| Document and Entity Information Document - shares | 3 Months Ended | |
| Mar. 31, 2016 | Apr. 29, 2016 | |
| Document Information [Line Items] | ||
| Entity Registrant Name | CPG OpCo GP LLC | |
| Entity Central Index Key | 1,667,153 | |
| Current Fiscal Year End Date | --12-31 | |
| Entity Filer Category | Non-accelerated Filer | |
| Document Type | 10-Q | |
| Document Period End Date | Mar. 31, 2016 | |
| Document Fiscal Year Focus | 2,016 | |
| Document Fiscal Period Focus | Q1 | |
| Amendment Flag | false | |
| Entity Common Stock, Shares Outstanding | 0 |
| Balance Sheets - USD ($) $ in Millions | Mar. 31, 2016 | Dec. 31, 2015 |
| Assets | ||
| Total Assets | $ 0 | $ 0 |
| Liabilities and Equity | ||
| Total Liabilities | 0 | 0 |
| Total Equity | 0 | 0 |
| Total Liabilities and Equity | $ 0 | $ 0 |
| Basis of Accounting Presentation | 3 Months Ended |
| Mar. 31, 2016 | |
| Organization, Consolidation and Presentation of Financial Statements [Abstract] | |
| Basis of Accounting Presentation | Basis of Accounting Presentation CPG OpCo GP LLC ("OpCo GP") is a subsidiary of CPG formed in Delaware on December 5, 2007. Through the registration of CPG's senior notes, OpCo GP, as a guarantor of the notes, must comply with the reporting requirements of Rule 3-10 of Regulation S-X. OpCo GP has not had any assets, liabilities, operations or cash flows since its formation date. The formation transactions that occurred as part of the initial public offering of CPPL resulted in OpCo GP becoming the general partner of Columbia OpCo. Columbia OpCo owns substantially all of CPG's natural gas transmission and storage assets. CPPL owns a 15.7% limited partner interest in Columbia OpCo. The remaining 84.3% limited partner interest in Columbia OpCo is owned by CEG, a 100% owned sub sidiary of CPG. Consolidation of Columbia OpCo Under ASC 810, Columbia OpCo is determined to be a VIE. Additionally, it is determined that OpCo GP is not the primary beneficiary of Columbia OpCo because OpCo GP lacks a direct economic interest in Columbia OpCo and the various related party limited partner interest holders were determined not to be so closely associated that they should be aggregated. As CPPL has a significant economic interest in Columbia OpCo and has the power to direct the activities of Columbia OpCo through that interest and it s 100% ownership interest in OpCo GP, CPPL is determined to be the primary beneficiary of Columbia OpCo and consolidates Columbia OpCo in its financial statements. |
| Other Commitments and Contingencies | 3 Months Ended |
| Mar. 31, 2016 | |
| Commitments and Contingencies Disclosure [Abstract] | |
| Commitments and Contingencies | Other Commitments and Contingencies Guarantees and Indemnities. OpCo GP entered into various agreements providing financial or performance assurance to third parties on behalf of CPG. 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 the parent on a stand-alone basis, thereby facilitating the extension of sufficient credit to accomplish the parent's intended commercial purposes. The total guarantees and indemnities in existence at March 31, 2016 and the years in which they expire were: (in millions) Total 2016 2017 2018 2019 2020 After Guarantees of debt $ 2,750.0 $ — $ — $ 500.0 $ — $ 750.0 $ 1,500.0 Lines and letters of credit 33.1 33.1 — — — — — Total commercial commitments $ 2,783.1 $ 33.1 $ — $ 500.0 $ — $ 750.0 $ 1,500.0 Guarantees of Debt. OpCo GP and Columbia OpCo (together with CEG, the "Guarantors") have guaranteed payment of $2,750.0 million in aggregated principal amount of CPG's senior notes. Each Guarantor is required to comply with covenants under the debt indenture and in the event of default the Guarantors would be obligated to pay the debt's principal and related interest. OpCo GP does not anticipate that any of the Guarantors will have any difficulty maintaining compliance. The guarantees of any Guarantor may be released under certain circumstances. First, if CPG discharges or defeases its obligations with respect to any series of CPG’s senior notes, then any guarantee will be released with respect to that series. Second, if no event of default has occurred and is continuing under the indenture, a Guarantor will be automatically and unconditionally released and discharged from its guarantee (i) at any time after June 1, 2018, upon any sale, exchange or transfer, whether by way of merger or otherwise, to any person that is not CPG’s affiliate, of all of CPG’s direct or indirect limited partnership, limited liability or other equity interests in the Guarantor; (ii) upon the merger of a guarantor into CPG or any other Guarantor or the liquidation and dissolution of such Guarantor; or (iii) at any time after June 1, 2018, upon release of all guarantees or other obligations of the Guarantor with respect to any of CPG’s funded debt, except CPG’s senior notes. Lines and Letters of Credit. CPPL maintains a $500.0 million senior revolving credit facility, of which $50.0 million is available for issuance of letters of credit. The purpose of the facility is to provide cash for general partnership purposes, including working capital, capital expenditures and the funding of capital calls. OpCo GP, together with CPG, CEG and Columbia OpCo, have each fully guaranteed the CPPL credit facility. As of March 31, 2016 , CPPL had $15.0 million in outstanding borrowings and no letters of credit under its revolving credit facility. CPG maintains a $1,500.0 million senior revolving credit facility, of which $250.0 million in letters of credit is available. CPG expects that $750.0 million of the facility will be utilized as credit support for Columbia OpCo and its subsidiaries and the remaining $750.0 million of the facility will be available for CPG's general corporate purposes, including working capital. The revolving credit facility will provide liquidity support for CPG's $1,000.0 million commercial paper program. OpCo GP, together with CEG and Columbia OpCo, have each fully guaranteed the CPG credit facility. As of March 31, 2016 , CPG had no borrowings outstanding and $18.1 million in letters of credit outstanding under its revolving credit facility. On March 17, 2016, CPG entered into an Agreement and Plan of Merger (the “Merger Agreement”), among CPG, TransCanada PipeLines Limited, a Canadian corporation (“Parent”), TransCanada PipeLine USA Ltd., a Nevada corporation and a wholly owned subsidiary of Parent (“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 Merger Agreement, TransCanada Corporation, a Canadian corporation and the direct parent company of Parent (“TransCanada”). Upon the terms and subject to the conditions set forth in the Merger Agreement, Merger Sub will be merged with and into CPG (the “Merger”) with CPG surviving the Merger as an indirect wholly owned subsidiary of TransCanada. The Merger, if completed, would constitute a change of control under the terms of the CPG and CPPL revolving credit facilities. Accordingly, in connection with the closing of the Merger, OpCo GP expects the CPG and CPPL revolving credit facilities to be terminated and all outstanding amounts thereunder to be repaid and any existing letters of credit to be replaced. CPG has established a commercial paper program (the "Program") pursuant to which CPG may issue short-term promissory notes (the "Promissory Notes") pursuant to the exemption from registration contained in Section 4(a)(2) of the Securities Act of 1933, as amended (the "Securities Act"). Amounts available under the Program may be borrowed, repaid and re-borrowed from time to time, with the aggregate face or principal amount of the Promissory Notes outstanding under the Program at any time not to exceed $1,000.0 million . OpCo GP, together with CEG and Columbia OpCo, have each agreed, jointly and severally, unconditionally and irrevocably to guarantee payment in full of the principal of and interest (if any) on the Promissory Notes. The net proceeds of issuances of the Promissory Notes are expected to be used for general corporate purposes. As of March 31, 2016 , CPG had no Promissory Notes outstanding under the Program. |
| Other Commitments and Contingencies (Tables) | 3 Months Ended |
| Mar. 31, 2016 | |
| Commitments and Contingencies Disclosure [Abstract] | |
| Schedule of Guarantor Obligations | The total guarantees and indemnities in existence at March 31, 2016 and the years in which they expire were: (in millions) Total 2016 2017 2018 2019 2020 After Guarantees of debt $ 2,750.0 $ — $ — $ 500.0 $ — $ 750.0 $ 1,500.0 Lines and letters of credit 33.1 33.1 — — — — — Total commercial commitments $ 2,783.1 $ 33.1 $ — $ 500.0 $ — $ 750.0 $ 1,500.0 |
| Basis of Accounting Presentation (Details) | 3 Months Ended |
| Mar. 31, 2016 | |
| Basis Of Accounting Presentation [Line Items] | |
| Limited Liability Company (LLC) or Limited Partnership (LP), Managing Member or General Partner, Ownership Interest | 100.00% |
| Columbia Pipeline Partners LP | |
| Basis Of Accounting Presentation [Line Items] | |
| Limited Liability Company (LLC) or Limited Partnership (LP), Managing Member or General Partner, Ownership Interest | 15.70% |
| Columbia Energy Group | |
| Basis Of Accounting Presentation [Line Items] | |
| Limited Liability Company (LLC) or Limited Partnership (LP), Members or Limited Partners, Ownership Interest | 84.30% |
| Other Commitments and Contingencies (Narrative) (Details) $ in Millions | 3 Months Ended |
| Mar. 31, 2016USD ($) | |
| Purchase Commitment, Excluding Long-term Commitment [Line Items] | |
| Guarantor Obligations, Maximum Exposure, Undiscounted | $ 2,783.1 |
| Columbia Pipeline Partners LP | Revolving Credit Facility | |
| Purchase Commitment, Excluding Long-term Commitment [Line Items] | |
| Line of Credit Facility, Maximum Borrowing Capacity | 500 |
| Columbia Pipeline Partners LP | Letter of Credit | |
| Purchase Commitment, Excluding Long-term Commitment [Line Items] | |
| Line of Credit Facility, Maximum Borrowing Capacity | 50 |
| Columbia Pipeline Group | |
| Purchase Commitment, Excluding Long-term Commitment [Line Items] | |
| Line of Credit Facility Used For Credit Support for Subsidiaries | 750 |
| Line of Credit Facility Used For Corporate Purposes | 750 |
| Commercial Paper | 0 |
| Columbia Pipeline Group | Revolving Credit Facility | |
| Purchase Commitment, Excluding Long-term Commitment [Line Items] | |
| Line of Credit Facility, Maximum Borrowing Capacity | 1,500 |
| Columbia Pipeline Group | Letter of Credit | |
| Purchase Commitment, Excluding Long-term Commitment [Line Items] | |
| Line of Credit Facility, Maximum Borrowing Capacity | 250 |
| Commercial Paper | Columbia Pipeline Group | |
| Purchase Commitment, Excluding Long-term Commitment [Line Items] | |
| Line of Credit Facility, Maximum Borrowing Capacity | 1,000 |
| Letter of Credit | Columbia Pipeline Partners LP | |
| Purchase Commitment, Excluding Long-term Commitment [Line Items] | |
| Guarantee Obligations Maximum Exposure Year One | 0 |
| Letter of Credit | Columbia Pipeline Group | |
| Purchase Commitment, Excluding Long-term Commitment [Line Items] | |
| Guarantee Obligations Maximum Exposure Year One | 18.1 |
| Revolving Credit Facility | Columbia Pipeline Partners LP | |
| Purchase Commitment, Excluding Long-term Commitment [Line Items] | |
| Line of Credit | 15 |
| Revolving Credit Facility | Columbia Pipeline Group | |
| Purchase Commitment, Excluding Long-term Commitment [Line Items] | |
| Line of Credit | 0 |
| Guarantee of Indebtedness of Others | |
| Purchase Commitment, Excluding Long-term Commitment [Line Items] | |
| Guarantor Obligations, Maximum Exposure, Undiscounted | $ 2,750 |
| Other Commitments and Contingencies (Existence and Expiration of Commercial Commitments) (Details) $ in Millions | Mar. 31, 2016USD ($) |
| Other Commitments and Contingencies [Line Items] | |
| Guarantor Obligations, Maximum Exposure, Undiscounted | $ 2,783.1 |
| Guarantee Obligations Maximum Exposure Year One | 33.1 |
| Guarantee Obligations Maximum Exposure Year Two | 0 |
| Guarantee Obligations Maximum Exposure Year Three | 500 |
| Guarantee Obligations Maximum Exposure Year Four | 0 |
| Guarantee Obligations Maximum Exposure Year Five | 750 |
| Guarantee Obligations Maximum Exposure Thereafter | 1,500 |
| Guarantee of Indebtedness of Others | |
| Other Commitments and Contingencies [Line Items] | |
| Guarantor Obligations, Maximum Exposure, Undiscounted | 2,750 |
| Guarantee Obligations Maximum Exposure Year One | 0 |
| Guarantee Obligations Maximum Exposure Year Two | 0 |
| Guarantee Obligations Maximum Exposure Year Three | 500 |
| Guarantee Obligations Maximum Exposure Year Four | 0 |
| Guarantee Obligations Maximum Exposure Year Five | 750 |
| Guarantee Obligations Maximum Exposure Thereafter | 1,500 |
| Letter of Credit | |
| Other Commitments and Contingencies [Line Items] | |
| Guarantor Obligations, Maximum Exposure, Undiscounted | 33.1 |
| Guarantee Obligations Maximum Exposure Year One | 33.1 |
| Guarantee Obligations Maximum Exposure Year Two | 0 |
| Guarantee Obligations Maximum Exposure Year Three | 0 |
| Guarantee Obligations Maximum Exposure Year Four | 0 |
| Guarantee Obligations Maximum Exposure Year Five | 0 |
| Guarantee Obligations Maximum Exposure Thereafter | $ 0 |
