Information by Operating Segment - Schedule of Operating Segment Information (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Sep. 30, 2019
Operating Segment Information [Abstract]
Total Operating Revenues	$ 566.9	$ 602.0
Gas Utility
Natural and propane gas	214.6	251.7
Operation and maintenance	106.0	102.5
Depreciation and amortization	46.4	43.7
Taxes, other than income taxes	37.9	39.2
Total Gas Utility Operating Expenses	404.9	437.1
Gas Marketing and other	59.7	59.8
Total Operating Expenses	464.6	496.9
Operating Income	102.3	105.1
Net Economic Earnings (Loss)	71.8	65.9
Total Assets	7,961.0	7,232.2	$ 7,619.2
Eliminations
Operating Segment Information [Abstract]
Total Operating Revenues	(7.2)	(4.4)
Gas Utility
Natural and propane gas	(26.9)	(40.1)
Operation and maintenance	(2.6)	(2.4)
Depreciation and amortization	0.0	0.0
Taxes, other than income taxes	0.0	0.0
Total Gas Utility Operating Expenses	(29.5)	(42.5)
Gas Marketing and other	22.3	38.1
Total Operating Expenses	(7.2)	(4.4)
Operating Income	0.0	0.0
Net Economic Earnings (Loss)	0.0	0.0
Total Assets	(1,038.2)	(1,563.8)	(1,380.4)
Gas Utility
Operating Segment Information [Abstract]
Total Operating Revenues	530.6	573.8
Gas Utility | Eliminations
Operating Segment Information [Abstract]
Total Operating Revenues	0.1	1.4
Gas Utility | Operating Segments
Operating Segment Information [Abstract]
Total Operating Revenues	530.7	575.2
Gas Utility
Natural and propane gas	241.5	291.8
Operation and maintenance	108.6	104.9
Depreciation and amortization	46.4	43.7
Taxes, other than income taxes	37.9	39.2
Total Gas Utility Operating Expenses	434.4	479.6
Gas Marketing and other	0.0	0.0
Total Operating Expenses	434.4	479.6
Operating Income	96.3	95.6
Net Economic Earnings (Loss)	69.1	66.4
Total Assets	6,306.0	5,843.7	6,094.6
Gas Marketing
Operating Segment Information [Abstract]
Total Operating Revenues	32.3	25.8
Gas Marketing | Eliminations
Operating Segment Information [Abstract]
Total Operating Revenues	0.0	0.0
Gas Marketing | Operating Segments
Operating Segment Information [Abstract]
Total Operating Revenues	32.3	25.8
Gas Utility
Natural and propane gas	0.0	0.0
Operation and maintenance	0.0	0.0
Depreciation and amortization	0.0	0.0
Taxes, other than income taxes	0.0	0.0
Total Gas Utility Operating Expenses	0.0	0.0
Gas Marketing and other	27.9	13.3
Total Operating Expenses	27.9	13.3
Operating Income	4.4	12.5
Net Economic Earnings (Loss)	6.1	8.3
Total Assets	242.7	416.7	212.3
Corporate, Non-Segment
Operating Segment Information [Abstract]
Total Operating Revenues	4.0	2.4
Corporate, Non-Segment | Eliminations
Operating Segment Information [Abstract]
Total Operating Revenues	7.1	3.0
Corporate, Non-Segment | Operating Segments
Operating Segment Information [Abstract]
Total Operating Revenues	11.1	5.4
Gas Utility
Natural and propane gas	0.0	0.0
Operation and maintenance	0.0	0.0
Depreciation and amortization	0.0	0.0
Taxes, other than income taxes	0.0	0.0
Total Gas Utility Operating Expenses	0.0	0.0
Gas Marketing and other	9.5	8.4
Total Operating Expenses	9.5	8.4
Operating Income	1.6	(3.0)
Net Economic Earnings (Loss)	(3.4)	(8.8)
Total Assets	$ 2,450.5	$ 2,535.6	$ 2,692.7