Unaudited Interim Financial Information (Details) - USD ($) $ in Thousands		Dec. 31, 2017		Sep. 30, 2017
Regulatory Asset [Line Items]
Regulatory assets		$ 134,059		$ 176,798
Regulatory asset - future recoverable pension costs		8,600		9,400
Regulatory Liabilities [Line Items]
Regulatory liabilities		1,308,688		555,657
Deferred Income Tax Charge [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities		746,246	[1]	0
Regulatory Cost of Removal [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities		520,483		521,330
Deferred Gas Costs [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities		19,739		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		1,720		0
Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities		7,673		5,941
Pension And Postretirement Benefit Costs [Member]
Regulatory Asset [Line Items]
Regulatory assets	[2]	24,598		26,826
Infrastructure Mechanisms [Member]
Regulatory Asset [Line Items]
Regulatory assets	[3]	54,571		46,437
Deferred Gas Costs [Member]
Regulatory Asset [Line Items]
Regulatory assets		18,505		65,714
Recoverable Loss On Reacquired Debt [Member]
Regulatory Asset [Line Items]
Regulatory assets		10,580		11,208
Deferred Project Costs [Member]
Regulatory Asset [Line Items]
Regulatory assets		12,942		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		3,160		2,629
Other [Member]
Regulatory Asset [Line Items]
Regulatory assets		$ 9,703		$ 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] Includes $8.6 million and $9.4 million of pension and postretirement expense deferred pursuant to regulatory authorization.

[3] 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