| **Equity Method Investments (Details) - USD ($)**           |                                                                                                                                                                                                                                                                                                                                                                                                            | 3 Months Ended  | 9 Months Ended |                 |              |
|-------------------------------------------------------------|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|-----------------|----------------|-----------------|--------------|
||
| **Schedule of Equity Method Investments [Line Items]**      |                                                                                                                                                                                                                                                                                                                                                                                                       |            |           |            |         |   |   |
| Related party costs                                         |                                                                                                                                                                                                                                                                                                                                                                                                       | $ 7,055,000     | $ 7,362,000    | $ 21,658,000    | $ 22,132,000 |   |   |
| Trade accounts payable                                      |                                                                                                                                                                                                                                                                                                                                                                                                       | 2,369,000       |           | 2,369,000       |         |   | $ 2,473,000 |
| Equity method investments in non-utility activities         |                                                                                                                                                                                                                                                                                                                                                                                                       | $ 239,650,000   |           | 239,650,000     |         |   | 206,956,000 |
| Contributions to equity method investments                  |                                                                                                                                                                                                                                                                                                                                                                                                       |            |           | $ 30,642,000    | 20,063,000   |   |   |
| Cardinal Pipeline Company                                   |                                                                                                                                                                                                                                                                                                                                                                                                       |            |           |            |         |   |   |
| **Schedule of Equity Method Investments [Line Items]**      |                                                                                                                                                                                                                                                                                                                                                                                                       |            |           |            |         |   |   |
| Membership interest percentage                              |                                                                                                                                                                                                                                                                                                                                                                                                       | 21.49%          |           | 21.49%          |         |   |   |
| Related party costs                                         |                                                                                                                                                                                                                                                                                                                                                                                                       | $ 2,198,000     | 2,207,000      | $ 6,550,000     | 6,556,000    |   |   |
| Trade accounts payable                                      |                                                                                                                                                                                                                                                                                                                                                                                                       | $ 741,000       |           | $ 741,000       |         |   | 744,000 |
| Pine Needle Company                                         |                                                                                                                                                                                                                                                                                                                                                                                                       |            |           |            |         |   |   |
| **Schedule of Equity Method Investments [Line Items]**      |                                                                                                                                                                                                                                                                                                                                                                                                       |            |           |            |         |   |   |
| Membership interest percentage                              |                                                                                                                                                                                                                                                                                                                                                                                                       | 45.00%          |           | 45.00%          |         |   |   |
| Related party costs                                         |                                                                                                                                                                                                                                                                                                                                                                                                       | $ 2,535,000     | 2,833,000      | $ 8,141,000     | 8,609,000    |   |   |
| Trade accounts payable                                      |                                                                                                                                                                                                                                                                                                                                                                                                       | $ 854,000       |           | $ 854,000       |         |   | 955,000 |
| South Star Energy Services                                  |                                                                                                                                                                                                                                                                                                                                                                                                       |            |           |            |         |   |   |
| **Schedule of Equity Method Investments [Line Items]**      |                                                                                                                                                                                                                                                                                                                                                                                                       |            |           |            |         |   |   |
| Membership interest percentage                              |                                                                                                                                                                                                                                                                                                                                                                                                       | 15.00%          |           | 15.00%          |         |   |   |
| Operating revenues                                          |                                                                                                                                                                                                                                                                                                                                                                                                       | $ 34,000        | 475,000        | $ 286,000       | 1,058,000    |   |   |
| Trade accounts receivable                                   |                                                                                                                                                                                                                                                                                                                                                                                                       | $ 2,000         |           | $ 2,000         |         |   | 183,000 |
| Equity Method Investment Agreement To Sell Interest         |                                                                                                                                                                                                                                                                                                                                                                                                       |            |           |            |         | $ 160,000,000 |   |
| Hardy Storage                                               |                                                                                                                                                                                                                                                                                                                                                                                                       |            |           |            |         |   |   |
| **Schedule of Equity Method Investments [Line Items]**      |                                                                                                                                                                                                                                                                                                                                                                                                       |            |           |            |         |   |   |
| Membership interest percentage                              |                                                                                                                                                                                                                                                                                                                                                                                                       | 50.00%          |           | 50.00%          |         |   |   |
| Related party costs                                         |                                                                                                                                                                                                                                                                                                                                                                                                       | $ 2,322,000     | $ 2,322,000    | $ 6,967,000     | $ 6,967,000  |   |   |
| Trade accounts payable                                      |                                                                                                                                                                                                                                                                                                                                                                                                       | $ 774,000       |           | $ 774,000       |         |   | $ 774,000 |
| Constitution Pipeline Company                               |                                                                                                                                                                                                                                                                                                                                                                                                       |            |           |            |         |   |   |
| **Schedule of Equity Method Investments [Line Items]**      |                                                                                                                                                                                                                                                                                                                                                                                                       |            |           |            |         |   |   |
| Membership interest percentage                              |                                                                                                                                                                                                                                                                                                                                                                                                       | 24.00%          |           | 24.00%          |         |   |   |
| Estimated Pipeline Costs Including AFUDC                    |                                                                                                                                                                                                                                                                                                                                                                                                       | $ 1,100,000,000 |           | $ 1,100,000,000 |         |   |   |
| Target Pipeline In Service Date                             |                                                                                                                                                                                                                                                                                                                                                                                                       |            |           | 2018            |         |   |   |
| Equity method investments in non-utility activities         |                                                                                                                                                                                                                                                                                                                                                                                                       | 93,600,000      |           | $ 93,600,000    |         |   |   |
| Other than Temporary Impairment Losses, Investments         |                                                                                                                                                                                                                                                                                                                                                                                                       |            |           | 0               |         |   |   |
| Estimated Pipeline Costs Excluding AFUDC                    |                                                                                                                                                                                                                                                                                                                                                                                                       | 955,000,000     |           | 955,000,000     |         |   |   |
| Estimated Contributions For Pipeline And Construction Costs |                                                                                                                                                                                                                                                                                                                                                                                                       | 229,300,000     |           | 229,300,000     |         |   |   |
| Contributions to equity method investments                  |                                                                                                                                                                                                                                                                                                                                                                                                       | 500,000         |           | 10,200,000      |         |   |   |
| Total equity contribution for the project                   |                                                                                                                                                                                                                                                                                                                                                                                                       | $ 82,900,000    |           | $ 82,900,000    |         |   |   |
| Atlantic Coast Pipeline                                     |                                                                                                                                                                                                                                                                                                                                                                                                       |            |           |            |         |   |   |
| **Schedule of Equity Method Investments [Line Items]**      |                                                                                                                                                                                                                                                                                                                                                                                                       |            |           |            |         |   |   |
| Membership interest percentage                              |                                                                                                                                                                                                                                                                                                                                                                                                       | 10.00%          |           | 10.00%          |         |   |   |
| Target Pipeline In Service Date                             |                                                                                                                                                                                                                                                                                                                                                                                                       |            |           | 2019            |         |   |   |
| Contributions to equity method investments                  |                                                                                                                                                                                                                                                                                                                                                                                                       | $ 10,000,000    |           | $ 20,400,000    |         |   |   |
| Total equity contribution for the project                   |                                                                                                                                                                                                                                                                                                                                                                                                       | 31,000,000      |           | $ 31,000,000    |         |   |   |
| Long term purchase commitment time period                   |                                                                                                                                                                                                                                                                                                                                                                                                       |            |           | 20 years        |         |   |   |
| Estimated Percentage Project Financing                      |                                                                                                                                                                                                                                                                                                                                                                                                       |            |           | 60.00%          |         |   |   |
| Estimated Percentage Debt Financing                         |                                                                                                                                                                                                                                                                                                                                                                                                       |            |           | 50.00%          |         |   |   |
| Estimated Percentage Equity Financing                       |                                                                                                                                                                                                                                                                                                                                                                                                       |            |           | 50.00%          |         |   |   |
| Funding Obligation Under Equity Contribution Agreements     |                                                                                                                                                                                                                                                                                                                                                                                                       |            |           | $ 15,200,000    |         |   |   |
| Minimum | Atlantic Coast Pipeline                           |                                                                                                                                                                                                                                                                                                                                                                                                       |            |           |            |         |   |   |
| **Schedule of Equity Method Investments [Line Items]**      |                                                                                                                                                                                                                                                                                                                                                                                                       |            |           |            |         |   |   |
| Estimated Pipeline Development And Construction Costs       |                                                                                                                                                                                                                                                                                                                                                                                                       | 4,500,000,000   |           | 4,500,000,000   |         |   |   |
| Maximum | Atlantic Coast Pipeline                           |                                                                                                                                                                                                                                                                                                                                                                                                       |            |           |            |         |   |   |
| **Schedule of Equity Method Investments [Line Items]**      |                                                                                                                                                                                                                                                                                                                                                                                                       |            |           |            |         |   |   |
| Estimated Pipeline Development And Construction Costs       |                                                                                                                                                                                                                                                                                                                                                                                                       | $ 5,000,000,000 |           | $ 5,000,000,000 |         |   |   |
| Subsequent Event | Atlantic Coast Pipeline                  |                                                                                                                                                                                                                                                                                                                                                                                                       |            |           |            |         |   |   |
| **Schedule of Equity Method Investments [Line Items]**      |                                                                                                                                                                                                                                                                                                                                                                                                       |            |           |            |         |   |   |
| Equity Method Investment, Additional Information            | On August 12, 2016, the FERC issued its notice of schedule for environmental review of the project. Under the notice of schedule, we anticipate that the FERC will issue a draft of its environmental impact statement (EIS) in December 2016 and the final EIS in the summer of 2017. Based on this schedule, ACP is reviewing the timing of the project, as well as refining the project cost estimates. |            |           |            |         |   |   |

| X                                                                                                                                                                                                                                                                                                                                         |
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| - Definition
This item represents the agreed upon selling price to sell our interest in an equity method investment.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                     |
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| - Definition
The estimated cash outflow associated with contributions to an equity method investment (EMI) to fund the EMI's development and construction of a new natural gas pipeline.
+ References
No definition available.
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| - Definition
The estimated percentage of debt financing the joint members expect to obtain for equity method investment (EMI) when EMI is operational.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                            |
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| - Definition
The estimated percentage of equity financing the joint members expect to obtain for equity method investment (EMI) when EMI is operational.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                          |
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| - Definition
The estimated percentage of financing the joint venture members expect to obtain to finance the project.
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| X                                                                                                                                                                                                                                                                                                                         |
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| - Definition
Total estimated cost pertaining to the development and construction of new pipeline.
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| X                                                                                                                                                                                                                                                                                                                                                                                                              |
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| - Definition
Total estimated cost pertaining to the development and construction of the new pipeline, excluding an allowance for funds used during construction (AFUDC).
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| X                                                                                                                                                                                                                                                                                                                                                                                                              |
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| - Definition
Total estimated cost pertaining to the development and construction of the new pipeline, including an allowance for funds used during construction (AFUDC).
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No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
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| - Definition
Maximum funding obligation, per Equity Contribution Agreement, that Company has committed to make funds available to Company's subsidiary for it to pay and perform its obligations under an Limited Liability Company Agreement.
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| X                                                                                                                                                                                                                                                                     |
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| - Definition
The target date for the pipeline to be placed into service.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                   |
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| - Definition
The total cash outflow associated with advances to an equity method investments, which are investments in joint ventures.
+ References
No definition available.
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| - Definition
The aggregate amount of receivables to be collected from related parties where one party can exercise control or significant influence over another party; including affiliates, owners or officers and their immediate families, pension trusts, and so forth, at the financial statement date. which are usually due within one year (or one business cycle).
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| - Definition
Carrying amount as of the balance sheet date of obligations due all related parties. For classified balance sheets, represents the current portion of such liabilities (due within one year or within the normal operating cycle if longer).
+ References
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| - Definition
This serves as a place to record data that is not required by accounting literature but is useful for readers of the financial statements as it relates to the details of an equity method investment in common stock. Such information may include the amount of change for a change in ownership (equity) percentage in a given period and the amount and type of the consideration given or received in relation to the change in ownership.
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No definition available.
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| - Definition
The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.
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| - Definition
This item represents the carrying amount on the entity's balance sheet of its investment in common stock of an equity method investee. This is not an indicator of the fair value of the investment, rather it is the initial cost adjusted for the entity's share of earnings and losses of the investee, adjusted for any distributions (dividends) and other than temporary impairment (OTTI) losses recognized.
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| - Definition
The amount by which the fair value of an investment is less than the amortized cost basis or carrying amount of that investment at the balance sheet date and the decline in fair value is deemed to be other than temporary, before considering whether or not such amount is recognized in earnings or other comprehensive income.
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| - Definition
Period covered by the long-term purchase commitment, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.
+ References
No definition available.
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| - Definition
The cash outflow associated with the purchase of or advances to an equity method investments, which are investments in joint ventures and entities in which the entity has an equity ownership interest normally of 20 to 50 percent and exercises significant influence.
+ References
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| - Definition
Expenses recognized resulting from transactions (excluding transactions that are eliminated in consolidated or combined financial statements) with related party.
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| - Definition
Amount of revenue, fees and commissions earned from transactions between (a) a parent company and its subsidiaries; (b) subsidiaries of a common parent; (c) an entity and trusts for the benefit of employees, for example, but not limited to, pension and profit-sharing trusts that are managed by or under the trusteeship of the entity's management; (d) an entity and its principal, owners, management, or members of their immediate families; and (e) affiliates.
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| - Definition
Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.
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No definition available.
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