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Unaudited Interim Financial Information (Details) - USD ($) $ in Thousands | 9 Months Ended |  |  |
--- | --- | --- | --- | ---
Jun. 30, 2017 | Aug. 01, 2017 | Jan. 06, 2017 | Sep. 30, 2016
Regulatory Asset [Line Items] |  |  |  |
Regulatory assets | $ 216,832 |  |  | $ 263,623
Regulatory asset - future recoverable pension costs | 11,500 |  |  | 12,400
Regulatory Liabilities [Line Items] |  |  |  |
Regulatory liabilities | 529,290 |  |  | 514,725
Atmos Pipeline - Texas 2017 Rate Case [Member] |  |  |  |
Subsequent Event [Line Items] |  |  |  |
Annual Operating Income Increase, Statement of Intent |  |  | $ 63,600 |
State of Texas Use Tax [Member] |  |  |  |
Use Tax Examination [Line Items] |  |  |  |
Use Tax Refund | 29,800 |  |  |
Use Tax Refund, Amount Received | 18,700 |  |  |
Use Tax Refund Receivable | 11,100 |  |  |
Subsequent Event [Member] | Atmos Pipeline - Texas 2017 Rate Case [Member] |  |  |  |
Subsequent Event [Line Items] |  |  |  |
Annual Operating Income Increase, Final Order |  | $ 13,000 |  |
Regulatory Cost of Removal [Member] |  |  |  |
Regulatory Liabilities [Line Items] |  |  |  |
Regulatory liabilities | 492,404 |  |  | 476,891
Deferred Gas Costs [Member] |  |  |  |
Regulatory Liabilities [Line Items] |  |  |  |
Regulatory liabilities | 16,753 |  |  | 20,180
Asset Retirement Obligation Costs [Member] |  |  |  |
Regulatory Liabilities [Line Items] |  |  |  |
Regulatory liabilities | 13,404 |  |  | 13,404
Other [Member] |  |  |  |
Regulatory Liabilities [Line Items] |  |  |  |
Regulatory liabilities | 6,729 |  |  | 4,250
Pension And Postretirement Benefit Costs [Member] |  |  |  |
Regulatory Asset [Line Items] |  |  |  |
Regulatory assets | 122,202 |  |  | 132,348
Infrastructure Mechanisms [Member] |  |  |  |
Regulatory Asset [Line Items] |  |  |  |
Regulatory assets | 38,653 |  |  | 42,719
Deferred Gas Costs [Member] |  |  |  |
Regulatory Asset [Line Items] |  |  |  |
Regulatory assets | 16,405 |  |  | 45,184
Recoverable Loss On Reacquired Debt [Member] |  |  |  |
Regulatory Asset [Line Items] |  |  |  |
Regulatory assets | 11,843 |  |  | 13,761
Deferred Project Costs [Member] |  |  |  |
Regulatory Asset [Line Items] |  |  |  |
Regulatory assets | 10,327 |  |  | 7,336
APT Annual Adjustment Mechanism [Member] |  |  |  |
Regulatory Asset [Line Items] |  |  |  |
Regulatory assets | 4,973 |  |  | 7,171
Rate Case Costs [Member] |  |  |  |
Regulatory Asset [Line Items] |  |  |  |
Regulatory assets | 2,480 |  |  | 1,539
Other [Member] |  |  |  |
Regulatory Asset [Line Items] |  |  |  |
Regulatory assets | $ 9,949 |  |  | $ 13,565

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