Unaudited Interim Financial Information (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended	9 Months Ended
		Jun. 30, 2016	Jun. 30, 2015	Mar. 31, 2016	Jun. 30, 2016	Jun. 30, 2015	Sep. 30, 2015
New Accounting Pronouncement, Early Adoption [Line Items]
Retained earnings		$ 1,273,057			$ 1,273,057		$ 1,073,029
Income tax expense (benefit)		(39,106)	$ (34,005)		(180,719)	$ (176,182)
Regulatory Asset [Line Items]
Regulatory assets		184,828			184,828		192,339
Regulatory asset - future recoverable pension costs		12,900			12,900		16,600
Regulatory Liabilities [Line Items]
Regulatory liabilities		535,198			535,198		524,532
Regulatory Cost of Removal [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities		486,290			486,290		483,676
Deferred Gas Costs [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities		34,362			34,362		28,100
Asset Retirement Obligation Costs [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities		9,063			9,063		9,063
Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities		5,483			5,483		3,693
Pension And Postretirement Benefit Costs [Member]
Regulatory Asset [Line Items]
Regulatory assets	[1]	110,425			110,425		121,183
Infrastructure Mechanisms [Member]
Regulatory Asset [Line Items]
Regulatory assets	[2]	31,090			31,090		32,813
Deferred Gas Costs [Member]
Regulatory Asset [Line Items]
Regulatory assets		3,390			3,390		9,715
Recoverable Loss On Reacquired Debt [Member]
Regulatory Asset [Line Items]
Regulatory assets		14,401			14,401		16,319
APT Annual Adjustment Mechanism [Member]
Regulatory Asset [Line Items]
Regulatory assets		2,976			2,976		1,002
Rate Case Costs [Member]
Regulatory Asset [Line Items]
Regulatory assets		1,640			1,640		1,533
Other [Member]
Regulatory Asset [Line Items]
Regulatory assets		20,906			20,906		$ 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		$ 4,900			4,900
Income tax expense (benefit)				$ 3,300	$ 4,900
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
Retained earnings				$ 14,500

[1] Includes $12.9 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 consist 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