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Shareholders' Equity - Reclassification out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Operation and maintenance expense									$ 599,595	$ 546,798	$ 538,592
Income from continuing operations before income taxes									611,144	604,094	542,184
Income tax (benefit) expense									(8,080)	(221,383)	(196,642)
Interest charges									(106,646)	(120,182)	(114,812)
Amount Reclassified from Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net of tax									69	4,141
Amount Reclassified from Accumulated Other Comprehensive Income \| Available- for-Sale Securities
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Operation and maintenance expense									2,360	(97)
Income from continuing operations before income taxes									2,360	(97)
Income tax (benefit) expense									(539)	35
Net of tax									1,821	(62)
Amount Reclassified from Accumulated Other Comprehensive Income \| Cash Flow Hedges
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income from continuing operations before income taxes									(2,375)	6,924
Income tax (benefit) expense									623	(2,721)
Net of tax									(1,752)	4,203
Amount Reclassified from Accumulated Other Comprehensive Income \| Cash Flow Hedges \| Interest rate agreements
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest charges									(2,375)	(1,043)
Natural Gas, US Regulated
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Purchased gas cost	$ (43,085)	$ (130,886)	$ (626,960)	$ (366,917)	$ (72,561)	$ (114,176)	$ (427,494)	$ (311,305)	$ (1,167,848)	(925,536)	$ (746,192)
Natural Gas, US Regulated \| Amount Reclassified from Accumulated Other Comprehensive Income \| Cash Flow Hedges \| Commodity contracts
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Purchased gas cost										$ 7,967