Unaudited Interim Financial Information (Details) (USD $)	3 Months Ended
	Jun. 30, 2013		Sep. 30, 2012
Regulatory Asset [Line Items]
Regulatory assets	$ 327,743,000		$ 368,319,000
Regulatory Liabilities [Line Items]
Regulatory liabilities	464,924,000		488,397,000
Regulatory asset - future recoverable pension costs	15,500,000		7,600,000
Regulatory noncurrent asset, amortization period	20
Deferred Gas Costs [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	30,773,000		23,072,000
Deferred Franchise Fees [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	2,097,000		0
Regulatory Cost of Removal Obligation [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	426,656,000		459,688,000
Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	5,398,000		5,637,000
Pension And Postretirement Benefit Costs [Member]
Regulatory Asset [Line Items]
Regulatory assets	280,136,000	[1]	296,160,000	[1]
Merger And Integration Costs Net [Member]
Regulatory Asset [Line Items]
Regulatory assets	5,376,000		5,754,000
Deferred Gas Costs Asset [Member]
Regulatory Asset [Line Items]
Regulatory assets	1,271,000		31,359,000
Regulatory Cost of Removal Asset [Member]
Regulatory Asset [Line Items]
Regulatory assets	6,058,000		10,500,000
Rate Case Costs [Member]
Regulatory Asset [Line Items]
Regulatory assets	6,207,000		4,661,000
Deferred Franchise Fees [Member]
Regulatory Asset [Line Items]
Regulatory assets	242,000		2,714,000
Texas 8.209 [Member]
Regulatory Asset [Line Items]
Regulatory assets	21,351,000	[2]	5,370,000	[2]
APT Annual Adjustment Mechanism [Member]
Regulatory Asset [Line Items]
Regulatory assets	5,167,000		4,539,000
Other [Member]
Regulatory Asset [Line Items]
Regulatory assets	$ 1,935,000		$ 7,262,000

[1] Includes $15.5 million and $7.6 million of pension and postretirement expense deferred pursuant to

[2] Texas Rule 8.209 is a Railroad Commission rule that allows for the deferral of all expenses associated with capital expenditures incurred pursuant to this rule, 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.