Information by Operating Segment - Schedule of Operating Segment Information (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020	Sep. 30, 2020
Operating Segment Information [Abstract]
Revenues from external customers	$ 1,104.9	$ 715.5	$ 1,617.5	$ 1,282.4
Total operating revenues	1,104.9	715.5	1,617.5	1,282.4
Operating Expenses:
Natural gas	619.1	297.1	800.3	559.0
Operation and maintenance	119.0	105.7	230.6	222.3
Depreciation and amortization	51.5	49.2	102.3	96.7
Taxes, other than income taxes	57.9	53.0	94.0	91.6
Total Operating Expenses	847.5	505.0	1,227.2	969.6
Net Economic Earnings (Loss)	195.6	144.0	272.5	215.8
Operating Income	257.4	210.5	390.3	312.8
Total Assets	8,936.8	8,018.0	8,936.8	8,018.0	$ 8,241.2
Eliminations
Operating Segment Information [Abstract]
Total operating revenues	2.8	(11.5)	(24.3)	(18.7)
Operating Expenses:
Natural gas	6.0	(8.2)	(17.8)	(12.4)
Operation and maintenance	(3.2)	(3.3)	(6.5)	(6.3)
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 Operating Expenses	2.8	(11.5)	(24.3)	(18.7)
Net Economic Earnings (Loss)	0.0	0.0	0.0	0.0
Operating Income	0.0	0.0	0.0	0.0
Total Assets	(1,182.9)	(667.6)	(1,182.9)	(667.6)	(1,101.2)
Other
Operating Segment Information [Abstract]
Revenues from external customers	4.5	3.1	9.2	7.1
Intersegment revenues	11.1	11.6	23.1	18.7
Total operating revenues	15.6	14.7	32.3	25.8
Operating Expenses:
Natural gas	0.1	0.1	0.1	0.2
Operation and maintenance	11.1	9.6	19.7	17.5
Depreciation and amortization	1.7	2.1	3.6	3.2
Taxes, other than income taxes	1.0	0.9	1.4	1.3
Total Operating Expenses	13.9	12.7	24.8	22.2
Net Economic Earnings (Loss)	(3.9)	(5.4)	(6.7)	(8.8)
Operating Income	1.7	2.0	7.5	3.6
Total Assets	2,513.7	2,214.7	2,513.7	2,214.7	2,443.5
Gas Utility
Operating Segment Information [Abstract]
Revenues from external customers	1,053.0	679.1	1,550.2	1,209.7
Intersegment revenues	0.1	(0.1)	1.1	0.0
Gas Utility | Operating Segments
Operating Segment Information [Abstract]
Total operating revenues	1,053.1	679.0	1,551.3	1,209.7
Operating Expenses:
Natural gas	619.2	271.6	823.5	513.1
Operation and maintenance	104.0	95.8	207.0	204.4
Depreciation and amortization	49.5	47.0	98.1	93.4
Taxes, other than income taxes	56.4	51.7	91.9	89.6
Total Operating Expenses	829.1	466.1	1,220.5	900.5
Net Economic Earnings (Loss)	159.7	144.3	236.1	213.4
Operating Income	224.0	212.9	330.8	309.2
Total Assets	7,265.0	6,311.6	7,265.0	6,311.6	6,716.2
Gas Marketing
Operating Segment Information [Abstract]
Revenues from external customers	47.4	33.3	58.1	65.6
Intersegment revenues	(14.0)	0.0	0.1	0.0
Gas Marketing | Operating Segments
Operating Segment Information [Abstract]
Total operating revenues	33.4	33.3	58.2	65.6
Operating Expenses:
Natural gas	(6.2)	33.6	(5.5)	58.1
Operation and maintenance	7.1	3.6	10.4	6.7
Depreciation and amortization	0.3	0.1	0.6	0.1
Taxes, other than income taxes	0.5	0.4	0.7	0.7
Total Operating Expenses	1.7	37.7	6.2	65.6
Net Economic Earnings (Loss)	39.8	5.1	43.1	11.2
Operating Income	31.7	(4.4)	52.0	0.0
Total Assets	$ 341.0	$ 159.3	$ 341.0	$ 159.3	$ 182.7