Unaudited Interim Financial Information (Details) - USD ($) $ in Thousands		Dec. 31, 2015	Sep. 30, 2015
Regulatory Asset [Line Items]
Regulatory assets		$ 205,306	$ 192,339
Regulatory asset - future recoverable pension costs		14,300	16,600
Regulatory Liabilities [Line Items]
Regulatory liabilities		529,638	524,532
Regulatory Cost of Removal [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities		482,544	483,676
Deferred Gas Costs [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities		32,895	28,100
Asset Retirement Obligation Costs [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities		9,063	9,063
Revenue Subject to Refund [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities		1,721	0
Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities		3,415	3,693
Pension And Postretirement Benefit Costs [Member]
Regulatory Asset [Line Items]
Regulatory assets	[1]	116,485	121,183
Infrastructure Mechanisms [Member]
Regulatory Asset [Line Items]
Regulatory assets	[2]	43,385	32,813
Deferred Gas Costs [Member]
Regulatory Asset [Line Items]
Regulatory assets		16,310	9,715
Recoverable Loss On Reacquired Debt [Member]
Regulatory Asset [Line Items]
Regulatory assets		15,680	16,319
APT Annual Adjustment Mechanism [Member]
Regulatory Asset [Line Items]
Regulatory assets		0	1,002
Rate Case Costs [Member]
Regulatory Asset [Line Items]
Regulatory assets		1,568	1,533
Other [Member]
Regulatory Asset [Line Items]
Regulatory assets		$ 11,878	$ 9,774

[1] Includes $14.3 million and $16.6 million of pension and postretirement expense deferred pursuant to regulatory authorization.

[2] Infrastructure mechanisms in Texas and Louisiana allow for the deferral of all expenses associated with capital expenditures incurred pursuant to these rules, which primarily consists of interest, depreciation and other taxes, until the next rate proceeding (rate case or annual rate filing), at which time investment and costs would be recoverable through base rates