minimum | Real Estate [Member] | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	0.00%
Minimum | Real Estate/Private Equity/Hedge Funds | Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage		0.00%
Minimum | Real Estate/Private Equity/Hedge Funds | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage		0.00%
Minimum | Short-Term Investments | Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	0.00%	0.00%
Minimum | Short-Term Investments | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	0.00%	0.00%
Maximum | Domestic Equities | Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	36.00%	45.00%
Maximum | Domestic Equities | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	55.00%	55.00%
Maximum | International Equities | Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	18.00%	25.00%
Maximum | International Equities | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	25.00%	25.00%
Maximum | Fixed Income | Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	51.00%
Maximum | Fixed Income | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	100.00%
Maximum | Diversified Credit [Member] | Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	13.00%
Maximum | Diversified Credit [Member] | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	0.00%
Maximum | Real Estate [Member] | Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	13.00%
Maximum | Real Estate [Member] | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	0.00%
Maximum | Real Estate/Private Equity/Hedge Funds | Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage		15.00%
Maximum | Real Estate/Private Equity/Hedge Funds | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage		0.00%
Maximum | Short-Term Investments | Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	10.00%	10.00%
Maximum | Short-Term Investments | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	10.00%	10.00%

Pension And Other Postretirem81

Pension And Other Postretirement Benefits (Schedule of Plan Asset Mix Prior Year) (Details)	Dec. 31, 2017	Dec. 31, 2016
Pension Plan | Domestic Equities | Minimum
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	16.00%	25.00%
Pension Plan | Domestic Equities | Maximum
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	36.00%	45.00%
Pension Plan | International Equities | Minimum
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	8.00%	15.00%
Pension Plan | International Equities | Maximum
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	18.00%	25.00%
Pension Plan | Fixed Income | Minimum
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage		23.00%
Pension Plan | Fixed Income | Maximum
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage		37.00%
Pension Plan | Real Estate/Private Equity/Hedge Funds | Minimum
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage		0.00%
Pension Plan | Real Estate/Private Equity/Hedge Funds | Maximum
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage		15.00%
Pension Plan | Short-Term Investments | Minimum
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	0.00%	0.00%
Pension Plan | Short-Term Investments | Maximum
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	10.00%	10.00%
Other Postretirement Benefits | Domestic Equities | Minimum
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	0.00%	35.00%
Other Postretirement Benefits | Domestic Equities | Maximum
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	55.00%	55.00%
Other Postretirement Benefits | International Equities | Minimum
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	0.00%	15.00%
Other Postretirement Benefits | International Equities | Maximum
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	25.00%	25.00%
Other Postretirement Benefits | Fixed Income | Minimum
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage		20.00%
Other Postretirement Benefits | Fixed Income | Maximum
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage		50.00%
Other Postretirement Benefits | Real Estate/Private Equity/Hedge Funds | Minimum
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage		0.00%
Other Postretirement Benefits | Real Estate/Private Equity/Hedge Funds | Maximum
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage		0.00%
Other Postretirement Benefits | Short-Term Investments | Minimum
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	0.00%	0.00%
Other Postretirement Benefits | Short-Term Investments | Maximum
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	10.00%	10.00%

Pension And Other Postretirem82

Pension And Other Postretirement Benefits (Schedule Of Pension Plan And Postretirement Plan Asset Mix) (Details) - USD ($) $ in Millions	Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	$ 2,422.5		$ 1,982.3
Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	$ 2,160		$ 1,750.9		$ 1,747.1
Percentage of total asset	100.00%		100.00%
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	$ 262.5		$ 231.4		$ 225.9
Percentage of total asset	100.00%		100.00%
Domestic Equities | Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	$ 698.2		$ 755.2
Percentage of total asset	32.30%		43.10%
Domestic Equities | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	$ 96		$ 97.9
Percentage of total asset	36.60%		42.30%
International Equities | Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	$ 351		$ 339.9
Percentage of total asset	16.20%		19.40%
International Equities | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	$ 39.8		$ 41.8
Percentage of total asset	15.20%		18.00%
Fixed Income | Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	$ 977.6		$ 565.8
Percentage of total asset	45.30%		32.30%
Fixed Income | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	$ 117.5		$ 87
Percentage of total asset	44.80%		37.60%
Real Estate/Private Equity/Hedge Funds | Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	$ 49.9		$ 74.8
Percentage of total asset	2.30%		4.30%
Real Estate/Private Equity/Hedge Funds | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	$ 0		$ 0
Percentage of total asset	0.00%		0.00%
Cash/Other [Member] | Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	$ 83.3		$ 15.2
Percentage of total asset	3.90%		0.90%
Cash/Other [Member] | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	$ 9.2		$ 4.7
Percentage of total asset	3.40%		2.10%
Commingled Funds | Short-Term Money Markets | Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	$ 34.1	[1]	$ 16.6	[2]
Commingled Funds | Short-Term Money Markets | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	9.4	[1]	9.5	[2]
Commingled Funds | International Equities | Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	265.1	[1]	223.2	[2]
Commingled Funds | Fixed Income | Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	244.9	[1]	280.7	[2]
Commingled Funds | United States Equities | Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	466.6	[1]	472	[2]
Commingled Funds | United States Equities | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	$ 12.2	[1]	$ 12.5	[2]
[1]	This class of investments is measured at fair value using the net asset value per share and has not been classified in the fair value hierarchy.
[2]	This class of investments is measured at fair value using the net asset value per share and has not been classified in the fair value hierarchy.

Pension And Other Postretirem83

Pension And Other Postretirement Benefits (Schedule Of Fair Value Of Pension Plan Assets) (Details) - USD ($) $ in Millions	Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	$ 2,422.5		$ 1,982.3
Quoted Prices In Active Markets For Identical Assets (Level 1) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	810.5		751.3
Significant Other Observable Inputs (Level 2) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	476		146.3
Significant Unobservable Inputs (Level 3) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	98.9		73.1		$ 95.3
Due To Brokers Net [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	(2.5)	[1]	(5)	[2]
Receivables/Payables [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	7.3		2.1
Fixed Income Securities [Member] | Other Fixed Income [Member] | Significant Unobservable Inputs (Level 3) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	0		0.1		0.1
Private Equity Limited Partnerships [Member] | U.S. Multi-Strategy [Member] | Significant Unobservable Inputs (Level 3) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	26.7		34.8		46.4
Private Equity Limited Partnerships [Member] | International Multi-Strategy [Member] | Significant Unobservable Inputs (Level 3) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	19.1		24.9		29.3
Private Equity Limited Partnerships [Member] | Distressed Opportunities [Member] | Significant Unobservable Inputs (Level 3) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	3.2		4.1		5.9
Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	2,160		1,750.9		1,747.1
Defined Benefit Plan Fair Value Of Plan Assets Before Pending Items	2,155.2		1,749.2
Pension Plan | Quoted Prices In Active Markets For Identical Assets (Level 1) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan Fair Value Of Plan Assets Before Pending Items	569.6		537.3
Pension Plan | Significant Other Observable Inputs (Level 2) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan Fair Value Of Plan Assets Before Pending Items	476		146.3
Pension Plan | Significant Unobservable Inputs (Level 3) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan Fair Value Of Plan Assets Before Pending Items	98.9		73.1
Pension Plan | Cash [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	9.7		1.9
Pension Plan | Cash [Member] | Quoted Prices In Active Markets For Identical Assets (Level 1) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	9.7		1.9
Pension Plan | Cash [Member] | Significant Other Observable Inputs (Level 2) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	0		0
Pension Plan | Cash [Member] | Significant Unobservable Inputs (Level 3) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	0		0
Pension Plan | International Equities
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	351		339.9
Pension Plan | Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	977.6		565.8
Pension Plan | Fixed Income Securities [Member] | Government
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	143.4		42.2
Pension Plan | Fixed Income Securities [Member] | Government | Quoted Prices In Active Markets For Identical Assets (Level 1) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	0		0
Pension Plan | Fixed Income Securities [Member] | Government | Significant Other Observable Inputs (Level 2) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	143.4		42.2
Pension Plan | Fixed Income Securities [Member] | Government | Significant Unobservable Inputs (Level 3) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	0		0
Pension Plan | Fixed Income Securities [Member] | Corporate [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	332.6		104.1
Pension Plan | Fixed Income Securities [Member] | Corporate [Member] | Quoted Prices In Active Markets For Identical Assets (Level 1) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	0		0
Pension Plan | Fixed Income Securities [Member] | Corporate [Member] | Significant Other Observable Inputs (Level 2) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	332.6		104.1
Pension Plan | Fixed Income Securities [Member] | Corporate [Member] | Significant Unobservable Inputs (Level 3) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	0		0
Pension Plan | Fixed Income Securities [Member] | Other Fixed Income [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets			0.1
Pension Plan | Fixed Income Securities [Member] | Other Fixed Income [Member] | Quoted Prices In Active Markets For Identical Assets (Level 1) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets			0
Pension Plan | Fixed Income Securities [Member] | Other Fixed Income [Member] | Significant Other Observable Inputs (Level 2) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets			0
Pension Plan | Fixed Income Securities [Member] | Other Fixed Income [Member] | Significant Unobservable Inputs (Level 3) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets			0.1
Pension Plan | Commingled Funds | International Equities
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	265.1	[3]	223.2	[4]
Pension Plan | Commingled Funds | Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	244.9	[3]	280.7	[4]
Pension Plan | Commingled Funds | United States Equities
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	466.6	[3]	472	[4]
Pension Plan | Commingled Funds | Short-Term Money Markets
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	34.1	[3]	16.6	[4]
Pension Plan | Private Equity Limited Partnerships [Member] | U.S. Multi-Strategy [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	26.7	[5]	34.8	[6]
Pension Plan | Private Equity Limited Partnerships [Member] | U.S. Multi-Strategy [Member] | Quoted Prices In Active Markets For Identical Assets (Level 1) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	0	[5]	0	[6]
Pension Plan | Private Equity Limited Partnerships [Member] | U.S. Multi-Strategy [Member] | Significant Other Observable Inputs (Level 2) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	0	[5]	0	[6]
Pension Plan | Private Equity Limited Partnerships [Member] | U.S. Multi-Strategy [Member] | Significant Unobservable Inputs (Level 3) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	26.7	[5]	34.8	[6]
Pension Plan | Private Equity Limited Partnerships [Member] | International Multi-Strategy [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	19.1	[7]	24.9	[8]
Pension Plan | Private Equity Limited Partnerships [Member] | International Multi-Strategy [Member] | Quoted Prices In Active Markets For Identical Assets (Level 1) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	0	[7]	0	[8]
Pension Plan | Private Equity Limited Partnerships [Member] | International Multi-Strategy [Member] | Significant Other Observable Inputs (Level 2) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	0	[7]	0	[8]
Pension Plan | Private Equity Limited Partnerships [Member] | International Multi-Strategy [Member] | Significant Unobservable Inputs (Level 3) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	19.1	[7]	24.9	[8]
Pension Plan | Private Equity Limited Partnerships [Member] | Distressed Opportunities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	3.2		4.1
Pension Plan | Private Equity Limited Partnerships [Member] | Distressed Opportunities [Member] | Quoted Prices In Active Markets For Identical Assets (Level 1) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	0		0
Pension Plan | Private Equity Limited Partnerships [Member] | Distressed Opportunities [Member] | Significant Other Observable Inputs (Level 2) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	0		0
Pension Plan | Private Equity Limited Partnerships [Member] | Distressed Opportunities [Member] | Significant Unobservable Inputs (Level 3) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	3.2		4.1
Pension Plan | Real Estate [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	49.9		9.2
Pension Plan | Real Estate [Member] | Quoted Prices In Active Markets For Identical Assets (Level 1) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	0		0
Pension Plan | Real Estate [Member] | Significant Other Observable Inputs (Level 2) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	0		0
Pension Plan | Real Estate [Member] | Significant Unobservable Inputs (Level 3) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	49.9		9.2
Pension Plan | Mutual Funds [Member] | International Equities
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	85.8		116.6
Pension Plan | Mutual Funds [Member] | International Equities | Quoted Prices In Active Markets For Identical Assets (Level 1) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	85.8		116.6
Pension Plan | Mutual Funds [Member] | U.S. Multi-Strategy [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	231.5		283.2
Pension Plan | Mutual Funds [Member] | U.S. Multi-Strategy [Member] | Quoted Prices In Active Markets For Identical Assets (Level 1) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	231.5		283.2
Pension Plan | Mutual Funds [Member] | Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	242.3		135.6
Pension Plan | Mutual Funds [Member] | Fixed Income | Quoted Prices In Active Markets For Identical Assets (Level 1) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	242.3		135.6
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	262.5		231.4		$ 225.9
Defined Benefit Plan Fair Value Of Plan Assets Before Pending Items	262.5		236
Other Postretirement Benefits | Quoted Prices In Active Markets For Identical Assets (Level 1) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets			214
Defined Benefit Plan Fair Value Of Plan Assets Before Pending Items	240.9
Other Postretirement Benefits | Significant Other Observable Inputs (Level 2) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets			0
Defined Benefit Plan Fair Value Of Plan Assets Before Pending Items	0
Other Postretirement Benefits | Significant Unobservable Inputs (Level 3) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets			0
Defined Benefit Plan Fair Value Of Plan Assets Before Pending Items	0
Other Postretirement Benefits | International Equities
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	39.8		41.8
Other Postretirement Benefits | Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	117.5		87
Other Postretirement Benefits | Commingled Funds | United States Equities
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	12.2	[3]	12.5	[4]
Other Postretirement Benefits | Commingled Funds | Short-Term Money Markets
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	9.4	[3]	9.5	[4]
Other Postretirement Benefits | Mutual Funds [Member] | International Equities
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	39.8		41.8
Other Postretirement Benefits | Mutual Funds [Member] | International Equities | Quoted Prices In Active Markets For Identical Assets (Level 1) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	39.8		41.8
Other Postretirement Benefits | Mutual Funds [Member] | International Equities | Significant Other Observable Inputs (Level 2) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	0		0
Other Postretirement Benefits | Mutual Funds [Member] | International Equities | Significant Unobservable Inputs (Level 3) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	0		0
Other Postretirement Benefits | Mutual Funds [Member] | Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	117.3		86.8
Other Postretirement Benefits | Mutual Funds [Member] | Fixed Income | Quoted Prices In Active Markets For Identical Assets (Level 1) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	117.3		86.8
Other Postretirement Benefits | Mutual Funds [Member] | Fixed Income | Significant Other Observable Inputs (Level 2) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	0		0
Other Postretirement Benefits | Mutual Funds [Member] | Fixed Income | Significant Unobservable Inputs (Level 3) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	0		0
Other Postretirement Benefits | Mutual Funds [Member] | United States Equities
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	83.8		85.4
Other Postretirement Benefits | Mutual Funds [Member] | United States Equities | Quoted Prices In Active Markets For Identical Assets (Level 1) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	83.8		85.4
Other Postretirement Benefits | Mutual Funds [Member] | United States Equities | Significant Other Observable Inputs (Level 2) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	0		0
Other Postretirement Benefits | Mutual Funds [Member] | United States Equities | Significant Unobservable Inputs (Level 3) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	$ 0		$ 0
[1]	This class represents pending trades with brokers.
[2]	This class represents pending trades with brokers.
[3]	This class of investments is measured at fair value using the net asset value per share and has not been classified in the fair value hierarchy.
[4]	This class of investments is measured at fair value using the net asset value per share and has not been classified in the fair value hierarchy.
[5]	This class includes limited partnerships/fund of funds that invest in a diverse portfolio of private equity strategies, including buy-outs, venture capital, growth capital, special situations and secondary markets, primarily inside the United States.
[6]	This class includes limited partnerships/fund of funds that invest in a diverse portfolio of private equity strategies, including buy-outs, venture capital, growth capital, special situations and secondary markets, primarily in the United States.
[7]	This class includes limited partnerships/fund of funds that invest in diverse portfolio of private equity strategies, including buy-outs, venture capital, growth capital, special situations and secondary markets, primarily outside the United States.
[8]	This class includes limited partnerships/fund of funds that invest in a diverse portfolio of private equity strategies, including buy-outs, venture capital, growth capital, special situations and secondary markets, primarily outside the United States.

Pension And Other Postretirem84

Pension And Other Postretirement Benefits (Schedule Of Changes In The Fair Value Of The Plan Level Three Assets) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at beginning of year	$ 1,982.3
Fair value of plan assets at end of year	2,422.5	$ 1,982.3
Significant Unobservable Inputs (Level 3) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at beginning of year	73.1	95.3
Total gains or losses (unrealized / realized)	2.9	3.8
Purchases	43.1	2
(Sales)	(20.2)	(28)
Fair value of plan assets at end of year	98.9	73.1
Real Estate [Member] | Significant Unobservable Inputs (Level 3) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at beginning of year	9.2	13.6
Total gains or losses (unrealized / realized)	(0.6)	0.1
Purchases	42.1	0.1
(Sales)	(0.8)	(4.6)
Fair value of plan assets at end of year	49.9	9.2
Private Equity Limited Partnerships [Member] | U.S. Multi-Strategy [Member] | Significant Unobservable Inputs (Level 3) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at beginning of year	34.8	46.4
Total gains or losses (unrealized / realized)	2.1	2.1
Purchases	0.9	0.8
(Sales)	(11.1)	(14.5)
Fair value of plan assets at end of year	26.7	34.8
Private Equity Limited Partnerships [Member] | International Multi-Strategy [Member] | Significant Unobservable Inputs (Level 3) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at beginning of year	24.9	29.3
Total gains or losses (unrealized / realized)	1.1	2
Purchases	0.1	1
(Sales)	(7)	(7.4)
Fair value of plan assets at end of year	19.1	24.9
Private Equity Limited Partnerships [Member] | Distressed Opportunities [Member] | Significant Unobservable Inputs (Level 3) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at beginning of year	4.1	5.9
Total gains or losses (unrealized / realized)	0.4	(0.4)
Purchases	0	0.1
(Sales)	(1.3)	(1.5)
Fair value of plan assets at end of year	3.2	4.1
Fixed Income Securities [Member] | Other Fixed Income [Member] | Significant Unobservable Inputs (Level 3) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at beginning of year	0.1	0.1
Total gains or losses (unrealized / realized)	(0.1)	0
Purchases	0	0
(Sales)	0	0
Fair value of plan assets at end of year	$ 0	$ 0.1

Pension and Other Postretirem85

Pension and Other Postretirement Benefits (Schedule of Net Asset Value Per Share) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Net Asset Value Excluded From Fair Value By Input	$ 1,032.3	$ 1,014.5
Commingled Funds | Short-Term Money Markets
Defined Benefit Plan Disclosure [Line Items]
Net Asset Value Excluded From Fair Value By Input	$ 43.5	$ 26.1
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Investment Redemption, Frequency	Daily	Daily
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Investment Redemption, Notice Period	1 day	1 day
Commingled Funds | United States Equities
Defined Benefit Plan Disclosure [Line Items]
Net Asset Value Excluded From Fair Value By Input	$ 478.8	$ 484.5
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Investment Redemption, Frequency	Monthly	Monthly
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Investment Redemption, Notice Period	3 days	3 days
Commingled Funds | International Equities
Defined Benefit Plan Disclosure [Line Items]
Net Asset Value Excluded From Fair Value By Input	$ 265.1	$ 223.2
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Investment Redemption, Frequency	Monthly	Monthly
Commingled Funds | Fixed Income Funds
Defined Benefit Plan Disclosure [Line Items]
Net Asset Value Excluded From Fair Value By Input	$ 244.9	$ 280.7
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Investment Redemption, Frequency	Monthly	Monthly
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Investment Redemption, Notice Period	3 days	3 days
Minimum | Commingled Funds | International Equities
Defined Benefit Plan Disclosure [Line Items]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Investment Redemption, Notice Period	10 days	14 days
Maximum | Commingled Funds | International Equities
Defined Benefit Plan Disclosure [Line Items]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Investment Redemption, Notice Period	30 days	30 days

Pension And Other Postretirem86

Pension And Other Postretirement Benefits (Schedule Of Reconciliation Of The Plans Funded Status And Amounts Reflected) (Details) - USD ($) $ in Millions		9 Months Ended	12 Months Ended
		Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016		Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at beginning of year		$ 1,982.3	$ 1,982.3
Fair value of plan assets at end of year			2,422.5	$ 1,982.3
Noncurrent liabilities			(293.1)	(296.9)
Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation at beginning of year	[1]	2,165.8	2,165.8	2,206.7
Service cost			30	30.7		$ 34.8
Interest cost			68.3	89.7		95.9
Plan participants' contributions			0	0
Plan amendments			0.9	0
Actuarial loss (gain)			98.3	(2.7)
Settlement Loss			1.6	0
Defined Benefit Plan, Benefit Obligation, Benefits Paid			172.3	158.6
Benefits paid			(172.3)	(158.6)
Estimated benefits paid by incurred subsidy			0	0
Projected benefit obligation at end of year			2,192.6	2,165.8	[1]	2,206.7	[1]
Fair value of plan assets at beginning of year		1,750.9	1,750.9	1,747.1
Actual return on plan assets			299.1	159.1
Employer contributions		277	282.3	3.3
Defined Benefit Plan, Plan Assets, Contributions by Plan Participant			0	0
Fair value of plan assets at end of year			2,160	1,750.9		1,747.1
Defined Benefit Plan, Funded (Unfunded) Status of Plan			(32.6)	(414.9)
Noncurrent assets			9.8	0
Current liabilities			(2.8)	(2.9)
Noncurrent liabilities			(39.6)	(412)
Net amount recognized at end of year	[2]		(32.6)	(414.9)
Unrecognized prior service cost	[3]		2.5	1
Unrecognized actuarial loss	[3]		692.9	835.5
Defined Benefit Plan Amounts Recognized In Other Comprehensive Income Or Regulatory Asset Or Liability	[3]		695.4	836.5
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation at beginning of year	[1]	529	529	525.8
Service cost			4.8	5		6.4
Interest cost			17.8	22		24.9
Plan participants' contributions			5.7	5.9
Plan amendments			1.6	7.5
Actuarial loss (gain)			36.2	1
Settlement Loss			0	0
Defined Benefit Plan, Benefit Obligation, Benefits Paid			39.3	38.9
Benefits paid			(39.3)	(38.9)
Estimated benefits paid by incurred subsidy			0.5	0.7
Projected benefit obligation at end of year			556.3	529	[1]	525.8	[1]
Fair value of plan assets at beginning of year		$ 231.4	231.4	225.9
Actual return on plan assets			33.1	13
Employer contributions			31.6	25.5
Defined Benefit Plan, Plan Assets, Contributions by Plan Participant			5.7	5.9
Fair value of plan assets at end of year			262.5	231.4		$ 225.9
Defined Benefit Plan, Funded (Unfunded) Status of Plan			(293.8)	(297.6)
Noncurrent assets			0	0
Current liabilities			(0.7)	(0.7)
Net amount recognized at end of year	[2]		(293.8)	(297.6)
Unrecognized prior service cost	[3]		(23.1)	(29.2)
Unrecognized actuarial loss	[3]		84.2	68.3
Defined Benefit Plan Amounts Recognized In Other Comprehensive Income Or Regulatory Asset Or Liability	[3]		$ 61.1	$ 39.1
[1]	The change in benefit obligation for Pension Benefits represents the change in Projected Benefit Obligation while the change in benefit obligation for Other Postretirement Benefits represents the change in accumulated postretirement benefit obligation.
[2]	NiSource recognizes in its Consolidated Balance Sheets the underfunded and overfunded status of its various defined benefit postretirement plans, measured as the difference between the fair value of the plan assets and the benefit obligation.
[3]	NiSource determined that for certain rate-regulated subsidiaries the future recovery of pension and other postretirement benefits costs is probable. These rate-regulated subsidiaries recorded regulatory assets and liabilities of $733.5 million and $0.1 million, respectively, as of December 31, 2017, and $847.5 million and $0.3 million, respectively, as of December 31, 2016 that would otherwise have been recorded to accumulated other comprehensive loss.

Pension and Other Postretirem87

Pension and Other Postretirement Benefits (Schedule of Benefit Obligations in Excess of Fair Value) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016		Dec. 31, 2015
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	$ 2,422.5	$ 1,982.3
Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Accumulated Benefit Obligation	2,170.4	2,148.9
Defined Benefit Plan, Benefit Obligation	2,192.6	2,165.8	[1]	$ 2,206.7	[1]
Defined Benefit Plan, Fair Value of Plan Assets	2,160	1,750.9		$ 1,747.1
Defined Benefit Plan, Funded (Unfunded) Status of Plan	(32.6)	(414.9)
Underfunded Plan [Member] | Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Accumulated Benefit Obligation	1,502.5	2,148.9
Defined Benefit Plan, Benefit Obligation	1,524.7	2,165.8
Defined Benefit Plan, Fair Value of Plan Assets	1,482.3	1,750.9
Defined Benefit Plan, Funded (Unfunded) Status of Plan	(42.4)	(414.9)
Overfunded Plan [Member] | Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Accumulated Benefit Obligation	667.9	0
Defined Benefit Plan, Benefit Obligation	667.9	0
Defined Benefit Plan, Fair Value of Plan Assets	677.7	0
Defined Benefit Plan, Funded (Unfunded) Status of Plan	$ 9.8	$ 0
[1]	The change in benefit obligation for Pension Benefits represents the change in Projected Benefit Obligation while the change in benefit obligation for Other Postretirement Benefits represents the change in accumulated postretirement benefit obligation.

Pension And Other Postretirem88

Pension And Other Postretirement Benefits (Schedule Of Significant Actuarial Assumptions In Determining Funded Status Plan) (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Discount Rate	3.58%	4.03%
Rate of Compensation Increases	4.00%	4.00%
Trend for Next Year	0.00%	0.00%
Ultimate Trend	0.00%	0.00%
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Discount Rate	3.67%	4.12%
Rate of Compensation Increases	0.00%	0.00%
Trend for Next Year	8.52%	8.43%
Ultimate Trend	4.50%	4.50%
Year Ultimate Trend Reached	2,025	2,024

Pension And Other Postretirem89

Pension And Other Postretirement Benefits (Schedule Of One-Percentage-Point Change In Assumed Health Care Cost Trend Rates) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017USD ($)
Defined Benefit Plan Disclosure [Line Items]
Effect on service and interest components of net periodic cost, 1% point increase	$ 1.1
Effect on service and interest components of net periodic cost, 1% point decrease	(0.9)
Effect on accumulated postretirement benefit obligation, 1% point increase	29.7
Effect on accumulated postretirement benefit obligation, 1% point decrease	$ (25.9)

Pension And Other Postretirem90

Pension And Other Postretirement Benefits (Schedule Of Expected Payments To Participants In Pension Plan) (Details) $ in Millions	Dec. 31, 2017USD ($)
Pension Plan
Defined Benefit Plan Disclosure [Line Items]
2,017	$ 176.2
2,018	173.7
2,019	172.1
2,020	172
2,021	171.3
2022-2026	784.7
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
2,017	34.3
2,018	35.3
2,019	36.3
2,020	36.9
2,021	36.9
2022-2026	178.9
Postretirement Health Coverage [Member]
Defined Benefit Plan Disclosure [Line Items]
2,017	0.5
2,018	0.5
2,019	0.5
2,020	0.5
2,021	0.5
2022-2026	$ 1.9

Pension And Other Postretirem91

Pension And Other Postretirement Benefits (Components Of The Plans' Net Periodic Benefits Cost) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Service Cost	$ 30	$ 30.7	$ 34.8
Interest cost	68.3	89.7	95.9
Expected return on assets	(123.1)	(132.9)	(167.2)
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)	(0.7)	(0.2)	0.1
Recognized actuarial loss	52.9	61.2	59.3
Net Periodic Benefit Costs	27.4	48.5	22.9
Settlement loss	13.7	0	2.5
Total Net Periodic Benefits Cost	41.1	48.5	25.4
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service Cost	4.8	5	6.4
Interest cost	17.8	22	24.9
Expected return on assets	(15.9)	(17.2)	(28.2)
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)	(4.4)	(4.9)	(5.2)
Recognized actuarial loss	3	3.1	3.4
Net Periodic Benefit Costs	5.3	8	1.3
Settlement loss	0	0	0
Total Net Periodic Benefits Cost	$ 5.3	$ 8	$ 1.3

Pension And Other Postretirem92

Pension And Other Postretirement Benefits (Schedule Of Key Assumptions That Were Used To Calculate The Net Periodic Benefits Cost) (Details)		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Expected Long-Term Rate of Return on Plan Assets		7.25%
Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Discount Rate	[1]	4.40%	4.24%	3.81%
Defined Benefit Plan Assumptions Used In Calculating Net Periodic Cost Discount Rate for Interest Cost	[1]	3.31%	4.24%	3.81%
Expected Long-Term Rate of Return on Plan Assets		7.25%	8.00%	8.30%
Rate of Compensation Increases		4.00%	4.00%	4.00%
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Discount Rate	[1]	4.58%	4.33%	3.94%
Defined Benefit Plan Assumptions Used In Calculating Net Periodic Cost Discount Rate for Interest Cost	[1]	3.48%	4.33%	3.94%
Expected Long-Term Rate of Return on Plan Assets		6.99%	7.85%	8.15%
Rate of Compensation Increases		0.00%	0.00%	0.00%
[1]	In January 2017, NiSource changed the method used to estimate the service and interest components of net periodic benefit cost for pension and other postretirement benefits. This change, compared to the previous method, resulted in a decrease in the actuarially-determined service and interest cost components. Historically, NiSource estimated service and interest cost utilizing a single weighted-average discount rate derived from the yield curve used to measure the benefit obligation at the beginning of the period. For fiscal 2017 and beyond, NiSource now utilizes a full yield curve approach to estimate these components by applying the specific spot rates along the yield curve used in the determination of the benefit obligation to the relevant projected cash flows.

Pension And Other Postretirem93

Pension And Other Postretirement Benefits (Schedule Of Changes In Plan Assets And Projected Benefit Obligations Recognized In Other Comprehensive Income) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Plan
Net prior service cost/(credit)	$ 0.9	$ 0
Net actuarial (gain)/loss	(76.1)	(28.9)
Settlements	(13.7)	0	$ (2.5)
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)	0.7	0.2	(0.1)
Less: amortization of net actuarial (gain) loss	(52.9)	(61.2)	(59.3)
Total Recognized in Other Comprehensive Income or Regulatory Asset or Liability	(141.1)	(89.9)
Amount Recognized in Net Periodic Benefits Cost and Other Comprehensive Income or Regulatory Asset or Liability	(100)	(41.4)
Other Postretirement Benefits
Net prior service cost/(credit)	1.6	7.5
Net actuarial (gain)/loss	18.9	5.3
Settlements	0	0	0
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)	4.4	4.9	5.2
Less: amortization of net actuarial (gain) loss	(3)	(3.1)	$ (3.4)
Total Recognized in Other Comprehensive Income or Regulatory Asset or Liability	21.9	14.6
Amount Recognized in Net Periodic Benefits Cost and Other Comprehensive Income or Regulatory Asset or Liability	$ 27.2	$ 22.6

Common Stock (Details)

Common Stock (Details) - USD ($)	1 Months Ended	12 Months Ended
	Dec. 08, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Forward Contract Indexed to Issuer's Equity [Line Items]
Common stock, shares authorized		400,000,000	400,000,000
Common stock, par value		$ 0.01	$ 0.01
Common Stock, Dividends, Per Share, Cash Paid		$ 0.70	$ 0.64	$ 0.83
Dividend Declared		$ 0.195
Dividends Payable, Date to be Paid		Feb. 20, 2018
Dividends Payable, Date of Record		Feb. 9, 2018
At The Market Program [Member]
Forward Contract Indexed to Issuer's Equity [Line Items]
Common Stock Aggregate Sale Price		$ 500,000,000
ATM Program Equity Remaining Available for Issuance		$ 10,000,000
Forward Agreement [Member]
Forward Contract Indexed to Issuer's Equity [Line Items]
Forward Contract Indexed to Issuer's Equity, Indexed Shares		6,345,860
Forward Contract Indexed to Issuer's Equity, Settlement Alternatives, Cash, at Fair Value		$ 171,200,000
Derivative, Forward Price	$ 27.24	$ 26.98

Common Stock (Schedule of Stock

Common Stock (Schedule of Stock Offering Program) (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
At The Market Stock Offering [Line Items]
Proceeds, net of fees (in millions)	$ 336.7	$ 23.1	$ 22.5
At The Market Program [Member]
At The Market Stock Offering [Line Items]
Stock Issued During Period, Shares, New Issues	11,931,376	0	0
Common Stock Issued Average Price Per Share	$ 26.58	$ 0	$ 0
Proceeds, net of fees (in millions)	$ 314.7	$ 0	$ 0

Share-Based Compensation (Narra

Share-Based Compensation (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	May 12, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock available for awards, shares				8,000,000
Common stock reserved for future awards, shares	4,455,389
Stock-based employee compensation expense	$ 15.3	$ 15.1	$ 18.8
Related tax benefits	5.9	5.8	7.2
Excess Tax Benefit from Share-based Compensation, Operating Activities	4.4	7.2
Unrecognized compensation cost related to nonvested awards	$ 19.4
Weighted-average remaining requisite service period, years	1 year 8 months
401(k) match, profit sharing and non-elective expense	$ 37.6	$ 32.3	$ 27.4
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay	3.00%
Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted	660,750	647,305
Fair value of shares granted	$ 12.9	$ 12.6
Shares nonvested	1,200,000	647,305
Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted	10,983		660,230
Fair value of shares granted			$ 23.9
Shares vesting period, (years)	3 years
Shares nonvested	698,126	1,642,030
Omnibus Plan | Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Units issued	225,613
Director Plan | Non-Employee Director Award
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares nonvested	54,964
2014 Award | Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares nonvested	635,795
2017 Performance Awards | Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares nonvested	604,944
2016 Performance Awards | Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares nonvested	579,829

Share-Based Compensation (Sched

Share-Based Compensation (Schedule Of Transactions Of Restricted Stock Unit) (Details) - Restricted Stock Units (RSUs) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Nonvested, Other Than Options	1,642,030
Nonvested, Weighted Average Grant Date Fair Value	$ 12.05
Granted, Other Than Options	10,983	660,230
Granted, Weighted Average Grant Date Fair Value	$ 22.87
Forfeited, Other Than Options	(85,436)
Forfeited, Weighted Average Grant Date Fair Value	$ 14.64
Vested, Other Than Options	(869,451)
Vested, Weighted Average Grant Date Fair Value	$ 9.33
Nonvested, Other Than Options	698,126
Nonvested, Weighted Average Grant Date Fair Value	$ 15.09

Share-Based Compensation (Sch98

Share-Based Compensation (Schedule Of Transactions Of Contingent Awards) (Details) - Performance Shares - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Nonvested, Other Than Options	647,305
Nonvested, Weighted Average Grant Date Fair Value	$ 19.50
Granted, Other Than Options	660,750	647,305
Granted, Weighted Average Grant Date Fair Value	$ 19.50
Forfeited, Other Than Options	(123,282)
Forfeited, Weighted Average Grant Date Fair Value	$ 19.45
Vested, Other Than Options	0
Vested, Weighted Average Grant Date Fair Value	$ 0
Nonvested, Other Than Options	1,200,000	647,305
Nonvested, Weighted Average Grant Date Fair Value	$ 19.52	$ 19.50

Long-Term Debt (Narrative) (Det

Long-Term Debt (Narrative) (Details) - USD ($) $ in Millions

12 Months Ended

Dec. 31, 2017

Dec. 31, 2016

Dec. 31, 2015

Nov. 17, 2017

Sep. 15, 2017

Sep. 14, 2017

Sep. 05, 2017

Aug. 04, 2017

Jul. 01, 2017

Jun. 12, 2017

May 22, 2017

May 11, 2017

Apr. 03, 2017

Mar. 27, 2017

Nov. 28, 2016

Nov. 01, 2016

Jun. 30, 2016

Mar. 31, 2016

Mar. 15, 2016

May 22, 2015

Debt Instrument [Line Items]

Debt Instrument, Convertible, Conversion Ratio

67.6

Face amount of notes

$ 500

Debt Instrument Tendered

$ 990.7

Payment for Debt Extinguishment or Debt Prepayment Cost

111.5

$ 0

$ 97.2

Loss on early extinguishment of long-term debt

(111.5)

$ 0

$ (97.2)

Cross default provision on default relating to any indebtedness

50

Security interest and other subset of asset

$ 150

Asset sale covenant percentage of consolidated total assets

10.00%

Percentage of additional subset of assets capped

10.00%

3.26% Notes Due 2019

Debt Instrument [Line Items]

Debt Instrument, Maturity Date

Oct. 31, 2019

4.375% Notes due 2047

Debt Instrument [Line Items]

Debt Instrument, Maturity Date

Dec. 31, 2027

NiSource Finance | Senior Notes

Debt Instrument [Line Items]

Face amount of notes

$ 2,000

NiSource Finance | Variable Rate Term Loan

Debt Instrument [Line Items]

Long-term Debt

$ 500

$ 500

NiSource Finance | Interest Rate Swap

Debt Instrument [Line Items]

Notional Amount of Forward-Starting Interest rate Swap Agreements

$ 500

NiSource Finance | Long Term Unsecured Notes Due 2016

Debt Instrument [Line Items]

Debt Instrument, Repurchased Face Amount

$ 90

$ 201.5

Interest rate on debt

5.41%

10.75%

NIPSCO | 3.95% Notes Due 2018

Debt Instrument [Line Items]

Debt, Weighted Average Interest Rate

3.95%

NIPSCO | 7.61% Notes Due 2027

Debt Instrument [Line Items]

Debt, Weighted Average Interest Rate

7.61%

NIPSCO | Pollution Control Bond

Debt Instrument [Line Items]

Debt Instrument, Repurchased Face Amount

$ 130

Interest rate on debt

5.60%

Columbia Pipeline Group

Debt Instrument [Line Items]

Face amount of notes

$ 2,750

Ni Source Corporate Services | 3.26% Notes Due 2019

Debt Instrument [Line Items]

Debt, Weighted Average Interest Rate

3.26%

Columbia Of Ohio | 6.53% Notes Due 2038

Debt Instrument [Line Items]

Debt, Weighted Average Interest Rate

6.41%

Columbia Of Virginia | 12.27% Notes Due 2029

Debt Instrument [Line Items]

Debt, Weighted Average Interest Rate

12.21%

Columbia Of Kentucky | 3.79% Notes Due 2027 [Member]

Debt Instrument [Line Items]

Debt Instrument, Maturity Date

May 31, 2027

Columbia Of Pennsylvania | 5.45% Notes Due 2036

Debt Instrument [Line Items]

Debt, Weighted Average Interest Rate

5.45%

Maximum

Debt Instrument [Line Items]

Cross default provision on default relating to any indebtedness

$ 50

Maximum | NIPSCO | 7.61% Notes Due 2027

Debt Instrument [Line Items]

Debt Instrument, Maturity Date

Aug. 31, 2027

Maximum | Columbia Of Ohio | 6.53% Notes Due 2038

Debt Instrument [Line Items]

Debt Instrument, Maturity Date

Jun. 30, 2038

Maximum | Columbia Of Virginia | 12.27% Notes Due 2029

Debt Instrument [Line Items]

Debt Instrument, Maturity Date

Jul. 31, 2029

Maximum | Columbia Of Pennsylvania | 5.45% Notes Due 2036

Debt Instrument [Line Items]

Debt Instrument, Maturity Date

Jun. 30, 2036

Minimum

Debt Instrument [Line Items]

Cross default provision on default relating to any indebtedness

$ 5

Minimum | NIPSCO | 7.61% Notes Due 2027

Debt Instrument [Line Items]

Debt Instrument, Maturity Date

Aug. 31, 2022

Minimum | Columbia Of Ohio | 6.53% Notes Due 2038

Debt Instrument [Line Items]

Debt Instrument, Maturity Date

Oct. 31, 2021

Minimum | Columbia Of Virginia | 12.27% Notes Due 2029

Debt Instrument [Line Items]

Debt Instrument, Maturity Date

Aug. 31, 2024

Minimum | Columbia Of Pennsylvania | 5.45% Notes Due 2036

Debt Instrument [Line Items]

Debt Instrument, Maturity Date

Aug. 31, 2027

Revolving Credit Facility | Maximum

Debt Instrument [Line Items]

Debt Instrument, Convertible, Conversion Ratio

70

Private Placement | Maximum

Debt Instrument [Line Items]

Debt Instrument, Convertible, Conversion Ratio

75

Interest Rate Swap

Debt Instrument [Line Items]

Derivative, Notional Amount

$ 1,000

Interest Rate Swap | NiSource Finance

Debt Instrument [Line Items]

Derivative, Notional Amount

$ 950

$ 950

Treasury Lock | NiSource Finance

Debt Instrument [Line Items]

Derivative, Notional Amount

$ 250

$ 750

$ 750

Medium Term Note Redeemed 2017 Note 9 [Member] [Domain] | NiSource Finance | Medium-term Notes [Member]

Debt Instrument [Line Items]

Debt Instrument, Repurchased Face Amount

$ 210.4

Medium Term Note Redeemed 2017 Note 9 [Member] [Domain] | NIPSCO | Medium-term Notes [Member]

Debt Instrument [Line Items]

Debt Instrument, Repurchased Face Amount

$ 5

Debt, Weighted Average Interest Rate

5.25%

7.02%

3.95% Notes Due 2048

Debt Instrument [Line Items]

Interest rate on debt

3.95%

3.95% Notes Due 2048 | NiSource Finance | Senior Notes

Debt Instrument [Line Items]

Face amount of notes

$ 750

Interest rate on debt

3.95%

Medium Term Note Redeemed 2017 Note 7 [Member] | NIPSCO | Medium-term Notes [Member]

Debt Instrument [Line Items]

Debt Instrument, Repurchased Face Amount

$ 22.5

Debt, Weighted Average Interest Rate

7.59%

5.45% Notes due 2020

Debt Instrument [Line Items]

Interest rate on debt

5.45%

6.40% Notes Due 2018

Debt Instrument [Line Items]

Interest rate on debt

6.40%

4.375% Notes due 2047 | NiSource Finance | Senior Notes

Debt Instrument [Line Items]

Face amount of notes

$ 1,000

Interest rate on debt

4.375%

Medium Term Note Redeemed 2017 Note 1 [Member] | Nisource Capital Markets Inc | Medium-term Notes [Member]

Debt Instrument [Line Items]

Debt Instrument, Repurchased Face Amount

$ 30

Interest rate on debt

7.86%

6.80% Notes Due 2019

Debt Instrument [Line Items]

Interest rate on debt

6.80%

Medium Term Note Redeemed 2017 Note 2 [Member] | Nisource Capital Markets Inc | Medium-term Notes [Member]

Debt Instrument [Line Items]

Debt Instrument, Repurchased Face Amount

$ 2

Interest rate on debt

7.85%

Medium Term Note Redeemed 2017 Note 3 [Member] | Nisource Capital Markets Inc | Medium-term Notes [Member]

Debt Instrument [Line Items]

Debt Instrument, Repurchased Face Amount

$ 12

Interest rate on debt

7.82%

Medium Term Note Redeemed 2017 Note 4 [Member] | Nisource Capital Markets Inc | Medium-term Notes [Member]

Debt Instrument [Line Items]

Debt Instrument, Repurchased Face Amount

$ 10

Interest rate on debt

7.92%

Medium Term Note Redeemed 2017 Note 5 [Member] | Nisource Capital Markets Inc | Medium-term Notes [Member]

Debt Instrument [Line Items]

Debt Instrument, Repurchased Face Amount

$ 2

Interest rate on debt

7.93%

Medium Term Note Redeemed 2017 Note 6 [Member] | Nisource Capital Markets Inc | Medium-term Notes [Member]

Debt Instrument [Line Items]

Debt Instrument, Repurchased Face Amount

$ 1

Interest rate on debt

7.94%

6.13% Notes Due 2022

Debt Instrument [Line Items]

Interest rate on debt

6.125%

Medium Term Note Redeemed 2017 Note 8 [Member] | NIPSCO | Medium-term Notes [Member]

Debt Instrument [Line Items]

Debt Instrument, Repurchased Face Amount

$ 55

Debt, Weighted Average Interest Rate

5.70%

3.49% Notes due 2027 | NiSource Finance | Senior Notes

Debt Instrument [Line Items]

Face amount of notes

$ 1,000

Interest rate on debt

3.49%

Long-Term Debt (Schedule of Con

Long-Term Debt (Schedule of Consolidated Long-Term Debt (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Mar. 27, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Senior Notes		$ 7,516.9		$ 5,968
Medium-Term Notes		157		241.5
Capital Lease Obligations		153.1		157.4
Long-term Debt, Excluding Current Maturities		7,796.5		6,421.3
Debt Instrument, Unamortized Discount (Premium) and Debt Issuance Costs, Net		$ (71.5)		(41.6)
4.375% Notes due 2047
Debt Instrument [Line Items]
Debt Instrument, Maturity Date		Dec. 31, 2027
3.26% Notes Due 2019
Debt Instrument [Line Items]
Debt Instrument, Maturity Date		Oct. 31, 2019
NiSource Finance | 5.25% Notes Due 2017
Debt Instrument [Line Items]
Senior Notes				210.4
NiSource Finance | 6.40% Notes Due 2018
Debt Instrument [Line Items]
Senior Notes				476
NiSource Finance | 6.80% Notes Due 2019
Debt Instrument [Line Items]
Senior Notes				500
NiSource Finance | Variable Rate due 2019
Debt Instrument [Line Items]
Senior Notes	[1]			500
NiSource Finance | 5.45% Notes due 2020
Debt Instrument [Line Items]
Senior Notes				550
NiSource Finance | 4.45% Notes Due 2021
Debt Instrument [Line Items]
Senior Notes				63.6
NiSource Finance | 6.13% Notes Due 2022
Debt Instrument [Line Items]
Senior Notes				500
NiSource Finance | 2.65% Notes Due 2022
Debt Instrument [Line Items]
Senior Notes				0
NiSource Finance | 3.85% Notes Due 2023
Debt Instrument [Line Items]
Senior Notes				250
NiSource Finance | 5.89% Notes Due 2025
Debt Instrument [Line Items]
Senior Notes				265
NiSource Finance | 3.49% Notes due 2027
Debt Instrument [Line Items]
Senior Notes				0
NiSource Finance | 6.25% Notes Due 2040
Debt Instrument [Line Items]
Senior Notes				250
NiSource Finance | 5.95% Notes Due 2041
Debt Instrument [Line Items]
Senior Notes				400
NiSource Finance | 5.80% Notes Due 2042
Debt Instrument [Line Items]
Senior Notes				250
NiSource Finance | 5.25% Notes Due 2043
Debt Instrument [Line Items]
Senior Notes				500
NiSource Finance | 4.80% Notes due 2044
Debt Instrument [Line Items]
Senior Notes				750
NiSource Finance | 5.65% Notes Due 2045
Debt Instrument [Line Items]
Senior Notes				500
Nisource Capital Markets Inc | 6.78% Notes Due 2027
Debt Instrument [Line Items]
Senior Notes				3
Nisource Capital Markets Inc | 4.375% Notes due 2047
Debt Instrument [Line Items]
Senior Notes				0
Nisource Capital Markets Inc | 3.95% Notes Due 2048
Debt Instrument [Line Items]
Senior Notes				0
NiSource | 5.25% Notes Due 2017
Debt Instrument [Line Items]
Debt Instrument, Maturity Date		Sep. 30, 2017
Debt, Weighted Average Interest Rate		5.25%
Senior Notes		$ 0
NiSource | 6.40% Notes Due 2018
Debt Instrument [Line Items]
Debt Instrument, Maturity Date		Mar. 31, 2018
Debt, Weighted Average Interest Rate		6.40%
Senior Notes		$ 275.1
NiSource | 6.80% Notes Due 2019
Debt Instrument [Line Items]
Debt Instrument, Maturity Date		Jan. 31, 2019
Debt, Weighted Average Interest Rate		6.80%
Senior Notes		$ 255.1
NiSource | Variable Rate due 2019
Debt Instrument [Line Items]
Debt Instrument, Maturity Date		Mar. 29, 2019
Senior Notes	[1]	$ 0
NiSource | 5.45% Notes due 2020
Debt Instrument [Line Items]
Debt Instrument, Maturity Date		Sep. 30, 2020
Debt, Weighted Average Interest Rate		5.45%
Senior Notes		$ 325.1
NiSource | 4.45% Notes Due 2021
Debt Instrument [Line Items]
Debt Instrument, Maturity Date		Dec. 31, 2021
Debt, Weighted Average Interest Rate		4.45%
Senior Notes		$ 63.6
NiSource | 6.13% Notes Due 2022
Debt Instrument [Line Items]
Debt Instrument, Maturity Date		Mar. 31, 2022
Debt, Weighted Average Interest Rate		6.13%
Senior Notes		$ 180
NiSource | 2.65% Notes Due 2022
Debt Instrument [Line Items]
Debt Instrument, Maturity Date		Nov. 30, 2022
Debt, Weighted Average Interest Rate		2.65%
Senior Notes		$ 500
NiSource | 3.85% Notes Due 2023
Debt Instrument [Line Items]
Debt Instrument, Maturity Date		Feb. 28, 2023
Debt, Weighted Average Interest Rate		3.85%
Senior Notes		$ 250
NiSource | 5.89% Notes Due 2025
Debt Instrument [Line Items]
Debt Instrument, Maturity Date		Nov. 30, 2025
Debt, Weighted Average Interest Rate		5.89%
Senior Notes		$ 265
NiSource | 3.49% Notes due 2027
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate		3.49%
Senior Notes		$ 1,000
NiSource | 6.78% Notes Due 2027
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate		6.78%
Senior Notes		$ 3
NiSource | 6.25% Notes Due 2040
Debt Instrument [Line Items]
Debt Instrument, Maturity Date		Dec. 31, 2040
Debt, Weighted Average Interest Rate		6.25%
Senior Notes		$ 250
NiSource | 5.95% Notes Due 2041
Debt Instrument [Line Items]
Debt Instrument, Maturity Date		Jun. 30, 2041
Debt, Weighted Average Interest Rate		5.95%
Senior Notes		$ 400
NiSource | 5.80% Notes Due 2042
Debt Instrument [Line Items]
Debt Instrument, Maturity Date		Feb. 28, 2042
Debt, Weighted Average Interest Rate		5.80%
Senior Notes		$ 250
NiSource | 5.25% Notes Due 2043
Debt Instrument [Line Items]
Debt Instrument, Maturity Date		Feb. 28, 2043
Debt, Weighted Average Interest Rate		5.25%
Senior Notes		$ 500
NiSource | 4.80% Notes due 2044
Debt Instrument [Line Items]
Debt Instrument, Maturity Date		Feb. 29, 2044
Debt, Weighted Average Interest Rate		4.80%
Senior Notes		$ 750
NiSource | 5.65% Notes Due 2045
Debt Instrument [Line Items]
Debt Instrument, Maturity Date		Feb. 28, 2045
Debt, Weighted Average Interest Rate		5.65%
Senior Notes		$ 500
NiSource | 4.375% Notes due 2047
Debt Instrument [Line Items]
Debt Instrument, Maturity Date		May 31, 2047
Debt, Weighted Average Interest Rate		4.38%
Senior Notes		$ 1,000
NiSource | 3.95% Notes Due 2048
Debt Instrument [Line Items]
Debt Instrument, Maturity Date		Mar. 31, 2048
Debt, Weighted Average Interest Rate		3.95%
Senior Notes		$ 750
NiSource | 7.92% Notes Due 2027
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate		7.99%
Medium-Term Notes		$ 49		106
NiSource | 7.92% Notes Due 2027 | Maximum
Debt Instrument [Line Items]
Debt Instrument, Maturity Date		May 31, 2027
NiSource | 7.92% Notes Due 2027 | Minimum
Debt Instrument [Line Items]
Debt Instrument, Maturity Date		Mar. 31, 2017
NiSource | 3.95% Notes Due 2018
Debt Instrument [Line Items]
Debt Instrument, Maturity Date		May 31, 2018
Columbia Of Massachusetts | 6.30% Notes Due 2028 [Member]
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate		6.30%
Medium-Term Notes		$ 40		40
Columbia Of Massachusetts | 6.30% Notes Due 2028 [Member] | Maximum
Debt Instrument [Line Items]
Debt Instrument, Maturity Date		Feb. 28, 2028
Columbia Of Massachusetts | 6.30% Notes Due 2028 [Member] | Minimum
Debt Instrument [Line Items]
Debt Instrument, Maturity Date		Dec. 31, 2025
Columbia Of Massachusetts | 5.33% notes Due 2036
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate		4.37%
Capital Lease Obligations		$ 22.8		23.7
Columbia Of Massachusetts | 5.33% notes Due 2036 | Maximum
Debt Instrument [Line Items]
Debt Instrument, Maturity Date		Jul. 31, 2036
Columbia Of Massachusetts | 5.33% notes Due 2036 | Minimum
Debt Instrument [Line Items]
Debt Instrument, Maturity Date		Dec. 31, 2033
NIPSCO | 7.61% Notes Due 2027
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate		7.61%
Medium-Term Notes		$ 68		95.5
NIPSCO | 7.61% Notes Due 2027 | Maximum
Debt Instrument [Line Items]
Debt Instrument, Maturity Date		Aug. 31, 2027
NIPSCO | 7.61% Notes Due 2027 | Minimum
Debt Instrument [Line Items]
Debt Instrument, Maturity Date		Aug. 31, 2022
NIPSCO | 3.95% Notes Due 2018
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate		3.95%
Capital Lease Obligations		$ 3.8		12.7
NIPSCO | 5.76% Notes Due 2019
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate		5.85%
Long-term Pollution Control Bond		$ 41		96
NIPSCO | 5.76% Notes Due 2019 | Maximum
Debt Instrument [Line Items]
Debt Instrument, Maturity Date		Apr. 30, 2019
NIPSCO | 5.76% Notes Due 2019 | Minimum
Debt Instrument [Line Items]
Debt Instrument, Maturity Date		Jul. 31, 2017
Columbia Of Ohio | 6.53% Notes Due 2038
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate		6.41%
Capital Lease Obligations		$ 88.5		80.1
Columbia Of Ohio | 6.53% Notes Due 2038 | Maximum
Debt Instrument [Line Items]
Debt Instrument, Maturity Date		Jun. 30, 2038
Columbia Of Ohio | 6.53% Notes Due 2038 | Minimum
Debt Instrument [Line Items]
Debt Instrument, Maturity Date		Oct. 31, 2021
Ni Source Corporate Services | 3.26% Notes Due 2019
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate		3.26%
Capital Lease Obligations		$ 1.4		3.5
Columbia Of Virginia | 12.27% Notes Due 2029
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate		12.21%
Capital Lease Obligations		$ 5.2		5.5
Columbia Of Virginia | 12.27% Notes Due 2029 | Maximum
Debt Instrument [Line Items]
Debt Instrument, Maturity Date		Jul. 31, 2029
Columbia Of Virginia | 12.27% Notes Due 2029 | Minimum
Debt Instrument [Line Items]
Debt Instrument, Maturity Date		Aug. 31, 2024
Columbia Of Kentucky | 5.33% notes Due 2036
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate		3.79%
Capital Lease Obligations		$ 0.4		0
Columbia Of Pennsylvania | 5.45% Notes Due 2036
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate		5.45%
Capital Lease Obligations		$ 31		$ 31.9
Columbia Of Pennsylvania | 5.45% Notes Due 2036 | Maximum
Debt Instrument [Line Items]
Debt Instrument, Maturity Date		Jun. 30, 2036
Columbia Of Pennsylvania | 5.45% Notes Due 2036 | Minimum
Debt Instrument [Line Items]
Debt Instrument, Maturity Date		Aug. 31, 2027
Medium Term Note Redeemed 2017 Note 1 [Member] | Nisource Capital Markets Inc | Medium-term Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage			7.86%
Debt Instrument, Repurchased Face Amount			$ 30
Medium Term Note Redeemed 2017 Note 2 [Member] | Nisource Capital Markets Inc | Medium-term Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage			7.85%
Debt Instrument, Repurchased Face Amount			$ 2
[1]	Rate of one month Libor plus 95 basis points.

Short-Term Borrowings (Narrativ

Short-Term Borrowings (Narrative) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Short-term Debt [Line Items]
Commercial paper outstanding	$ 869	$ 1,178
Line of Credit Facility, Amount Outstanding	0
Accounts receivable securitization facility borrowings	336.7	310
Commercial Paper
Short-term Debt [Line Items]
Revolving credit facility, maximum	1,500
Revolving Credit Facility
Short-term Debt [Line Items]
Revolving credit facility, maximum	1,850
Standby Letters of Credit
Short-term Debt [Line Items]
Line of Credit Facility, Amount Outstanding	$ 11.1	$ 14.7

Short-Term Borrowings (Schedule

Short-Term Borrowings (Schedule Of Short-Term Borrowings) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Short-term Debt [Line Items]
Commercial paper	$ 869	$ 1,178
Credit facilities borrowings	0
Accounts receivable securitization facility borrowings	336.7	310
Total short-term borrowings	$ 1,205.7	$ 1,488
Commercial Paper
Short-term Debt [Line Items]
Commercial Paper/credit facilities borrowings, weighted average interest rate	1.97%	1.24%
Revolving Credit Facility
Short-term Debt [Line Items]
Commercial Paper/credit facilities borrowings, weighted average interest rate	0.00%	0.00%
Standby Letters of Credit
Short-term Debt [Line Items]
Credit facilities borrowings	$ 11.1	$ 14.7

Fair Value (Narrative) (Details

Fair Value (Narrative) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017USD ($)
Fair Value Disclosure [Line Items]
Transfers between Fair Value Hierarchies	0
Fair Value, Assets and Liabilities Measured on a Non-Recurring Basis	0
U.S. Treasury debt securities
Fair Value Disclosure [Line Items]
Available-for-sale securities, maturities of less than a year	$ 13.7
Corporate/Other debt securities
Fair Value Disclosure [Line Items]
Available-for-sale securities, maturities of less than a year	$ 2.9

Fair Value (Fair Value Of Finan

Fair Value (Fair Value Of Financial Assets And Liabilities Measured On A Recurring Basis) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Risk management assets	$ 21.1	$ 49
Available-for-sale securities	133.9	131.5
Fair value assets measured on recurring basis, total	155	180.5
Risk management liabilities	71.7	61.3
Fair value liabilities measured on recurring basis, total	71.7	61.3
Quoted Prices In Active Markets For Identical Assets (Level 1) [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Risk management assets	0	5.4
Available-for-sale securities	0	0
Fair value assets measured on recurring basis, total	0	5.4
Risk management liabilities	0	1.2
Fair value liabilities measured on recurring basis, total	0	1.2
Significant Other Observable Inputs (Level 2) [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Risk management assets	21.1	43.6
Available-for-sale securities	133.9	131.5
Fair value assets measured on recurring basis, total	155	175.1
Risk management liabilities	71.4	58.9
Fair value liabilities measured on recurring basis, total	71.4	58.9
Significant Unobservable Inputs (Level 3) [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Risk management assets	0	0
Available-for-sale securities	0	0
Fair value assets measured on recurring basis, total	0	0
Risk management liabilities	0.3	1.2
Fair value liabilities measured on recurring basis, total	$ 0.3	$ 1.2

Fair Value (Available-For-Sale

Fair Value (Available-For-Sale Securities) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Fair Value Disclosure [Line Items]
Amortized Cost	$ 133.7	$ 133.7
Gross Unrealized Gains	0.9	0.4
Gross Unrealized Losses	(0.7)	(2.6)
Available-for-sale Securities	133.9	131.5
U.S. Treasury debt securities
Fair Value Disclosure [Line Items]
Amortized Cost	26.9	35
Gross Unrealized Gains	0	0.1
Gross Unrealized Losses	(0.1)	(0.6)
Available-for-sale Securities	26.8	34.5
Corporate/Other debt securities
Fair Value Disclosure [Line Items]
Amortized Cost	106.8	98.7
Gross Unrealized Gains	0.9	0.3
Gross Unrealized Losses	(0.6)	(2)
Available-for-sale Securities	$ 107.1	$ 97

Fair Value (Carrying Amount And

Fair Value (Carrying Amount And Estimated Fair Values Of Financial Instruments) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Fair Value Disclosures [Abstract]
Long-term debt (including current portion), Carrying Amount	$ 7,796.5	$ 6,421.3
Long-term debt (including current portion), Estimated Fair Value	$ 8,603.4	$ 7,064.1

Transfers Of Financial Asset107

Transfers Of Financial Assets (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Assets and Associated Liabilities of Transfers Accounted for as Secured Borrowings [Line Items]
Cash from financing activities	$ 26.7	$ 64
Securitization transaction fees	2.5	$ 2.3	$ 2.5
Accounts Receivable Program
Assets and Associated Liabilities of Transfers Accounted for as Secured Borrowings [Line Items]
Seasonal Limit	$ 375

Transfers Of Financial Asset108

Transfers Of Financial Assets (Schedule Of Gross And Net Receivables Transferred As Well As Short-Term Borrowings Related To The Securitization Transactions) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Transfers and Servicing [Abstract]
Gross Receivables interest	$ 635.3	$ 618.3
Less: Receivables not transferred	298.6	308.3
Net receivables transferred	336.7	310
Accounts receivable securitization agreements outstanding	$ 336.7	$ 310

Other Commitments And Contin109

Other Commitments And Contingencies (Narrative) (Details) $ in Millions	Jul. 31, 2008MW	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)
Other Commitments And Contingencies [Line Items]
Payments made in connection with operating leases		$ 49.5	$ 52	$ 47.5
Capital lease payments		171.2	167
Capital leases related accumulated depreciation		32.4	20.6
Wind power purchase agreement capacity | MW	100
Each wind power purchase agreement capacity | MW	50
Line of Credit Facility, Amount Outstanding		0
Recorded reserves to cover environmental remediation at various sites		111.4	111.4
Estimated Cost of Compliance with Coal Combustion Residuals, Maximum		193
Estimated Cost of Compliance with Effluent Limitations Guidelines		170
Liability Associated with Bailey Generating Station Retirement			22.1
Variable Interest Entity, Reporting Entity Involvement, Maximum Loss Exposure, Amount		0
Payments Pure Air		$ 22	21.7
Liability Associated with Bailly Generating Station Retirement - Pure Air Contract Termination Portion			16
MGP Sites
Other Commitments And Contingencies [Line Items]
Number of waste disposal sites identified by program		64
Liability for estimated remediation costs		$ 106.9	105.5
Reasonably possible remediation costs variance from reserve		$ 25
Maximum
Other Commitments And Contingencies [Line Items]
Long term purchase commitment time period		20 years
Minimum
Other Commitments And Contingencies [Line Items]
Long term purchase commitment time period		15 years
Minimum | NIPSCO
Other Commitments And Contingencies [Line Items]
Cost to Maintain and Operate Current Fleet of Coal-Fired Generating Units		$ 1,000
Pipeline Capacity Transportation And Storage | Maximum
Other Commitments And Contingencies [Line Items]
Long Term Purchase Commitment Expiration Year		2,045
Pipeline Capacity Transportation And Storage | Minimum
Other Commitments And Contingencies [Line Items]
Long Term Purchase Commitment Expiration Year		2,018
Coal Transportation | Maximum
Other Commitments And Contingencies [Line Items]
Long Term Purchase Commitment Expiration Year		2,021
Standby Letters of Credit
Other Commitments And Contingencies [Line Items]
Line of Credit Facility, Amount Outstanding		$ 11.1	$ 14.7

Other Commitments And Contin110

Other Commitments And Contingencies Other Commitments and Contingencies (Contractual Obligation, Fiscal Year Maturity Schedule) (Details) - USD ($) $ in Millions		Dec. 31, 2017	Dec. 31, 2016
Long-term Purchase Commitment [Line Items]
Long-term Debt, Future Minimum Payments Due	[1]	$ 7,714.9
Long-term Debt, Future Minimum Payments Due, Next Twelve Months	[1]	275.1
Long-term Debt, Future Minimum Payments, Due in Two Years	[1]	296.1
Long-term Debt, Future Minimum Payments, Due in Three Years	[1]	325.1
Long-term Debt, Future Minimum Payments, Due in Four Years	[1]	63.6
Long-term Debt, Future Minimum Payments, Due in Five Years	[1]	710
Long-term Debt, Future Minimum Payments, Due Thereafter	[1]	6,045
Capital Leases, Future Minimum Payments Due	[2]	254.4
Capital Leases, Future Minimum Payments Due, Next Twelve Months	[2]	18.1
Capital Leases, Future Minimum Payments Due in Two Years	[2]	15.7
Capital Leases, Future Minimum Payments Due in Three Years	[2]	15.4
Capital Leases, Future Minimum Payments Due in Four Years	[2]	15.5
Capital Leases, Future Minimum Payments Due in Five Years	[2]	15.5
Capital Leases, Future Minimum Payments Due Thereafter	[2]	174.2
Interest Payments on Long-term Debt, Future Minimum Payments Due		6,701.2
Interest Payments on Long-term Debt, Future Minimum Payments Due, Next Twelve Months		364.4
Interest Payments on Long-term Debt, Future Minimum Payments, Due in Two Years		344.4
Interest Payments on Long-term Debt, Future Minimum Payments, Due in Three Years		334.6
Interest Payments on Long-term Debt, Future Minimum Payments, Due in Four Years		316.8
Interest Payments on Long-term Debt, Future Minimum Payments, Due in Five Years		307.7
Interest Payments on Long-term Debt, Future Minimum Payments, Due Thereafter		5,033.3
Operating Leases, Future Minimum Payments Due	[3]	57.2
Operating Leases, Future Minimum Payments Due, Next Twelve Months	[3]	13.8
Operating Leases, Future Minimum Payments, Due in Two Years	[3]	10.2
Operating Leases, Future Minimum Payments, Due in Three Years	[3]	7.3
Operating Leases, Future Minimum Payments, Due in Four Years	[3]	6.2
Operating Leases, Future Minimum Payments, Due in Five Years	[3]	4.4
Operating Leases, Future Minimum Payments, Due Thereafter	[3]	15.3
Energy Commodity Contracts, Future Minimum Payments Due		216.7
Energy Commodity Contracts, Future Minimum Payments Due, Next Twelve Months		102.5
Energy Commodity Contracts, Future Minimum Payments, Due in Two Years		57.3
Energy Commodity Contracts, Future Minimum Payments, Due in Three Years		56.9
Energy Commodity Contracts, Future Minimum Payments, Due in Four Years		0
Energy Commodity Contracts, Future Minimum Payments, Due in Five Years		0
Energy Commodity Contracts, Future Minimum Payments, Due Thereafter		0
Service Obligations, Pipeline Service Obligations, Future Minimum Payments Due		2,649.9
Service Obligations, Pipeline Service Obligations, Future Minimum Payments Due, Next Twelve Months		538.9
Service Obligations, Pipeline Service Obligations, Future Minimum Payments, Due in Two Years		520.5
Service Obligations, Pipeline Service Obligations, Future Minimum Payments, Due in Three Years		390.7
Service Obligations, Pipeline Service Obligations, Future Minimum Payments, Due in Four Years		344.7
Service Obligations, Pipeline Service Obligations, Future Minimum Payments, Due in Five Years		331
Service Obligations, Pipeline Service Obligations, Future Minimum Payments, Due Thereafter		524.1
Service Obligations, IBM Service Obligations, Future Minimum Payments Due		311.5
Service Obligations, IBM Service Obligations, Future Minimum Payments Due, Next Twelve Months		88.3
Service Obligations, IBM Service Obligations, Future Minimum Payments, Due in Two Years		71.5
Service Obligations, IBM Service Obligations, Future Minimum Payments, Due in Three Years		63.5
Service Obligations, IBM Service Obligations, Future Minimum Payments, Due in Four Years		50.7
Service Obligations, IBM Service Obligations, Future Minimum Payments, Due in Five Years		37.5
Service Obligations, IBM Service Obligations, Future Minimum Payments, Due Thereafter		0
Service Obligations, Other Service Obligations, Future Minimum Payments Due		178.2
Service Obligations, Other Service Obligations, Future Minimum Payments Due, Next Twelve Months		48.3
Service Obligations, Other Service Obligations, Future Minimum Payments, Due in Two Years		43.3
Service Obligations, Other Service Obligations, Future Minimum Payments, Due in Three Years		43.3
Service Obligations, Other Service Obligations, Future Minimum Payments, Due in Four Years		43.3
Service Obligations, Other Service Obligations, Future Minimum Payments, Due in Five Years		0
Service Obligations, Other Service Obligations, Future Minimum Payments, Due Thereafter		0
Other Liabilities, Future Minimum Payments Due		28.7
Other Liabilities, Future Minimum Payments Due, Next Twelve Months		28.7
Other Liabilities, Future Minimum Payments, Due in Two Years		0
Other Liabilities, Future Minimum Payments, Due in Three Years		0
Other Liabilities, Future Minimum Payments, Due in Four Years		0
Other Liabilities, Future Minimum Payments, Due in Five Years		0
Other Liabilities, Future Minimum Payments, Due Thereafter		0
Total Future Minimum Lease Payments Due		18,112.7
Total Future Minimum Lease Payments Due, Next Twelve Months		1,478.1
Total Future Minimum Lease Payments, Due in Two Years		1,359
Total Future Minimum Lease Payments, Due in Three Years		1,236.8
Total Future Minimum Lease Payments, Due in Four Years		840.8
Total Future Minimum Lease Payments, Due in Five Years		1,406.1
Total Future Minimum Lease Payments, Due Thereafter		11,791.9
Debt Instrument, Unamortized Discount (Premium) and Debt Issuance Costs, Net		(71.5)	$ (41.6)
Capital Leases, Future Minimum Payments, Interest Included in Payments		91.9
Accrual for Environmental Loss Contingencies		111.4	$ 111.4
Fleet Lease
Long-term Purchase Commitment [Line Items]
Operating Leases, Future Minimum Payments Due, Next Twelve Months		29.3
Operating Leases, Future Minimum Payments, Due in Two Years		27.5
Operating Leases, Future Minimum Payments, Due in Three Years		19.7
Operating Leases, Future Minimum Payments, Due in Four Years		13.9
Operating Leases, Future Minimum Payments, Due in Five Years		9.6
Operating Leases, Future Minimum Payments, Due Thereafter		$ 7.4
[1]	Long-term debt balance excludes unamortized issuance costs and discounts of $71.5 million.
[2]	Capital lease payments shown above are inclusive of interest totaling $91.9 million.
[3]	Operating lease balances do not include amounts for fleet leases that can be renewed beyond the initial lease term. The Company anticipates renewing the leases beyond the initial term, but the anticipated payments associated with the renewals do not meet the definition of expected minimum lease payments and therefore are not included above. Expected payments are $29.3 million in 2018, $27.5 million in 2019, $19.7 million in 2020, $13.9 million in 2021, $9.6 million in 2022 and $7.4 million thereafter.

Accumulated Other Comprehens111

Accumulated Other Comprehensive Loss (Components Of Accumulated Other Comprehensive Loss) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax		$ (21.7)	$ 7.6	$ (16.5)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		3.4	2.4	5.5
Net current-period other comprehensive income (loss)		(18.3)	10	(11)
Allocation of AOCI to Noncontrolling Interest	[1]			2
Distribution of CPG to shareholders				24.5
Accumulated other comprehensive loss		(43.4)	(25.1)	(35.1)	$ (50.6)
Gains and Losses on Securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax		0.6	0	(0.5)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		0.2	(0.1)	(0.3)
Net current-period other comprehensive income (loss)		0.8	(0.1)	(0.8)
Allocation of AOCI to Noncontrolling Interest				0
Distribution of CPG to shareholders				0
Accumulated other comprehensive loss		0.2	(0.6)	(0.5)	0.3
Gains and Losses on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax		(24.2)	7.1	(11)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		1.7	1.5	3.2
Net current-period other comprehensive income (loss)		(22.5)	8.6	(7.8)
Allocation of AOCI to Noncontrolling Interest				2
Distribution of CPG to shareholders				13.9
Accumulated other comprehensive loss		(29.4)	(6.9)	(15.5)	(23.6)
Pension and OPEB Items
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax		1.9	0.5	(5)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		1.5	1	2.6
Net current-period other comprehensive income (loss)		3.4	1.5	(2.4)
Allocation of AOCI to Noncontrolling Interest				0
Distribution of CPG to shareholders				10.6
Accumulated other comprehensive loss		$ (14.2)	$ (17.6)	$ (19.1)	$ (27.3)
[1]	This transaction, which occurred prior to the Separation, was distributed through retained earnings as part of the Separation on July 1, 2015.

Other, Net (Schedule Of Other N

Other, Net (Schedule Of Other Net) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other Nonoperating Income (Expense) [Abstract]
Interest income		$ 4.6	$ 3.4	$ 0.8
AFUDC Equity		12.6	11.6	11.5
Charitable Contributions		(19.9)	(4.5)	(4.8)
Miscellaneous	[1]	(0.1)	(9)	9.9
Total Other, net		(2.8)	$ 1.5	$ 17.4
TUA Related Charge		$ 8.6
[1]	Miscellaneous in 2016 primarily consists of a TUA-related charge of $8.6 million to reflect the estimated amount owed to the upgrade sponsors for the portion of the multiplier previously collected for taxes. In 2015, Miscellaneous primarily consisted of TUA income.

Interest Expense, Net (Details)

Interest Expense, Net (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Interest Expense [Abstract]
Interest on long-term debt	$ 354.8	$ 352.3	$ 377.5
Interest on short-term borrowings	14.9	9.2	2.2
Debt discount/cost amortization	7.2	7.6	8.7
Accounts receivable securitization fees	2.5	2.3	2.5
Allowance for borrowed funds used and interest capitalized during construction	(6.2)	(5.6)	(5.4)
Debt-based post-in-service carrying charges	(36.4)	(35.1)	(21.4)
Other	16.4	18.8	16.1
Total Interest Expense, net	$ 353.2	$ 349.5	$ 380.2

Segments Of Business (Narrative

Segments Of Business (Narrative) (Details)	12 Months Ended
	Dec. 31, 2017
Segment Reporting [Abstract]
Number of Reportable Segments	2
Number of counties in which electric service provided by Electric Operations	20

Segments Of Business (Schedule

Segments Of Business (Schedule Of Operating Income Derived From Revenues And Expenses By Segment) (Details) - USD ($) $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Unaffiliated										$ 4,874.6	$ 4,492.5	$ 4,651.8
Operating Income		$ 270	$ 99.6	$ 124.5	$ 416.5	$ 224.9	$ 113.7	$ 138.2	$ 381.4	910.6	858.2	799.9
Consolidated Depreciation and Amortization										570.3	547.1	524.4
Consolidated Assets		19,961.7				18,691.9				19,961.7	18,691.9	17,492.5
Payments to Acquire Property Plant and Equipment Including Captial Expenditures From Current Liabilities And Equity Method Investments	[1]									1,753.8	1,490.4	1,367.5
Gas Distribution Operations
Segment Reporting Information [Line Items]
Unaffiliated										3,102.1	2,830.6	3,069.1
Operating Income										545.6	574	555.8
Consolidated Depreciation and Amortization										269.3	252.9	232.6
Consolidated Assets		12,048.8				11,096.4				12,048.8	11,096.4	10,094.5
Payments to Acquire Property Plant and Equipment Including Captial Expenditures From Current Liabilities And Equity Method Investments	[1]									1,125.6	1,054.4	917
Electric Operations
Segment Reporting Information [Line Items]
Unaffiliated										1,786.5	1,661.6	1,574.4
Operating Income										364.8	291.4	264.4
Consolidated Depreciation and Amortization										277.8	274.5	267.7
Consolidated Assets		5,478.6				5,233.3				5,478.6	5,233.3	5,265.3
Payments to Acquire Property Plant and Equipment Including Captial Expenditures From Current Liabilities And Equity Method Investments	[1]									592.4	420.6	400.3
Corporate and Other
Segment Reporting Information [Line Items]
Unaffiliated										511.8	426.8	405.9
Operating Income										0.2	(7.2)	(20.3)
Consolidated Depreciation and Amortization										23.2	19.7	24.1
Consolidated Assets		$ 2,434.3				$ 2,362.2				2,434.3	2,362.2	2,132.7
Payments to Acquire Property Plant and Equipment Including Captial Expenditures From Current Liabilities And Equity Method Investments	[1]									35.8	15.4	50.2
Eliminations
Segment Reporting Information [Line Items]
Unaffiliated										(525.8)	(426.5)	(397.6)
Unaffiliated | Gas Distribution Operations
Segment Reporting Information [Line Items]
Unaffiliated										3,087.9	2,818.2	3,068.7
Unaffiliated | Electric Operations
Segment Reporting Information [Line Items]
Unaffiliated										1,785.7	1,660.8	1,573.6
Unaffiliated | Corporate and Other
Segment Reporting Information [Line Items]
Unaffiliated										1	13.5	9.5
Intersegment | Gas Distribution Operations
Segment Reporting Information [Line Items]
Unaffiliated										14.2	12.4	0.4
Intersegment | Electric Operations
Segment Reporting Information [Line Items]
Unaffiliated										0.8	0.8	0.8
Intersegment | Corporate and Other
Segment Reporting Information [Line Items]
Unaffiliated										$ 510.8	$ 413.3	$ 396.4
[1]	Amounts differ from those presented on the Statements of Consolidated Cash Flows primarily due to the inclusion of capital expenditures included in current liabilities and AFUDC Equity.

Quarterly Financial Data (Sched

Quarterly Financial Data (Schedule Of Quarterly Financial Data) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended										12 Months Ended
	Dec. 31, 2017		Sep. 30, 2017	Jun. 30, 2017		Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Data [Abstract]
Operating Revenues	$ 1,368.3		$ 917	$ 990.7		$ 1,598.6	$ 1,297	$ 861.3	$ 897.6	$ 1,436.6
Operating Income	270		99.6	124.5		416.5	224.9	113.7	138.2	381.4	$ 910.6	$ 858.2	$ 799.9
Income from Continuing Operations	(52.4)	[1]	14	(44.3)	[2]	211.3	88.8	23.7	29	186.6	128.6	328.1	198.6
Results from Discontinued Operations-net of taxes	0		0	(0.1)		0	0	3.5	(0.1)	0	0.1	(3.4)	(87.9)
Net Income	$ (52.4)		$ 14	$ (44.4)		$ 211.3	$ 88.8	$ 27.2	$ 28.9	$ 186.6	128.5	331.5	286.5
Net Income (Loss) attributable to NiSource											$ 128.5	$ 331.5	$ 302.1
Basic Earnings (Loss) Per Share, Continuing Operations	$ (0.16)		$ 0.04	$ (0.14)		$ 0.65	$ 0.28	$ 0.07	$ 0.09	$ 0.58	$ 0.39	$ 1.02	$ 0.63
Basic Earnings (Loss) Per Share, Discontinued Operations	0		0	0		0	0	0.01	0	0	0	0.01	0.27
Basic Earnings (Loss) Per Share	(0.16)		0.04	(0.14)		0.65	0.28	0.08	0.09	0.58	0.39	1.03	0.90
Diluted Earnings (Loss) Per Share, Continuing Operations	(0.16)		0.04	(0.14)		0.65	0.27	0.07	0.09	0.58	0.39	1.01	0.63
Diluted Earnings (Loss) Per Share, Discontinued Operations	0		0	0		0	0	0.01	0	0	0	0.01	0.27
Diluted Earnings (Loss) Per Share	$ (0.16)		$ 0.04	$ (0.14)		$ 0.65	$ 0.27	$ 0.08	$ 0.09	$ 0.58	$ 0.39	$ 1.02	$ 0.90
[1]	The decrease in income from continuing operations during the fourth quarter of 2017 was due primarily to increased tax expense as a result of the impact of implementing the provisions of the TCJA. See Note 10, "Income Taxes," for additional information.
[2]	The decrease in income from continuing operations during the second quarter of 2017 relates primarily to a $111.5 million loss on early extinguishment of long-term debt, primarily attributable to early redemption premiums. See Note 14, "Long-Term Debt," for additional information.

Supplemental Cash Flow Infor117

Supplemental Cash Flow Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Non-cash transactions:
Capital expenditures included in current liabilities	$ 173	$ 125.3	$ 121.6
Assets acquired under a capital lease	11.5	4	47.5
Schedule of interest and income taxes paid:
Cash paid for interest, net of interest capitalized amounts	339.9	337.8	390.4
Cash paid for income taxes, net of refunds	$ 5.5	$ 8	$ 21.3

Subsequent Event (Details)

Subsequent Event (Details) - Subsequent Event $ in Millions	Feb. 01, 2018USD ($)
Subsequent Event [Line Items]
Bailly Unit 8 Portion of Net Book Value in Net Utility Plant	$ 15
Remainder of Net Utility Plant Reclassified to Regulatory assets (noncurrent)	$ 143

Valuation And Qualifying Acc119

Valuation And Qualifying Accounts (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reserve For Accounts Receivable [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance		$ 23.3	$ 20.3	$ 24.9
Valuation Allowances and Reserves, Charged to Cost and Expense		14.8	19.7	22.5
Valuation Allowances and Reserves, Charged to Other Accounts	[1]	39.1	48.5	56.7
Deductions for Purposes for which Reserves were Created		58.9	65.2	83.8
Ending Balance		18.3	23.3	20.3
Reserve For Other Investments [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance		3	3	3
Valuation Allowances and Reserves, Charged to Cost and Expense		0	0	0
Valuation Allowances and Reserves, Charged to Other Accounts	[1]	0	0	0
Deductions for Purposes for which Reserves were Created		0	0	0
Ending Balance		$ 3	$ 3	$ 3
[1]	Charged to Other Accounts reflects the deferral of bad debt expense to a regulatory asset.