Accumulated Other Comprehensive Income Affected Line Item in the Statement of Income (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Dec. 31, 2013	Dec. 31, 2012
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Operation and maintenance	$ 115,757	$ 106,527
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest	136,473	125,098
Income Tax Expense (Benefit)	(49,457)	(47,750)
Income (loss) from continuing operations	87,016	77,348
Interest charges	(32,115)	(30,522)
Purchased gas cost	866,191	671,793
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income (loss) from continuing operations	(2,264)	(3,467)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Net Gain (Loss) from Interest Rate Hedges [Member] [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest charges	(1,058)	(502)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Net Gain (Loss) from Commodity Contracts Cash Flow Hedges [Member] [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Purchased gas cost	(2,609)	(5,160)
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	(3,667)	(5,662)
Income Tax Expense (Benefit)	$ 1,403	$ 2,195