Information by Operating Segment - Schedule of Operating Segment Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Sep. 30, 2019
Operating Segment Information [Abstract]
Total Operating Revenues	$ 321.1	$ 321.3	$ 1,603.5	$ 1,726.8
Gas Utility
Natural and propane gas	70.7	75.5	534.3	664.6
Operation and maintenance	112.5	111.2	311.6	323.2
Depreciation and amortization	47.8	45.1	141.2	133.2
Taxes, other than income taxes	31.7	29.7	121.3	126.3
Total Gas Utility Operating Expenses	262.7	261.5	1,108.4	1,247.3
Impairments	148.6	0.0	148.6	0.0
Gas Marketing and other	16.3	46.5	140.2	151.6
Total Operating Expenses	427.6	308.0	1,397.2	1,398.9
Operating (Loss) Income	(106.5)	13.3	206.3	327.9
Net Economic Earnings (Loss)	7.3	5.0	223.1	218.8
Total Assets	7,822.6	7,332.0	7,822.6	7,332.0	$ 7,619.2
Eliminations
Operating Segment Information [Abstract]
Total Operating Revenues	(11.9)	(3.2)	(30.6)	(10.7)
Gas Utility
Natural and propane gas	(19.9)	(12.6)	(69.4)	(82.0)
Operation and maintenance	(3.0)	(2.2)	(8.3)	(7.1)
Depreciation and amortization	0.0	0.0	0.0	0.0
Taxes, other than income taxes	0.0	0.0	0.0	0.0
Total Gas Utility Operating Expenses	(22.9)	(14.8)	(77.7)	(89.1)
Impairments	0.0		0.0
Gas Marketing and other	11.0	11.6	47.1	78.4
Total Operating Expenses	(11.9)	(3.2)	(30.6)	(10.7)
Operating (Loss) Income	0.0	0.0	0.0	0.0
Net Economic Earnings (Loss)	0.0	0.0	0.0	0.0
Total Assets	(990.9)	(1,267.8)	(990.9)	(1,267.8)	(1,380.4)
Gas Utility
Operating Segment Information [Abstract]
Total Operating Revenues	305.7	301.4	1,515.4	1,651.9
Gas Utility | Eliminations
Operating Segment Information [Abstract]
Total Operating Revenues	0.3	0.2	0.3	1.7
Gas Utility | Operating Segments
Operating Segment Information [Abstract]
Total Operating Revenues	306.0	301.6	1,515.7	1,653.6
Gas Utility
Natural and propane gas	90.6	88.1	603.7	746.6
Operation and maintenance	115.5	113.4	319.9	330.3
Depreciation and amortization	47.8	45.1	141.2	133.2
Taxes, other than income taxes	31.7	29.7	121.3	126.3
Total Gas Utility Operating Expenses	285.6	276.3	1,186.1	1,336.4
Impairments	0.0		0.0
Gas Marketing and other	0.0	0.0	0.0	0.0
Total Operating Expenses	285.6	276.3	1,186.1	1,336.4
Operating (Loss) Income	20.4	25.3	329.6	317.2
Net Economic Earnings (Loss)	8.4	7.6	221.8	220.7
Total Assets	6,330.1	5,830.1	6,330.1	5,830.1	6,094.6
Gas Marketing
Operating Segment Information [Abstract]
Total Operating Revenues	11.6	18.4	77.2	69.6
Gas Marketing | Eliminations
Operating Segment Information [Abstract]
Total Operating Revenues	0.0	(0.1)	0.0	0.0
Gas Marketing | Operating Segments
Operating Segment Information [Abstract]
Total Operating Revenues	11.6	18.3	77.2	69.6
Gas Utility
Natural and propane gas	0.0	0.0	0.0	0.0
Operation and maintenance	0.0	0.0	0.0	0.0
Depreciation and amortization	0.0	0.0	0.0	0.0
Taxes, other than income taxes	0.0	0.0	0.0	0.0
Total Gas Utility Operating Expenses	0.0	0.0	0.0	0.0
Impairments	0.0		0.0
Gas Marketing and other	(6.6)	25.3	59.0	47.3
Total Operating Expenses	(6.6)	25.3	59.0	47.3
Operating (Loss) Income	18.2	(7.0)	18.2	22.3
Net Economic Earnings (Loss)	0.1	3.3	11.3	17.8
Total Assets	158.6	232.3	158.6	232.3	212.3
Corporate, Non-Segment
Operating Segment Information [Abstract]
Total Operating Revenues	3.8	1.5	10.9	5.3
Corporate, Non-Segment | Eliminations
Operating Segment Information [Abstract]
Total Operating Revenues	11.6	3.1	30.3	9.0
Corporate, Non-Segment | Operating Segments
Operating Segment Information [Abstract]
Total Operating Revenues	15.4	4.6	41.2	14.3
Gas Utility
Natural and propane gas	0.0	0.0	0.0	0.0
Operation and maintenance	0.0	0.0	0.0	0.0
Depreciation and amortization	0.0	0.0	0.0	0.0
Taxes, other than income taxes	0.0	0.0	0.0	0.0
Total Gas Utility Operating Expenses	0.0	0.0	0.0	0.0
Impairments	148.6		148.6
Gas Marketing and other	11.9	9.6	34.1	25.9
Total Operating Expenses	160.5	9.6	182.7	25.9
Operating (Loss) Income	(145.1)	(5.0)	(141.5)	(11.6)
Net Economic Earnings (Loss)	(1.2)	(5.9)	(10.0)	(19.7)
Total Assets	$ 2,324.8	$ 2,537.4	$ 2,324.8	$ 2,537.4	$ 2,692.7