Information by Operating Segment - Schedule of Operating Segment Information (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Sep. 30, 2021
Operating Segment Information [Abstract]
Revenues from external customers	$ 555.4	$ 512.6
Total operating revenues	555.4	512.6
Operating Expenses:
Natural gas	249.2	181.2
Operation and maintenance	116.4	111.6
Depreciation and amortization	56.9	50.8
Taxes, other than income taxes	37.6	36.1
Total Operating Expenses	460.1	379.7
Operating Income	95.3	132.9
Net Economic Earnings (Loss)	62.6	76.9
Total Assets	9,650.5	8,515.6	$ 9,356.4
Eliminations
Operating Segment Information [Abstract]
Total operating revenues	(12.6)	(27.1)
Operating Expenses:
Natural gas	(9.0)	(23.8)
Operation and maintenance	(3.6)	(3.3)
Total Operating Expenses	(12.6)	(27.1)
Total Assets	(1,073.6)	(1,083.8)	(1,076.8)
Other
Operating Segment Information [Abstract]
Revenues from external customers	4.0	4.7
Intersegment revenues	12.6	12.0
Total operating revenues	16.6	16.7
Operating Expenses:
Operation and maintenance	10.0	8.6
Depreciation and amortization	2.0	1.9
Taxes, other than income taxes	0.6	0.4
Total Operating Expenses	12.6	10.9
Operating Income	4.0	5.8
Net Economic Earnings (Loss)	(5.1)	(2.8)
Total Assets	2,417.6	2,462.3	2,351.7
Gas Utility
Operating Segment Information [Abstract]
Revenues from external customers	503.5	497.2
Intersegment revenues		1.0
Gas Utility | Operating Segments
Operating Segment Information [Abstract]
Total operating revenues	503.5	498.2
Operating Expenses:
Natural gas	210.2	204.3
Operation and maintenance	107.3	103.0
Depreciation and amortization	54.6	48.6
Taxes, other than income taxes	37.0	35.5
Total Operating Expenses	409.1	391.4
Operating Income	94.4	106.8
Net Economic Earnings (Loss)	67.2	76.4
Total Assets	7,882.1	6,932.3	7,615.4
Gas Marketing
Operating Segment Information [Abstract]
Revenues from external customers	47.9	10.7
Intersegment revenues		14.1
Gas Marketing | Operating Segments
Operating Segment Information [Abstract]
Total operating revenues	47.9	24.8
Operating Expenses:
Natural gas	48.0	0.7
Operation and maintenance	2.7	3.3
Depreciation and amortization	0.3	0.3
Taxes, other than income taxes		0.2
Total Operating Expenses	51.0	4.5
Operating Income	(3.1)	20.3
Net Economic Earnings (Loss)	0.5	3.3
Total Assets	$ 424.4	$ 204.8	$ 466.1