Shareholders' Equity - Reclassification out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Operation and maintenance expense	$ 145,075	$ 128,690	$ 435,715	$ 385,867
Total before tax	97,641	111,877	547,089	548,832
Income tax (benefit) expense	(26,448)	(41,069)	17,228	(201,974)
Interest charges	(23,349)	(28,498)	(82,162)	(86,472)
Purchased gas cost	130,886	114,176	1,124,763	852,975
Amount Reclassified from Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net of tax	(454)	(113)	(410)	4,511
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	7		1,146	(107)
Total before tax	7		1,146	(107)
Income tax (benefit) expense	(2)		(262)	39
Net of tax	5		884	(68)
Amount Reclassified from Accumulated Other Comprehensive Income | Cash Flow Hedges
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total before tax	(594)	(177)	(1,781)	7,517
Income tax (benefit) expense	135	64	487	(2,938)
Net of tax	(459)		(1,294)	4,579
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	$ (594)	$ (177)	$ (1,781)	(450)
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