Unaudited Interim Financial Information (Details) - USD ($) $ in Thousands		3 Months Ended
		Jun. 30, 2015	Sep. 30, 2014
Regulatory Asset [Line Items]
Regulatory assets		$ 202,508	$ 250,309
Regulatory Liabilities [Line Items]
Regulatory liabilities		581,363	545,759
Deferred Gas Costs [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities		81,134	35,063
Deferred Franchise Fees [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities		747	5,268
Regulatory Cost of Removal [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities		486,672	490,448
Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities		12,810	14,980
Pension And Postretirement Benefit Costs [Member]
Regulatory Asset [Line Items]
Regulatory assets	[1]	149,202	162,777
Regulatory asset - future recoverable pension costs		15,800	18,800
Merger And Integration Costs Net [Member]
Regulatory Asset [Line Items]
Regulatory assets		$ 4,327	4,730
Regulatory noncurrent asset, amortization period		P20Y
Deferred Gas Costs [Member]
Regulatory Asset [Line Items]
Regulatory assets		$ 1,494	20,069
Rate Case Costs [Member]
Regulatory Asset [Line Items]
Regulatory assets		1,354	3,757
Infrastructure Mechanisms [Member]
Regulatory Asset [Line Items]
Regulatory assets	[2]	24,228	26,948
APT Annual Adjustment Mechanism [Member]
Regulatory Asset [Line Items]
Regulatory assets		0	8,479
Recoverable Loss On Reacquired Debt [Member]
Regulatory Asset [Line Items]
Regulatory assets		16,959	18,877
Other [Member]
Regulatory Asset [Line Items]
Regulatory assets		$ 4,944	$ 4,672

[1] Includes $15.8 million and $18.8 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, including the recording of interest expense, until the next rate proceeding (rate case or annual rate filing), at which time investment and costs would be recoverable through base rates