Unaudited Interim Financial Information (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2016	Mar. 31, 2015	Sep. 30, 2015
New Accounting Pronouncement, Early Adoption [Line Items]
Retained earnings		$ 1,245,418		$ 1,245,418		$ 1,073,029
Income tax expense (benefit)		(79,961)	$ (83,518)	(141,613)	$ (142,177)
Regulatory Asset [Line Items]
Regulatory assets		201,450		201,450		192,339
Regulatory asset - future recoverable pension costs		13,600		13,600		16,600
Regulatory Liabilities [Line Items]
Regulatory liabilities		570,574		570,574		524,532
Regulatory Cost of Removal [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities		486,857		486,857		483,676
Deferred Gas Costs [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities		68,812		68,812		28,100
Asset Retirement Obligation Costs [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities		9,063		9,063		9,063
Revenue Subject to Refund [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities		640		640		0
Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities		5,202		5,202		3,693
Pension And Postretirement Benefit Costs [Member]
Regulatory Asset [Line Items]
Regulatory assets	[1]	113,483		113,483		121,183
Infrastructure Mechanisms [Member]
Regulatory Asset [Line Items]
Regulatory assets	[2]	53,698		53,698		32,813
Deferred Gas Costs [Member]
Regulatory Asset [Line Items]
Regulatory assets		1,136		1,136		9,715
Recoverable Loss On Reacquired Debt [Member]
Regulatory Asset [Line Items]
Regulatory assets		15,040		15,040		16,319
APT Annual Adjustment Mechanism [Member]
Regulatory Asset [Line Items]
Regulatory assets		0		0		1,002
Rate Case Costs [Member]
Regulatory Asset [Line Items]
Regulatory assets		1,796		1,796		1,533
Other [Member]
Regulatory Asset [Line Items]
Regulatory assets		16,297		16,297		$ 9,774
New Accounting Pronouncement, Early Adoption, Effect [Member] | Accounting Standards Update 2016-09 [Member]
New Accounting Pronouncement, Early Adoption [Line Items]
Deferred Tax Assets, Operating Loss Carryforwards		3,300		3,300
Income tax expense (benefit)				3,300
Adjustments for New Accounting Principle, Early Adoption [Member] | Accounting Standards Update 2016-09 [Member]
New Accounting Pronouncement, Early Adoption [Line Items]
Deferred Tax Assets, Operating Loss Carryforwards		14,500		14,500
Retained earnings		$ 14,500		$ 14,500

[1] Includes $13.6 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