Accumulated Other Comprehensive Income Affected Line Item in the Statement of Income (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Operation and maintenance	$ (132,447)	$ (125,559)	$ (384,489)	$ (365,991)
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest	90,286	74,391	467,742	427,827
Income Tax Expense	(34,005)	(28,670)	(176,182)	(161,723)
Interest charges	(27,955)	(31,840)	(85,166)	(95,556)
Purchased gas cost	(304,728)	(583,637)	(2,159,538)	(2,907,115)
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income (loss) from continuing operations	(9,823)	2,362	(17,955)	4,036
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Net Unrealized Investment Gain (Loss) [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Operation and maintenance	508	733	514	1,091
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest	508	733	514	1,091
Income Tax Expense	(186)	(267)	(188)	(398)
Income (loss) from continuing operations	322	466	326	693
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Net Gain (Loss) from Interest Rate Hedges [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest charges	(137)	(1,057)	(717)	(3,172)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Net Gain (Loss) from Commodity Contracts Cash Flow Hedges [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Purchased gas cost	(16,488)	4,209	(29,222)	8,783
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest	(16,625)	3,152	(29,939)	5,611
Income Tax Expense	6,480	(1,256)	11,658	(2,268)
Income (loss) from continuing operations	$ (10,145)	$ 1,896	$ (18,281)	$ 3,343