Information by Operating Segment - Schedule of Operating Segment Information (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Operating Segment Information [Abstract]
Revenues from external customers	$ 2,235.5	$ 1,855.4	$ 1,952.4
Total operating revenues	2,235.5	1,855.4	1,952.4
Operating Expenses:
Natural gas	946.3	696.1	840.3
Other operation and maintenance	465.8	458.6	474.1
Depreciation and amortization	213.1	197.3	181.7
Taxes, other than income taxes	160.1	148.4	154.0
Impairments	0.0	148.6	0.0
Total Operating Expenses	1,785.3	1,649.0	1,650.1
Operating Income	450.2	206.4	302.3
Net Economic Earnings (Loss)	266.3	207.8	195.1
Capital Expenditures	624.8	638.4	823.3
Total Assets	9,356.4	8,241.2	7,619.2
Eliminations
Operating Segment Information [Abstract]
Total operating revenues	(48.0)	(42.3)	(13.6)
Operating Expenses:
Natural gas	(34.3)	(29.6)	(2.7)
Other operation and maintenance	(13.7)	(12.7)	(10.9)
Total Operating Expenses	(48.0)	(42.3)	(13.6)
Total Assets	(1,076.8)	(1,101.2)	(1,380.4)
Other
Operating Segment Information [Abstract]
Revenues from external customers	20.8	15.7	9.5
Intersegment revenues	46.9	42.1	12.0
Total operating revenues	67.7	57.8	21.5
Operating Expenses:
Natural gas	0.1	0.4	0.5
Other operation and maintenance	40.2	38.2	31.6
Depreciation and amortization	7.5	7.0	2.2
Taxes, other than income taxes	2.2	0.8	1.5
Impairments		148.6
Total Operating Expenses	50.0	195.0	35.8
Operating Income	17.7	(137.2)	(14.3)
Net Economic Earnings (Loss)	(11.3)	(14.7)	(24.1)
Capital Expenditures	33.7	87.0	254.8
Total Assets	2,351.7	2,443.5	2,692.7
Gas Utility
Operating Segment Information [Abstract]
Revenues from external customers	2,118.2	1,751.8	1,859.2
Intersegment revenues	1.1	0.2	1.6
Gas Utility | Operating Segments
Operating Segment Information [Abstract]
Total operating revenues	2,119.3	1,752.0	1,860.8
Operating Expenses:
Natural gas	961.7	660.2	794.6
Other operation and maintenance	422.2	421.3	441.7
Depreciation and amortization	204.4	189.7	179.4
Taxes, other than income taxes	157.0	146.5	151.7
Total Operating Expenses	1,745.3	1,417.7	1,567.4
Operating Income	374.0	334.3	293.4
Net Economic Earnings (Loss)	230.6	213.4	199.8
Capital Expenditures	590.4	547.8	565.4
Total Assets	7,615.4	6,716.2	6,094.6
Gas Marketing
Operating Segment Information [Abstract]
Revenues from external customers	96.5	87.9	83.7
Gas Marketing | Operating Segments
Operating Segment Information [Abstract]
Total operating revenues	96.5	87.9	83.7
Operating Expenses:
Natural gas	18.8	65.1	47.9
Other operation and maintenance	17.1	11.8	11.7
Depreciation and amortization	1.2	0.6	0.1
Taxes, other than income taxes	0.9	1.1	0.8
Total Operating Expenses	38.0	78.6	60.5
Operating Income	58.5	9.3	23.2
Net Economic Earnings (Loss)	47.0	9.1	19.4
Capital Expenditures	0.7	3.6	3.1
Total Assets	$ 466.1	$ 182.7	$ 212.3