Unaudited Interim Financial Information (Details) - USD ($) $ in Thousands		6 Months Ended		12 Months Ended
		Mar. 31, 2018		Sep. 30, 2017
Regulatory Asset [Line Items]
Regulatory assets		$ 121,884		$ 176,798
Regulatory asset - future recoverable pension costs		7,800		9,400
Regulatory Liabilities [Line Items]
Regulatory liabilities		$ 1,493,007		$ 555,657
Federal Statutory Income Tax Rate, Pre TCJA, Percent		35.00%		35.00%
Federal Statutory Income Tax Rate, Post TCJA, Percent		21.00%
Deferred Income Tax Charge [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities		$ 737,798	[1]	$ 0
Regulatory Cost of Removal [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities		526,483		521,330
Deferred Gas Costs [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities		167,036		15,559
Asset Retirement Obligation Costs [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities		12,827		12,827
Revenue Subject to Refund [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities		5,081		0
Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities		14,740		5,941
Regulatory Cost of Service Reserve [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities		29,042	[2]	0
Pension And Postretirement Benefit Costs [Member]
Regulatory Asset [Line Items]
Regulatory assets	[3]	20,918		26,826
Infrastructure Mechanisms [Member]
Regulatory Asset [Line Items]
Regulatory assets	[4]	65,286		46,437
Deferred Gas Costs [Member]
Regulatory Asset [Line Items]
Regulatory assets		0		65,714
Recoverable Loss On Reacquired Debt [Member]
Regulatory Asset [Line Items]
Regulatory assets		9,954		11,208
Deferred Project Costs [Member]
Regulatory Asset [Line Items]
Regulatory assets		14,646		11,692
APT Annual Adjustment Mechanism [Member]
Regulatory Asset [Line Items]
Regulatory assets		0		2,160
Rate Case Costs [Member]
Regulatory Asset [Line Items]
Regulatory assets		3,016		2,629
Other [Member]
Regulatory Asset [Line Items]
Regulatory assets		$ 8,064		$ 10,132

[1] The TCJA resulted in the remeasurement of the net deferred tax liability included in our rate base. The excess deferred taxes will be returned to utility customers in accordance with regulatory requirements. See Note 6 for further information.

[2] Effective January 1, 2018, regulators in each of our service areas required us to establish a regulatory liability for the difference in recoverable federal taxes included in revenues based on the former 35% federal statutory rate and the new 21% federal statutory rate for service provided on or after January 1, 2018. This liability will be returned to utility customers in accordance with regulatory requirements. See Note 6 for further information.

[3] Includes $7.8 million and $9.4 million of pension and postretirement expense deferred pursuant to regulatory authorization.

[4] Infrastructure mechanisms in Texas and Louisiana allow for the deferral of all eligible expenses associated with capital expenditures incurred pursuant to these rules, including the recording of interest on deferred expenses until the next rate proceeding (rate case or annual rate filing), at which time investment and costs would be recoverable through base rates