Information by Operating Segment - Schedule of Operating Segment Information (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2022	Mar. 31, 2021	Mar. 31, 2022	Mar. 31, 2021	Sep. 30, 2021
Operating Segment Information [Abstract]
Revenues from external customers	$ 880.9	$ 1,104.9	$ 1,436.3	$ 1,617.5
Total operating revenues	880.9	1,104.9	1,436.3	1,617.5
Operating Expenses:
Natural gas	392.0	619.1	641.2	800.3
Operation and maintenance	113.2	119.0	229.6	230.6
Depreciation and amortization	58.9	51.5	115.8	102.3
Taxes, other than income taxes	71.6	57.9	109.2	94.0
Total Operating Expenses	635.7	847.5	1,095.8	1,227.2
Operating Income	245.2	257.4	340.5	390.3
Net Economic Earnings (Loss)	181.0	195.6	243.6	272.5
Total Assets	9,401.3	8,936.8	9,401.3	8,936.8	$ 9,356.4
Other
Operating Segment Information [Abstract]
Revenues from external customers	4.1	4.5	8.1	9.2
Intersegment revenues	14.0	11.1	26.6	23.1
Total operating revenues	18.1	15.6	34.7	32.3
Operating Expenses:
Natural gas		0.1		0.1
Operation and maintenance	10.0	11.1	20.0	19.7
Depreciation and amortization	2.0	1.7	4.0	3.6
Taxes, other than income taxes	0.9	1.0	1.5	1.4
Total Operating Expenses	12.9	13.9	25.5	24.8
Operating Income	5.2	1.7	9.2	7.5
Net Economic Earnings (Loss)	(2.6)	(3.9)	(7.7)	(6.7)
Total Assets	2,543.8	2,513.7	2,543.8	2,513.7	2,351.7
Eliminations
Operating Segment Information [Abstract]
Total operating revenues	(14.0)	2.8	(26.6)	(24.3)
Operating Expenses:
Natural gas	(9.8)	6.0	(18.8)	(17.8)
Operation and maintenance	(4.2)	(3.2)	(7.8)	(6.5)
Total Operating Expenses	(14.0)	2.8	(26.6)	(24.3)
Total Assets	(1,257.7)	(1,182.9)	(1,257.7)	(1,182.9)	(1,076.8)
Gas Utility
Operating Segment Information [Abstract]
Revenues from external customers	817.4	1,053.0	1,320.9	1,550.2
Intersegment revenues		0.1		1.1
Gas Utility | Operating Segments
Operating Segment Information [Abstract]
Total operating revenues	817.4	1,053.1	1,320.9	1,551.3
Operating Expenses:
Natural gas	356.0	619.2	566.2	823.5
Operation and maintenance	104.2	104.0	211.5	207.0
Depreciation and amortization	56.5	49.5	111.1	98.1
Taxes, other than income taxes	70.3	56.4	107.3	91.9
Total Operating Expenses	587.0	829.1	996.1	1,220.5
Operating Income	230.4	224.0	324.8	330.8
Net Economic Earnings (Loss)	169.2	159.7	236.4	236.1
Total Assets	7,684.1	7,265.0	7,684.1	7,265.0	7,615.4
Gas Marketing
Operating Segment Information [Abstract]
Revenues from external customers	59.4	47.4	107.3	58.1
Intersegment revenues		(14.0)		0.1
Gas Marketing | Operating Segments
Operating Segment Information [Abstract]
Total operating revenues	59.4	33.4	107.3	58.2
Operating Expenses:
Natural gas	45.8	(6.2)	93.8	(5.5)
Operation and maintenance	3.2	7.1	5.9	10.4
Depreciation and amortization	0.4	0.3	0.7	0.6
Taxes, other than income taxes	0.4	0.5	0.4	0.7
Total Operating Expenses	49.8	1.7	100.8	6.2
Operating Income	9.6	31.7	6.5	52.0
Net Economic Earnings (Loss)	14.4	39.8	14.9	43.1
Total Assets	$ 431.1	$ 341.0	$ 431.1	$ 341.0	$ 466.1