Regulatory Matters - Rate and Regulatory Actions (Details) - USD ($) $ in Thousands	1 Months Ended	6 Months Ended	9 Months Ended
	Aug. 31, 2016	Mar. 31, 2016	Jul. 31, 2016	Jul. 31, 2015
Public Utilities, Rate Matters, Approved [Abstract]
Increase (Decrease) Regulatory liabilities			$ 13,377	$ (5,180)
North Carolina Utilities Commission | IMR Petition Filed November 2015
Public Utilities, Rate Matters, Requested [Abstract]
Annual Requested Integrity Management Rider, Amount			40,900
North Carolina Utilities Commission | IMR Petition Filed May 2016
Public Utilities, Rate Matters, Requested [Abstract]
Public Utilities Requested Rate Increase (Decrease) Amount			7,400
North Carolina Utilities Commission | Proposed Stipulation And Settlement Agreement Filed June 2016
Public Utilities, Rate Matters, Approved [Abstract]
Annual Charitable Contributions Commitment			17,500
Low-Income Household Energy Assistance and Workforce Development Programs Commitment			7,500
North Carolina Utilities Commission | Subsequent Event | Joint Proposed Order Filed August 2016
Public Utilities, Rate Matters, Approved [Abstract]
Public Utilities, Disclosure of Regulatory Matters Pending	A hearing was held on July 18 and 19, 2016. On August 25, 2016, we, Duke Energy and the NCUC Public Staff filed a joint proposed order pursuant to the NCUC's procedural schedule. We are waiting on a ruling from the NCUC at this time.
North Carolina Utilities Commission | Subsequent Event | Gas Cost Purchasing Review August 2016
Public Utilities, Rate Matters, Approved [Abstract]
Public Utilities, Disclosure of Rate Matters	In August 2016, we filed testimony with the NCUC in support of our gas cost purchasing and accounting practices for the twelve months ended May 31, 2016. A hearing has been scheduled for October 3, 2016.
North Carolina Utilities Commission | Amounts due to customers | Proposed Stipulation And Settlement Agreement Filed June 2016
Public Utilities, Rate Matters, Approved [Abstract]
Increase (Decrease) Regulatory liabilities			$ 10,000
Public Service Commission of South Carolina | Subsequent Event | Approved Annual Purchase Gas Adjustment August 2016
Public Utilities, Rate Matters, Approved [Abstract]
Public Utilities, Disclosure of Rate Matters	In June 2016, we filed testimony with the Public Service Commission of South Carolina (PSCSC) in support of our annual review of purchased gas adjustment and gas purchasing policies for the twelve months ended March 31, 2016. In June 2016, a settlement agreement with the Office of Regulatory Staff (ORS) was filed. A hearing was held on July 14, 2016. In August 2016, 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.
Public Service Commission of South Carolina | Subsequent Event | Requested Rate Stabilization Agreement June 2016
Public Utilities, Rate Matters, Approved [Abstract]
Public Utilities, Disclosure of Rate Matters	In June 2016, we filed with the PSCSC a quarterly monitoring report for the twelve months ended March 31, 2016 and a cost and revenue study under the Rate Stabilization Act requesting a change in rates. In August 2016, a settlement agreement was filed with the ORS stipulating an $8.3 million annual increase in margin based on a return on equity of 10.2%, effective November 1, 2016. We are waiting on a ruling from the PSCSC at this time.
Public Utilities, Rate Matters, Requested [Abstract]
Public Utilities Requested Rate Increase (Decrease) Amount	$ 8,300
Public Utilities, Requested Return on Equity, Percentage	10.20%
Tennessee Regulatory Authority | Subsequent Event | Actual Cost Adjustment
Public Utilities, Rate Matters, Approved [Abstract]
Public Utilities, Disclosure of Rate Matters	In August 2016, we filed an annual report for the twelve months ended June 30, 2016 with the TRA that reflected the transactions in the deferred gas cost account for the ACA mechanism. We are waiting on a ruling from the TRA at this time.
Tennessee Regulatory Authority | Subsequent Event | Tennessee Incentive Plan
Public Utilities, Rate Matters, Approved [Abstract]
Public Utilities, Disclosure of Rate Matters	In August 2016, 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, 2016 under the Tennessee Incentive Plan (TIP). We are waiting on a ruling from the TRA at this time.
Tennessee Regulatory Authority | Subsequent Event | Weather Normalization Adjustment Rider
Public Utilities, Rate Matters, Approved [Abstract]
Public Utilities, Disclosure of Rate Matters	In July 2016, the TRA Staff filed its compliance audit report for operation of our Weather Normalization Adjustment (WNA) rider during the 2015 – 2016 heating season, concluding that we had correctly implemented the WNA rider in all material aspects. The TRA Staff identified an immaterial error that resulted in an under-collection of our WNA revenues and recommended a correcting adjustment through the ACA mechanism which we recorded in August 2016. In August 2016, the TRA approved and adopted the TRA Staff’s compliance audit report.
NORTH CAROLINA
Public Utilities, Rate Matters, Approved [Abstract]
Capital Investments In Integrity and Safety Projects		$ 74,900