Significant Accounting Policies [Text Block] | Nature of Operations and Summary of Significant Accounting Policies A. Company Structure and Basis of Presentation . CPG OpCo LP (the "Partnership") was formed in Delaware on September 26, 2014, as a subsidiary of NiSource. The general partner of the Partnership, OpCo GP, is 100% owned by its sole member CPPL. At or prior to the closing of CPPL's IPO the following transactions occurred: • CEG contributed $1,217.3 million of capital to certain subsidiaries of the Predecessor to repay intercompany debt owed to NiSource Finance. CEG entered into new intercompany debt agreements with NiSource Finance for $1,217.3 million ; • CEG and Columbia Hardy contributed substantially all of the subsidiaries in the Predecessor to the Partnership; • CEG assumed responsibility for all historical current and deferred income taxes other than Tennessee state income taxes that continue to be borne by the Partnership post-IPO, as well as associated regulatory assets and liabilities; • CEG contributed to CPPL a 7.3% limited partner interest in the Partnership in exchange for 46,811,398 subordinated units in CPPL and all of CPPL's incentive distribution rights; • CPPL purchased from the Partnership an additional 8.4% limited partner interest in exchange for $1,170.0 million from the net proceeds of CPPL's IPO, resulting in CPPL owning a 15.7% limited partner interest in the Partnership; • The Partnership distributed $500.0 million to CEG as a reimbursement of preformation capital expenditures with respect to the assets contributed to the Partnership. • Following CPPL's IPO, CPPL owns a 15.7% limited partner interest in the Partnership, Columbia Hardy owns a 0.77% limited partner interest and CEG owns the remaining 83.53% limited partner interest. On February 11, 2015, concurrent with the completion of CPPL's IPO, NiSource contributed its subsidiary, CEG, to CPG. Following this contribution, CPG owns and operates, through its subsidiaries, approximately 15,000 miles of strategically located interstate gas pipelines extending from New York to the Gulf of Mexico and one of the nation’s largest underground natural gas storage systems, with approximately 300 MMDth of working gas capacity, as well as related gathering and processing assets. CEG owns and operates, through its subsidiaries, substantially all of the natural gas transmission and storage assets of CPG. Prior to July 1, 2015, CPG was a wholly owned subsidiary of NiSource. On July 1, 2015, all the shares of CPG were distributed by NiSource to holders of NiSource common stock completing CPG's separation from NiSource ("the Separation"). As a result of the Separation, CPG became an independent publicly traded company. The Partnership is a guarantor of CPG's senior notes. Through the registration of CPG's senior notes, the Partnership must comply with the reporting requirements of Rule 3-10 of Regulation S-X. The Partnership has suspended its obligations to file periodic reports with the SEC based on CPG’s senior notes and once it has filed its Form 10-K for the year ended December 31, 2016, it will no longer have reporting obligations with respect to CPG’s senior notes. CPG OpCo LP Predecessor (the “Predecessor”) is comprised of NiSource’s Columbia Pipeline Group Operations reportable segment. The Partnership entered into an omnibus agreement with CEG and its affiliates (together with a services agreement with CPGSC) at the closing of CPPL's IPO that addresses (1) centralized corporate, general and administrative services to be provided by CEG for the Partnership and the reimbursement by the Partnership for the Partnership's portion of these services, (2) CPPL's right of first offer for CEG's 84.3% interest in the Partnership, (3) the indemnification of the Partnership for certain potential environmental and toxic tort claims losses and expenses associated with the operation of the assets and occurring before the closing date of the IPO and (4) the Partnership's requirement to guarantee future indebtedness that CPG incurs. The Partnership is engaged in regulated interstate gas transportation and storage services for LDCs, marketers, producers and industrial and commercial customers located in northeastern, mid-Atlantic, midwestern and southern states and the District of Columbia along with unregulated businesses such as midstream services, including gathering, treating, conditioning, processing, compression and liquids handling, and development of mineral rights positions. The regulated services are performed under tariffs at rates subject to FERC approval. CPG Merger. On March 17, 2016, CPG entered into an Agreement and Plan of Merger (the “CPG Merger Agreement”), among CPG, TCPL, US Parent, Taurus Merger Sub Inc., a Delaware corporation and a wholly owned subsidiary of US Parent (“Merger Sub”), and, solely for purposes of Section 3.02, Section 5.02, Section 5.09 and Article VIII of the CPG Merger Agreement, TransCanada. Upon the terms and subject to the conditions set forth in the CPG Merger Agreement, effective July 1, 2016, Merger Sub was merged with and into CPG (the “Merger”) with CPG surviving the Merger as an indirect, wholly owned subsidiary of TransCanada. The Merger did not affect the ownership of the Partnership's general partner or limited partner interests, but the Partnership is indirectly managed by TransCanada after the Merger. The Partnership incurred approximately $114.2 million of Merger related costs within operation and maintenance and property and other taxes, including approximately $104.7 million of employee related costs. Additionally, as a result of the Merger, the Partnership recognized an impairment charge of $18.8 million related to the cancellation of IT system upgrades that were in process prior to the Merger. CPPL Merger. On November 1, 2016 , CPPL entered into an agreement and plan of merger (the "CPPL Merger Agreement") with CPG, Pony Merger Sub LLC, a Delaware limited liability company and wholly owned subsidiary of CPG (“Pony Merger Sub”) and MLP GP. The conflicts committee of the board of directors of MLP GP (the “GP Conflicts Committee”) and the board of directors of the MLP GP (the "GP Board") approved the CPPL Merger Agreement and transactions contemplated by the CPPL Merger Agreement and determined that the CPPL Merger Agreement and the merger transactions are fair and reasonable to and in the best interests of CPPL and to the holders of CPPL common units unaffiliated with CPG, CEG and MLP GP (collectively, the “CPPL unaffiliated unitholders”). The GP Conflicts Committee recommended the GP Board approve the CPPL Merger Agreement and the merger transactions. The GP Board resolved that the CPPL Merger Agreement and the merger transactions be submitted to the unitholders of CPPL at a special meeting of the unitholders for approval. The GP Board recommended that the unitholders of CPPL vote in favor of the proposal to approve the CPPL Merger Agreement and the merger transactions at the special meeting of the unitholders. On February 16, 2017, the CPPL common unitholders voted to approve the CPPL Merger. The transaction closed on February 17, 2017 and as of that date CPPL became a wholly owned subsidiary of CPG. The CPPL Merger did not affect the ownership of the Partnership's general partner or limited partner interests. For periods subsequent to the closing of CPPL's IPO, the financial statements included in this annual report are the financial statements and accounting records of the Partnership. For periods prior to the closing of CPPL's IPO, the financial statements included in this annual report are the financial statements and accounting records of the Predecessor. The consolidated and combined financial statements were prepared as follows: • The Consolidated Balance Sheets consist of the consolidated balance sheet of the Partnership as of December 31, 2016 and December 31, 2015 . • The Statements of Consolidated and Combined Operations consist of the consolidated results of the Partnership for the year ended December 31, 2016 and for the period February 11, 2015 through December 31, 2015 , and the combined results of the Predecessor for the period from January 1, 2015 through February 10, 2015 and for the year ended December 31, 2014. • The Statements of Consolidated and Combined Comprehensive Income consist of the consolidated results of the Partnership for the year ended December 31, 2016 and for the period from February 11, 2015 through December 31, 2015 , and the combined results of the Predecessor for the period from January 1, 2015 through February 10, 2015 and for the year ended December 31, 2014. • The Statements of Consolidated and Combined Cash Flows consist of the consolidated cash flows of the Partnership for the year ended December 31, 2016 and for the period from February 11, 2015 through December 31, 2015 , and the combined cash flows of the Predecessor for the period from January 1, 2015 through February 10, 2015 and for the year ended December 31, 2014. • The Statements of Consolidated and Combined Equity and Partners' Capital consist of the consolidated activity of the Partnership for the year ended December 31, 2016 and for the period from February 11, 2015 through December 31, 2015 , and the combined activity of the Predecessor for the period from January 1, 2015 through February 10, 2015 and for the year ended December 31, 2014. The Partnership’s accompanying Consolidated and Combined Financial Statements have been prepared in accordance with GAAP. These financial statements include the accounts of the following subsidiaries: Columbia Gas Transmission, Columbia Gulf, Columbia Midstream, CEVCO and CNS Microwave. All intercompany transactions and balances have been eliminated. Also included in the Consolidated and Combined Financial Statements are equity method investments Hardy Storage, Millennium Pipeline and Pennant. B. Use of Estimates. The preparation of financial statements in conformity with accounting principles generally accepted in the United States 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 and Cash Equivalents . Cash and cash equivalents are liquid marketable securities with an original maturity date of less than three months. D. Allowance for Uncollectible Accounts. The reserve for uncollectible receivables is the Partnership's best estimate of the amount of probable credit losses in the existing accounts receivable. Collectability of accounts receivable is reviewed regularly and an allowance is established or adjusted, as necessary, using the specific identification method. Account balances are charged against the allowance when it is anticipated the receivable will not be recovered. E. 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 recognized in income as the related amounts are included in service rates and recovered from or refunded to customers. In the event that regulation significantly changes the opportunity for the Partnership to recover its costs in the future, all or a portion of the Partnership’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 the Partnership’s existing regulatory assets and liabilities could result. If unable to continue to apply the provisions of regulatory accounting, the Partnership would be required to apply the provisions of Discontinuation of Rate-Regulated Accounting. In management’s opinion, the Partnership’s regulated subsidiaries will be subject to regulatory accounting for the foreseeable future. Please see Note 8, "Regulatory Matters," in the Notes to Consolidated and Combined Financial Statements for further discussion. F. Property, Plant and Equipment and Related AFUDC and Maintenance . Property, plant and equipment is stated at cost. The Partnership's rate-regulated subsidiaries record depreciation using composite rates on a straight-line basis over the remaining service lives of the properties as approved by the appropriate regulators. The Partnership's non-regulated companies depreciate assets on a component basis on a straight-line basis over the remaining service lives of the properties. The Partnership capitalizes AFUDC 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. A combination of short-term borrowings, long-term debt and equity were used to fund construction efforts for all three years presented. The pre-tax rate for AFUDC debt and AFUDC equity are summarized in the table below: 2016 2015 2014 Debt Equity Debt Equity Debt Equity Predecessor Columbia Gas Transmission 0.6 % 4.8 % 1.8 % 6.3 % 0.9 % 3.0 % Columbia Gulf 0.6 % 3.7 % 2.9 % 6.3 % 2.1 % 9.4 % The Partnership follows the practice of charging maintenance and repairs, including the cost of removal of minor items of property, to expense as incurred. When regulated property that represents a retired unit is replaced or removed, the cost of such property is credited to utility plant, and such cost, net of salvage, is charged to the accumulated provision for depreciation in accordance with composite depreciation. G. Gas Stored-Base Gas. Base gas, which is valued at original cost, represents storage volumes that are maintained to ensure that adequate well pressure exists to deliver current gas inventory. There were no purchases of base gas during the years ended December 31, 2016 , 2015 and 2014 . Gas stored-base gas is included in Property, plant and equipment on the Consolidated Balance Sheets. H. Amortization of Software Costs. 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. The Partnership amortized $9.7 million in 2016 , $5.8 million in 2015 and $4.3 million in 2014 related to software costs. The Partnership’s unamortized software balance was $29.1 million and $27.1 million at December 31, 2016 and 2015 , respectively. The increase in software amortization and unamortized software balance is primarily due to software placed in service subsequent to the Separation. The additional software was necessary for CPG to operate as an independent company. I. Goodwill. The Partnership has $1,975.5 million in goodwill. All goodwill relates to the excess of cost over the fair value of the net assets acquired in the CEG acquisition on November 1, 2000. Please see Note 6, "Goodwill," in the Notes to Consolidated and Combined Financial Statements for further discussion. J. Impairments. An impairment loss on long-lived assets shall be recognized only if the carrying amount of a long-lived assets is not recoverable and exceeds its fair value. The test for impairment compares the carrying amount of the long-lived asset to the sum of the undiscounted cash flows expected to result from the use and eventual disposition of the asset. The Partnership recognized an impairment loss of $18.8 million , $0.6 million , and zero for the years ended December 31, 2016 , 2015 , and 2014 , respectively. As a result of the Merger, the Partnership recognized an impairment loss for the year ended December 31, 2016 related to the cancellation of IT system upgrades that were in process prior to the Merger. K. Revenue Recognition. Revenue is recorded as services are performed. Revenues are billed to customers monthly at rates established through the FERC's cost-based rate-making process or at rates less than those allowed by the FERC. Revenues are recorded on the accrual basis and include estimates for transportation provided but not billed. The demand and commodity charges for transportation of gas under long-term agreements are recognized separately. Demand revenues are recognized monthly over the term of the agreement with the customer regardless of the volume of natural gas transported. Commodity revenues for both firm and interruptible transportation are recognized in the period the transportation services are provided based on volumes of natural gas physically delivered at the agreed upon delivery point. The Partnership provides shorter term transportation and storage services for which cash is received at inception of the service period resulting in the recording of deferred revenues that are recognized in revenues over the period the services are provided. Storage capacity revenues are recognized monthly over the term of the agreement with the customer regardless of the volume of storage service actually utilized. Injection and withdrawal revenues are recognized in the period when volumes of natural gas are physically injected into or withdrawn from storage. The Partnership includes the subsidiary CEVCO, which owns the mineral rights to approximately 460,000 acres in the Marcellus and Utica shale areas. CEVCO leases or contributes the mineral rights to producers in return for royalty interest. Royalties from mineral interests are recognized on an accrual basis when earned and realized. Royalty revenue was $21.5 million , $26.5 million and $43.8 million for the years ended December 31, 2016 , 2015 and 2014 , respectively, and is included in "Other revenues" on the Statements of Consolidated and Combined Operations. The Partnership periodically recognizes gains on the conveyance of mineral interest related to pooling of assets (production rights) in joint undertakings intended to find, develop, or produce oil or gas from a particular property or group of properties. The gains are initially deferred if the Partnership has a substantial obligation for future performance. As the obligation for future performance is satisfied, the deferred revenue is relieved and the associated gain is recognized. Gains on conveyances amounted to $16.9 million , $52.3 million and $34.5 million for the years ended December 31, 2016 , 2015 and 2014 , respectively, and are included in "Gain on sale of assets" on the Statements of Consolidated and Combined Operations. L. Estimated Rate Refunds . The Partnership collects revenue subject to refund pending final determination in rate proceedings. 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. M. Accounting for Exchange and Balancing Arrangements of Natural Gas. The Partnership enters into balancing and exchange arrangements of natural gas as part of its operations. The Partnership records a receivable or payable for its respective cumulative gas imbalances. These receivables and payables are recorded as “Exchange gas receivable” or “Exchange gas payable” on the Partnership’s Consolidated Balance Sheets, as appropriate. N. Income Taxes and Investment Tax Credits. The Partnership is a limited partnership and is treated as a partnership for U.S. federal income tax purposes and therefore, is not liable for entity-level federal income taxes. The Predecessor's operating results were included in NiSource's consolidated U.S. federal and in consolidated, combined or stand-alone state income tax returns. Amounts presented in the combined financial statements prior to CPPL's IPO relate to income taxes that have been determined on a separate tax return basis, and the Predecessor's contribution to NiSource's net operating losses and tax credits have been included in the Predecessor's financial statements. O. Environmental Expenditures. The Partnership 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 “Other Accruals” for short-term portions of these liabilities and “Other noncurrent liabilities” for the respective long-term portions of these liabilities. The Partnership establishes regulatory assets on the Consolidated Balance Sheets to the extent that future recovery of environmental remediation costs is probable through the regulatory process. Please see Note 13, "Other Commitments and Contingencies" in the Notes to Consolidated and Combined Financial Statements for further discussion. P. Accounting for Investments. The Partnership accounts for its ownership interests in Millennium Pipeline using the equity method of accounting. Columbia Gas Transmission owns a 47.5% interest in Millennium Pipeline. The equity method of accounting is applied for investments in unconsolidated companies where the Partnership (or a subsidiary) owns 20 to 50 percent of the voting rights and can exercise significant influence. The Partnership has a 49.0% interest in Hardy Storage. The Predecessor had a 50.0% interest in Hardy Storage. The Partnership and the Predecessor reflect the investment in Hardy Storage as an equity method investment. Columbia Midstream entered into a 50:50 joint venture in 2012 with Hilcorp to construct Pennant, a new wet natural gas gathering infrastructure and NGL processing facilities to support natural gas production in the Utica Shale region of northeastern Ohio and western Pennsylvania. During the third quarter of 2015, an additional member, an affiliate of Williams Partners, joined the Pennant joint venture. Williams Partners' initial ownership investment in Pennant is 5.00% , and by funding specified investment amounts for future growth projects, Williams Partners can invest directly in the growth of Pennant. Such funding will potentially increase Williams Partners' ownership in Pennant up to 33.33% over a defined investment period. As a result of the buy-in, Columbia Midstream received $12.7 million in cash and recorded a gain of $2.9 million , and its ownership interest in Pennant decreased from 50.0% to 47.5% . During 2016, Williams Partners funded additional specified growth projects. As a result, Columbia Midstream's ownership interest decreased to 47.0% . The Partnership accounts for the joint venture under the equity method of accounting. Q. Natural Gas and Oil Properties. CEVCO participates as a working interest partner in the development of a broader acreage dedication. The working interest allows CEVCO to invest in the drilling operations of the partnership in addition to a royalty interest in well production. Please see Note 1K, “Revenue Recognition,” in the Notes to Consolidated and Combined Financial Statements for further discussion regarding the royalty revenue. CEVCO uses the successful efforts method of accounting for natural gas and oil producing activities for their portion of drilling activities. Capitalized well costs are depleted based on the units of production method. CEVCO’s portion of unproved property investment is periodically evaluated for impairment. The majority of these costs generally relate to CEVCO’s portion of the working interest. The costs are capitalized and evaluated as to recoverability, based on changes brought about by economic factors and potential shifts in business strategy employed by management. Impairment of individually significant unproved property is assessed on a field-by-field basis considering a combination of time, geologic and engineering factors. The following table reflects the changes in capitalized exploratory well costs for the years ended December 31, 2016 and 2015 : (in millions) 2016 2015 Beginning Balance $ 1.7 $ 14.9 Additions pending the determination of proved reserves — 1.3 Reclassifications of proved properties (1.3 ) (14.5 ) Ending Balance $ 0.4 $ 1.7 As of December 31, 2016 , there was $0.3 million of capitalized exploratory well costs that have been capitalized for more than one year relating to one project initiated in 2013. |