Unaudited Interim Financial Information (Details) (USD $)	3 Months Ended
	Mar. 31, 2014		Sep. 30, 2013
Regulatory Asset [Line Items]
Regulatory assets	$ 277,756,000		$ 286,748,000
Regulatory Liabilities [Line Items]
Regulatory liabilities	528,997,000		453,581,000
Regulatory asset - future recoverable pension costs	18,100,000		17,400,000
Regulatory noncurrent asset, amortization period	20
Deferred Gas Costs [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	80,330,000		16,481,000
Deferred Franchise Fees [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	11,523,000		1,689,000
Regulatory Cost of Removal Obligation [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	425,461,000		427,524,000
Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	11,683,000		7,887,000
Pension And Postretirement Benefit Costs [Member]
Regulatory Asset [Line Items]
Regulatory assets	176,616,000	[1]	187,977,000	[1]
Merger And Integration Costs Net [Member]
Regulatory Asset [Line Items]
Regulatory assets	4,990,000		5,250,000
Deferred Gas Costs Asset [Member]
Regulatory Asset [Line Items]
Regulatory assets	10,004,000		15,152,000
Regulatory Cost of Removal Asset [Member]
Regulatory Asset [Line Items]
Regulatory assets	9,716,000		10,008,000
Rate Case Costs [Member]
Regulatory Asset [Line Items]
Regulatory assets	5,037,000		6,329,000
Texas 8.209 [Member]
Regulatory Asset [Line Items]
Regulatory assets	40,760,000	[2]	30,364,000	[2]
APT Annual Adjustment Mechanism [Member]
Regulatory Asset [Line Items]
Regulatory assets	4,084,000		5,853,000
Recoverable Loss On Reacquired Debt [Member]
Regulatory Asset [Line Items]
Regulatory assets	20,156,000		21,435,000
Other [Member]
Regulatory Asset [Line Items]
Regulatory assets	$ 6,393,000		$ 4,380,000

[1] Includes $18.1 million and $17.4 million of pension and postretirement expense deferred pursuant to regulatory authorization.

[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.