Summary Of Significant Accounting Policies (Reclassification Out Of Accumulated Other Comprehensive Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Loss [Line Items]
Operating Revenues	$ 540,905	$ 522,075	$ 960,561	$ 944,576
Purchased Gas	176,608	147,971	270,642	218,214
Other Income	770	1,744	2,492	3,356
Income Before Income Taxes	130,116	142,255	247,494	287,594
Income Tax Expense	(38,269)	(52,971)	43,007	(109,403)
Net Income Available for Common Stock	91,847	89,284	290,501	178,191
Amount Of Gain Or (Loss) Reclassified From Accumulated Other Comprehensive Loss [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Loss [Line Items]
Income Before Income Taxes	(3,199)	10,472	9,779	41,930
Income Tax Expense	551	(4,414)	(4,804)	(17,641)
Net Income Available for Common Stock	(2,648)	6,058	4,975	24,289
Amount Of Gain Or (Loss) Reclassified From Accumulated Other Comprehensive Loss [Member] | Gains And Losses On Securities Available For Sale [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Loss [Line Items]
Other Income	0	0	430	741
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 Loss [Line Items]
Operating Revenues	(3,467)	12,109	9,375	43,429
Purchased Gas	750	(1,498)	947	(1,958)
Foreign Currency Contracts [Member] | 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 Loss [Line Items]
Operation and Maintenance	$ (482)	$ (139)	$ (973)	$ (282)