Summary Of Significant Accounting Policies (Reclassification Out Of Accumulated Other Comprehensive Loss) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2014	Mar. 31, 2013
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Operating Revenues	$ 756,242	$ 597,826	$ 1,306,314	$ 1,050,680
Purchased Gas	(322,772)	(209,817)	(490,378)	(331,735)
Income Before Income Taxes	161,306	140,906	298,983	252,399
Income Tax Expense	(66,095)	(55,186)	(121,520)	(98,735)
Net Income Available for Common Stock	95,211	85,720	177,463	153,664
Amount Of Gain Or (Loss) Reclassified From Accumulated Other Comprehensive Loss [Member] | Gains And Losses On Derivative Financial Instruments [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income Before Income Taxes	(26,640)		(16,457)
Income Tax Expense	11,170		6,872
Net Income Available for Common Stock	(15,470)		(9,585)
Amount Of Gain Or (Loss) Reclassified From Accumulated Other Comprehensive Loss [Member] | Commodity Contracts [Member] | Gains And Losses On Derivative Financial Instruments [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Operating Revenues	(22,611)		(12,825)
Purchased Gas	$ (4,029)		$ (3,632)