Summary of Significant Accounting Policies (Details) - USD ($) $ in Thousands		12 Months Ended
		Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014	Mar. 31, 2016
Regulatory Assets [Line Items]
Regulatory Assets		$ 263,623	$ 192,339
Regulatory Liabilities [Line Items]
Regulatory Liabilities		514,725	524,532
Regulatory Assets and Liabilities, Other Disclosures [Abstract]
Regulatory Asset - Future Recoverable Pension Costs		12,400	16,600
Property, Plant and Equipment [Line Items]
Interest Costs Capitalized		$ 2,800	$ 2,300	$ 1,500
Composite depreciation rate for regulated property, plant and equipment		3.20%	3.30%	3.30%
Derivative [Line Items]
Unrealized gain (loss) on open contracts		$ 1,300	$ (2,400)	$ 9,600
Asset Retirement Costs in Property, Plant and Equipment		8,100	4,800
Asset Retirement Obligation		13,400	11,100
New Accounting Pronouncement, Early Adoption [Line Items]
Deferred charges and other assets		305,285	275,484
Deferred Tax Assets, Operating Loss Carryforwards		514,391	313,224
Retained earnings		1,262,534	1,073,029
Long-term debt		(2,188,779)	(2,437,515)
Income tax expense		200,373	195,690	187,002
Regulated Distribution Segment [Member]
New Accounting Pronouncement, Early Adoption [Line Items]
Long-term debt		(2,188,779)	(2,437,515)
Income tax expense		128,265	130,827	117,684
Derivative Liability, Collateral, Right to Reclaim Cash, Offset		25,670	0
Nonregulated Segment [Member]
Derivative [Line Items]
Cash Held in Margin Accounts		24,700	43,500
New Accounting Pronouncement, Early Adoption [Line Items]
Long-term debt		0	0
Income tax expense		12,620	12,652	$ 22,151
Derivative Liability, Collateral, Right to Reclaim Cash, Offset		17,905	34,620
Regulatory Cost Of Removal Liability [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities		476,891	483,676
Deferred Gas Costs [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities		20,180	28,100
Asset Retirement Obligation Costs [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities		13,404	9,063
Other [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities		4,250	3,693
Pension And Postretirement Benefit Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets	[1]	132,348	121,183
Infrastructure Mechanisms [Member]
Regulatory Assets [Line Items]
Regulatory Assets	[2]	42,719	32,813
Deferred Gas Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets		45,184	9,715
Recoverable Loss On Reacquired Debt [Member]
Regulatory Assets [Line Items]
Regulatory Assets		13,761	16,319
Deferred Pipeline Record Collection Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets		7,336	3,118
APT Annual Adjustment Mechanism Asset [Member]
Regulatory Assets [Line Items]
Regulatory Assets		7,171	1,002
Rate Case Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets		1,539	1,533
Other [Member]
Regulatory Assets [Line Items]
Regulatory Assets		$ 13,565	6,656
Minimum [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives for nonregulated property, plant and equipment		3 years
Maximum [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives for nonregulated property, plant and equipment		43 years
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		$ 5,000
Income tax expense		(5,000)
New Accounting Pronouncement, Early Adoption, Effect [Member] | Accounting Standards Update 2015-03 [Member]
New Accounting Pronouncement, Early Adoption [Line Items]
Deferred charges and other assets		(17,000)	(17,900)
Long-term debt		$ (17,000)	$ (17,900)
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 $12.4 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 eligible expenses associated with capital expenditures incurred pursuant to these rules, including the recording of interest on the deferred expenses until the next rate proceeding (rate case or annual rate filing), at which time investment and costs would be recovered through base rates.