Information by Operating Segment - Schedule of Operating Segment Information (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Sep. 30, 2020
Operating Segment Information [Abstract]
Operating Revenues	$ 512.6	$ 566.9
Operating Expenses:
Natural gas	181.2	261.9
Operation and maintenance	111.6	116.6
Depreciation and amortization	50.8	47.5
Taxes, other than income taxes	36.1	38.6
Total Operating Expenses	379.7	464.6
Net Economic Earnings (Loss)	76.9	71.8
Operating Income	132.9	102.3
Total Assets	8,515.6	7,961.0	$ 8,241.2
Eliminations
Operating Segment Information [Abstract]
Operating Revenues	(27.1)	(7.2)
Operating Expenses:
Natural gas	(23.8)	(4.2)
Operation and maintenance	(3.3)	(3.0)
Depreciation and amortization	0.0	0.0
Taxes, other than income taxes	0.0	0.0
Total Operating Expenses	(27.1)	(7.2)
Net Economic Earnings (Loss)	0.0	0.0
Operating Income	0.0	0.0
Total Assets	(1,083.8)	(1,038.2)	(1,101.2)
Gas Utility
Operating Segment Information [Abstract]
Operating Revenues	497.2	530.6
Gas Utility | Eliminations
Operating Segment Information [Abstract]
Operating Revenues	1.0	0.1
Gas Utility | Operating Segments
Operating Segment Information [Abstract]
Operating Revenues	498.2	530.7
Operating Expenses:
Natural gas	204.3	241.5
Operation and maintenance	103.0	108.6
Depreciation and amortization	48.6	46.4
Taxes, other than income taxes	35.5	37.9
Total Operating Expenses	391.4	434.4
Net Economic Earnings (Loss)	76.4	69.1
Operating Income	106.8	96.3
Total Assets	6,932.3	6,306.0	6,716.2
Gas Marketing
Operating Segment Information [Abstract]
Operating Revenues	10.7	32.3
Gas Marketing | Eliminations
Operating Segment Information [Abstract]
Operating Revenues	14.1	0.0
Gas Marketing | Operating Segments
Operating Segment Information [Abstract]
Operating Revenues	24.8	32.3
Operating Expenses:
Natural gas	0.7	24.5
Operation and maintenance	3.3	3.1
Depreciation and amortization	0.3	0.0
Taxes, other than income taxes	0.2	0.3
Total Operating Expenses	4.5	27.9
Net Economic Earnings (Loss)	3.3	6.1
Operating Income	20.3	4.4
Total Assets	204.8	242.7	182.7
Corporate, Non-Segment
Operating Segment Information [Abstract]
Operating Revenues	4.7	4.0
Corporate, Non-Segment | Eliminations
Operating Segment Information [Abstract]
Operating Revenues	12.0	7.1
Corporate, Non-Segment | Operating Segments
Operating Segment Information [Abstract]
Operating Revenues	16.7	11.1
Operating Expenses:
Natural gas	0.0	0.1
Operation and maintenance	8.6	7.9
Depreciation and amortization	1.9	1.1
Taxes, other than income taxes	0.4	0.4
Total Operating Expenses	10.9	9.5
Net Economic Earnings (Loss)	(2.8)	(3.4)
Operating Income	5.8	1.6
Total Assets	$ 2,462.3	$ 2,450.5	$ 2,443.5