Information by Operating Segment - Schedule of Operating Segment Information (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Operating Segment Information [Abstract]
Operating Revenues	$ 251.9	$ 321.1	$ 715.5	$ 566.9	$ 225.6	$ 321.3	$ 803.5	$ 602.0	$ 1,855.4	$ 1,952.4	$ 1,965.0
Operating Expenses:
Natural gas									696.1	840.3	871.7
Other operation and maintenance									458.6	474.1	477.3
Depreciation and amortization									197.3	181.7	168.4
Taxes, other than income taxes									148.4	154.0	153.5
Impairments									148.6	0.0	0.0
Total Operating Expenses									1,649.0	1,650.1	1,670.9
Operating Income	0.1	$ (106.5)	$ 210.5	$ 102.3	(25.6)	$ 13.3	$ 209.5	$ 105.1	206.4	302.3	294.1
Net Economic Earnings (Loss)									207.8	195.1	183.7
Capital Expenditures									638.4	823.3	499.4
Total Assets	8,241.2				7,619.2				8,241.2	7,619.2	6,843.6
Eliminations
Operating Segment Information [Abstract]
Operating Revenues									(42.3)	(13.6)	(11.5)
Operating Expenses:
Natural gas									(29.6)	(2.7)	(1.4)
Other operation and maintenance									(12.7)	(10.9)	(10.1)
Total Operating Expenses									(42.3)	(13.6)	(11.5)
Total Assets	(1,101.2)				(1,380.4)				(1,101.2)	(1,380.4)	(1,566.4)
Gas Utility
Operating Segment Information [Abstract]
Operating Revenues									1,751.8	1,859.2	1,888.0
Gas Utility | Operating Segments
Operating Segment Information [Abstract]
Operating Revenues									1,752.0	1,860.8	1,888.4
Operating Expenses:
Natural gas									660.2	794.6	842.6
Other operation and maintenance									421.3	441.7	449.7
Depreciation and amortization									189.7	179.4	167.0
Taxes, other than income taxes									146.5	151.7	152.5
Total Operating Expenses									1,417.7	1,567.4	1,611.8
Operating Income									334.3	293.4	276.6
Net Economic Earnings (Loss)									213.4	199.8	183.1
Capital Expenditures									547.8	565.4	457.7
Total Assets	6,716.2				6,094.6				6,716.2	6,094.6	5,606.7
Gas Utility | Eliminations
Operating Segment Information [Abstract]
Operating Revenues									0.2	1.6	0.4
Gas Marketing
Operating Segment Information [Abstract]
Operating Revenues									87.9	83.7	71.6
Gas Marketing | Operating Segments
Operating Segment Information [Abstract]
Operating Revenues									87.9	83.7	71.6
Operating Expenses:
Natural gas									65.1	47.9	30.2
Other operation and maintenance									11.8	11.7	7.4
Depreciation and amortization									0.6	0.1
Taxes, other than income taxes									1.1	0.8	0.2
Total Operating Expenses									78.6	60.5	37.8
Operating Income									9.3	23.2	33.8
Net Economic Earnings (Loss)									9.1	19.4	22.9
Capital Expenditures									3.6	3.1
Total Assets	182.7				212.3				182.7	212.3	295.3
Corporate, Non-Segment
Operating Segment Information [Abstract]
Operating Revenues									15.7	9.5	5.4
Corporate, Non-Segment | Operating Segments
Operating Segment Information [Abstract]
Operating Revenues									57.8	21.5	16.5
Operating Expenses:
Natural gas									0.4	0.5	0.3
Other operation and maintenance									38.2	31.6	30.3
Depreciation and amortization									7.0	2.2	1.4
Taxes, other than income taxes									0.8	1.5	0.8
Impairments									148.6
Total Operating Expenses									195.0	35.8	32.8
Operating Income									(137.2)	(14.3)	(16.3)
Net Economic Earnings (Loss)									(14.7)	(24.1)	(22.3)
Capital Expenditures									87.0	254.8	41.7
Total Assets	$ 2,443.5				$ 2,692.7				2,443.5	2,692.7	2,508.0
Corporate, Non-Segment | Eliminations
Operating Segment Information [Abstract]
Operating Revenues									$ 42.1	$ 12.0	$ 11.1