Summary Of Significant Accounting Policies (Reclassification Out Of Accumulated Other Comprehensive Income (Loss)) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2015	Mar. 31, 2014
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Operating Revenues	$ 596,127	$ 756,242	$ 1,120,036	$ 1,306,314
Purchased Gas	(190,600)	(322,772)	(317,690)	(490,378)
Income Before Income Taxes	21,805	161,306	161,705	298,983
Income Tax Expense	(5,136)	(66,095)	(60,296)	(121,520)
Net Income Available for Common Stock	16,669	95,211	101,409	177,463
Amount Of Gain Or (Loss) Reclassified From Accumulated Other Comprehensive Income (Loss) [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income Before Income Taxes	54,130	(26,640)	78,395	(16,457)
Income Tax Expense	(22,917)	11,170	(33,006)	6,872
Net Income Available for Common Stock	31,213	(15,470)	45,389	(9,585)
Amount Of Gain Or (Loss) Reclassified From Accumulated Other Comprehensive Income (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	53,471	(22,611)	73,508	(12,825)
Purchased Gas	$ 659	$ (4,029)	$ 4,887	$ (3,632)