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