Summary Of Significant Accounting Policies (Reclassification Out Of Accumulated Other Comprehensive Income (Loss)) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2017	Mar. 31, 2016
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Operating Revenues	$ 522,075	$ 449,132	$ 944,576	$ 824,327
Purchased Gas	147,971	81,623	218,214	123,691
Other Income	1,744	3,236	3,356	5,654
Income Before Income Taxes	142,255	(263,717)	287,594	(597,176)
Income Tax Expense	(52,971)	116,030	(109,403)	260,380
Net Income (Loss) Available for Common Stock	89,284	(147,687)	178,191	(336,796)
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	10,472	61,623	41,930	118,793
Income Tax Expense	(4,414)	(25,763)	(17,641)	(43,769)
Net Income (Loss) Available for Common Stock	6,058	35,860	24,289	75,024
Amount Of Gain Or (Loss) Reclassified From Accumulated Other Comprehensive Income (Loss) [Member] | Gains And Losses On Securities Available For Sale [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other Income	0	388	741	388
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	12,109	57,914	43,429	114,242
Purchased Gas	(1,498)	3,530	(1,958)	4,450
Foreign Currency Contracts [Member] | Amount Of Gain Or (Loss) Reclassified From Accumulated Other Comprehensive Income (Loss) [Member] | Gains And Losses On Derivative Financial Instruments [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Operation and Maintenance	$ (139)	$ (209)	$ (282)	$ (287)