Information by Operating Segment - Schedule of Operating Segment Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Sep. 30, 2020
Operating Segment Information [Abstract]
Revenues from external customers	$ 327.8	$ 321.1	$ 1,945.3	$ 1,603.5
Total operating revenues	327.8	321.1	1,945.3	1,603.5
Operating Expenses:
Natural gas	96.9	72.9	897.2	631.9
Operation and maintenance	112.0	124.9	342.6	347.2
Depreciation and amortization	53.1	50.1	155.4	146.8
Taxes, other than income taxes	32.6	31.1	126.6	122.7
Impairments	0.0	148.6	0.0	148.6
Total Operating Expenses	294.6	427.6	1,521.8	1,397.2
Net Economic Earnings (Loss)	6.9	7.3	279.4	223.1
Operating Income (Loss)	33.2	(106.5)	423.5	206.3
Total Assets	8,893.3	7,822.6	8,893.3	7,822.6	$ 8,241.2
Eliminations
Operating Segment Information [Abstract]
Total operating revenues	(11.8)	(11.9)	(36.1)	(30.6)
Operating Expenses:
Natural gas	(8.2)	(8.6)	(26.0)	(21.0)
Operation and maintenance	(3.6)	(3.3)	(10.1)	(9.6)
Depreciation and amortization	0.0	0.0	0.0	0.0
Taxes, other than income taxes	0.0	0.0	0.0	0.0
Impairments		0.0		0.0
Total Operating Expenses	(11.8)	(11.9)	(36.1)	(30.6)
Net Economic Earnings (Loss)	0.0	0.0	0.0	0.0
Operating Income (Loss)	0.0	0.0	0.0	0.0
Total Assets	(1,014.6)	(990.9)	(1,014.6)	(990.9)	(1,101.2)
Other
Operating Segment Information [Abstract]
Revenues from external customers	5.9	3.8	15.1	10.9
Intersegment revenues	11.9	11.6	35.0	30.3
Total operating revenues	17.8	15.4	50.1	41.2
Operating Expenses:
Natural gas	0.0	0.1	0.1	0.3
Operation and maintenance	9.2	10.5	28.9	28.0
Depreciation and amortization	1.9	2.1	5.5	5.3
Taxes, other than income taxes	0.3	(0.8)	1.7	0.5
Impairments		148.6		148.6
Total Operating Expenses	11.4	160.5	36.2	182.7
Net Economic Earnings (Loss)	(0.2)	(1.2)	(6.9)	(10.0)
Operating Income (Loss)	6.4	(145.1)	13.9	(141.5)
Total Assets	2,256.3	2,324.8	2,256.3	2,324.8	2,443.5
Gas Utility
Operating Segment Information [Abstract]
Revenues from external customers	306.7	305.7	1,856.9	1,515.4
Intersegment revenues	0.0	0.3	1.1	0.3
Gas Utility | Operating Segments
Operating Segment Information [Abstract]
Total operating revenues	306.7	306.0	1,858.0	1,515.7
Operating Expenses:
Natural gas	84.9	90.6	908.4	603.7
Operation and maintenance	103.2	115.5	310.2	319.9
Depreciation and amortization	50.9	47.8	149.0	141.2
Taxes, other than income taxes	32.1	31.7	124.0	121.3
Impairments		0.0		0.0
Total Operating Expenses	271.1	285.6	1,491.6	1,186.1
Net Economic Earnings (Loss)	12.3	8.4	248.4	221.8
Operating Income (Loss)	35.6	20.4	366.4	329.6
Total Assets	7,302.4	6,330.1	7,302.4	6,330.1	6,716.2
Gas Marketing
Operating Segment Information [Abstract]
Revenues from external customers	15.2	11.6	73.3	77.2
Intersegment revenues	(0.1)	0.0	0.0	0.0
Gas Marketing | Operating Segments
Operating Segment Information [Abstract]
Total operating revenues	15.1	11.6	73.3	77.2
Operating Expenses:
Natural gas	20.2	(9.2)	14.7	48.9
Operation and maintenance	3.2	2.2	13.6	8.9
Depreciation and amortization	0.3	0.2	0.9	0.3
Taxes, other than income taxes	0.2	0.2	0.9	0.9
Impairments		0.0		0.0
Total Operating Expenses	23.9	(6.6)	30.1	59.0
Net Economic Earnings (Loss)	(5.2)	0.1	37.9	11.3
Operating Income (Loss)	(8.8)	18.2	43.2	18.2
Total Assets	$ 349.2	$ 158.6	$ 349.2	$ 158.6	$ 182.7