| **Regulatory Matters - Rate and Regulatory Actions (Details) - USD ($) $ in Millions**                                                                |                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              | 1 Months Ended                                                                                                                                                                                                                                                                                              | 9 Months Ended | 12 Months Ended |
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| NORTH CAROLINA                                                                                                                                        |                                                                                                                                                                                                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |                                                                                                                                                                                                                                                                                                        |           |            |
| **Public Utilities, General Disclosures [Line Items]**                                                                                                |                                                                                                                                                                                                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |                                                                                                                                                                                                                                                                                                        |           |            |
| Capital Investments In Integrity and Safety Projects                                                                                                  |                                                                                                                                                                                                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |                                                                                                                                                                                                                                                                                                        |           | $ 241.9         |
| North Carolina Utilities Commission | NCUC Authorized IMR Adjustment January 2015                                                                     |                                                                                                                                                                                                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |                                                                                                                                                                                                                                                                                                        |           |            |
| **Public Utilities, Rate Matters, Approved [Abstract]**                                                                                               |                                                                                                                                                                                                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |                                                                                                                                                                                                                                                                                                        |           |            |
| Public Utilities Approved Rate Increase (Decrease) Amount                                                                                             |                                                                                                                                                                                                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |                                                                                                                                                                                                                                                                                                        | $ 26.6         |            |
| Public Service Commission of South Carolina [Member] | PSCSC Requested Rate Stabilization Agreement June 2015 [Member]                                |                                                                                                                                                                                                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |                                                                                                                                                                                                                                                                                                        |           |            |
| **Public Utilities, Rate Matters, Requested [Abstract]**                                                                                              |                                                                                                                                                                                                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |                                                                                                                                                                                                                                                                                                        |           |            |
| Public Utilities Requested Rate Increase (Decrease) Amount                                                                                            |                                                                                                                                                                                                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |                                                                                                                                                                                                                                                                                                        | $ 1.7          |            |
| Public Utilities, Requested Return on Equity, Percentage                                                                                              |                                                                                                                                                                                                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |                                                                                                                                                                                                                                                                                                        | 10.20%         |            |
| Tennessee Regulatory Authority | Amortization And Refund For Excess Deferred Taxes                                                                    |                                                                                                                                                                                                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |                                                                                                                                                                                                                                                                                                        |           |            |
| **Public Utilities, Rate Matters, Requested [Abstract]**                                                                                              |                                                                                                                                                                                                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |                                                                                                                                                                                                                                                                                                        |           |            |
| Public Utilities Requested Rate Increase (Decrease) Amount                                                                                            |                                                                                                                                                                                                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |                                                                                                                                                                                                                                                                                                        | $ (4.7)        |            |
| Proposed Amortization Time Period For Specified Expenses                                                                                              |                                                                                                                                                                                                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |                                                                                                                                                                                                                                                                                                        | 3 years        |            |
| Tennessee Regulatory Authority | Tennessee Ruling Compressed Natural Gas February 2015                                                                |                                                                                                                                                                                                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |                                                                                                                                                                                                                                                                                                        |           |            |
| **Public Utilities, Rate Matters, Approved [Abstract]**                                                                                               |                                                                                                                                                                                                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |                                                                                                                                                                                                                                                                                                        |           |            |
| Deferral CNG Equipment In Utility Rate Base                                                                                                           |                                                                                                                                                                                                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |                                                                                                                                                                                                                                                                                                        | $ 4.7          |            |
| Subsequent Event [Member] | North Carolina Utilities Commission | NCUC Gas Cost Purchasing Review August 2015 [Member]                                |                                                                                                                                                                                                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |                                                                                                                                                                                                                                                                                                        |           |            |
| **Public Utilities, General Disclosures [Line Items]**                                                                                                |                                                                                                                                                                                                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |                                                                                                                                                                                                                                                                                                        |           |            |
| Public Utilities, Disclosure of Rate Matters                                                                                                          |                                                                                                                                                                                                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         | In August 2015, we filed testimony with the NCUC in support of our gas cost purchasing and accounting practices for the twelve months ended May 31, 2015. A hearing has been scheduled for October 6, 2015.                                                                                                 |           |            |
| Subsequent Event [Member] | Public Service Commission of South Carolina [Member] | PSCSC Approved Annual Purchase Gas Adjustment August 2015 [Member] |                                                                                                                                                                                                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |                                                                                                                                                                                                                                                                                                        |           |            |
| **Public Utilities, General Disclosures [Line Items]**                                                                                                |                                                                                                                                                                                                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |                                                                                                                                                                                                                                                                                                        |           |            |
| Public Utilities, Disclosure of Rate Matters                                                                                                          |                                                                                                                                                                                                                                                                                                                                                                                                                                                              | In June 2015, we filed testimony with the Public Service Commission of South Carolina (PSCSC) in support of our annual review of purchased gas adjustment (PGA) and gas purchasing policies for the twelve months ended March 31, 2015. On June 29, 2015, a settlement agreement with the Office of Regulatory Staff (ORS) was filed. On August 12, 2015, the PSCSC approved the settlement agreement finding that our gas purchasing policies were reasonable and prudent, that we properly adhered to the gas cost recovery provisions of our tariff and relevant PSCSC orders and that we managed our hedging program in a manner consistent with PSCSC orders. We are waiting on the PSCSC's written order at this time. |                                                                                                                                                                                                                                                                                                        |           |            |
| Subsequent Event [Member] | Public Service Commission of South Carolina [Member] | PSCSC Requested Rate Stabilization Agreement June 2015 [Member]    |                                                                                                                                                                                                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |                                                                                                                                                                                                                                                                                                        |           |            |
| **Public Utilities, General Disclosures [Line Items]**                                                                                                |                                                                                                                                                                                                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |                                                                                                                                                                                                                                                                                                        |           |            |
| Public Utilities, Disclosure of Rate Matters                                                                                                          | In June 2015, we filed with the PSCSC a quarterly monitoring report for the twelve months ended March 31, 2015 and a cost and revenue study under the Rate Stabilization Act requesting a change in rates. On September 1, 2015, a settlement agreement with the ORS was filed that stipulated a $1.7 million annual increase in margin based on a return on equity of 10.2%, effective November 1, 2015. We are waiting on a ruling from the PSCSC at this time. |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |                                                                                                                                                                                                                                                                                                        |           |            |
| Subsequent Event [Member] | Tennessee Regulatory Authority | Tennessee Incentive Plan [Member]                                                        |                                                                                                                                                                                                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |                                                                                                                                                                                                                                                                                                        |           |            |
| **Public Utilities, General Disclosures [Line Items]**                                                                                                |                                                                                                                                                                                                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |                                                                                                                                                                                                                                                                                                        |           |            |
| Public Utilities, Disclosure of Rate Matters                                                                                                          |                                                                                                                                                                                                                                                                                                                                                                                                                                                              |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         | In August 2015, we filed an annual report with the TRA reflecting the shared gas cost savings from gains and losses derived from gas purchase benchmarking and secondary market transactions for the twelve months ended June 30, 2015 under the TIP. We are waiting on a ruling from the TRA at this time. |           |            |

| X                                                                                                                                                                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount of the Company's capital investments in integrity and safety projects.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                            |
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| - Definition
The amount of prior investments in Compressed Natural Gas (CNG) equipment that we have put into service since our last rate case proceeding that we may seek recovery of in our next general rate case proceeding.
+ References
No definition available.
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Amortization time period proposed to the commission for specified expenses.
+ References
No definition available.
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| - Definition
Amount of public utility's approved rate increase (decrease) by regulatory agency.
+ References
No definition available.
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| - Definition
Describes all of the specific events that have or may impact rates and amortization of regulatory assets and liabilities (for example, pending or recently concluded regulatory proceedings, environmental requirements). Specifics may include quantifications related to rates, costs, and earnings. Events may relate to rate moratoriums or price caps.
+ References
No definition available.
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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.
+ References
No definition available.
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| - References
No definition available.
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| - Definition
Amount of public utility's requested rate increase (decrease) with regulatory agency.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                 |
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| - Definition
Percentage of public utility's requested return on equity.
+ References
No definition available.
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