Information by Operating Segment - Schedule of Operating Segment Information (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Operating Segment Information [Abstract]
Total Operating Revenues	$ 225.6	$ 321.3	$ 803.5	$ 602.0	$ 239.2	$ 350.6	$ 813.4	$ 561.8	$ 1,952.4	$ 1,965.0	$ 1,740.7
Gas Utility
Natural and propane gas									698.2	770.1	570.5
Other operation and maintenance expenses									432.3	441.2	385.8
Depreciation and amortization									179.4	167.0	153.5
Taxes, other than income taxes									151.7	152.5	137.8
Total Gas Utility Operating Expenses									1,461.6	1,530.8	1,247.6
Gas Marketing and other									188.5	140.1	152.2
Total Operating Expenses									1,650.1	1,670.9	1,399.8
Operating Income	(25.6)	$ 13.3	$ 209.5	$ 105.1	(18.9)	$ 58.9	$ 150.1	$ 104.0	302.3	294.1	340.9
Net Economic Earnings (Loss)									195.1	183.7	167.6
Capital Expenditures									823.3	499.4	438.1
Total Assets	7,619.2				6,843.6				7,619.2	6,843.6	6,546.7
Eliminations
Operating Segment Information [Abstract]
Total Operating Revenues									(13.6)	(11.5)	(14.2)
Gas Utility
Natural and propane gas									(96.4)	(72.5)	(75.4)
Other operation and maintenance expenses									(9.4)	(8.5)	(4.1)
Depreciation and amortization									0.0	0.0	0.0
Taxes, other than income taxes									0.0	0.0	0.0
Total Gas Utility Operating Expenses									(105.8)	(81.0)	(79.5)
Gas Marketing and other									92.2	69.5	65.3
Total Operating Expenses									(13.6)	(11.5)	(14.2)
Operating Income									0.0	0.0	0.0
Net Economic Earnings (Loss)									0.0	0.0	0.0
Capital Expenditures									0.0	0.0	0.0
Total Assets	(1,380.4)				(1,566.4)				(1,380.4)	(1,566.4)	(1,490.2)
Gas Utility
Operating Segment Information [Abstract]
Total Operating Revenues									1,859.2	1,888.0	1,660.0
Gas Utility | Eliminations
Operating Segment Information [Abstract]
Total Operating Revenues									1.6	0.4	7.9
Gas Utility | Operating Segments
Operating Segment Information [Abstract]
Total Operating Revenues									1,860.8	1,888.4	1,667.9
Gas Utility
Natural and propane gas									794.6	842.6	645.9
Other operation and maintenance expenses									441.7	449.7	389.9
Depreciation and amortization									179.4	167.0	153.5
Taxes, other than income taxes									151.7	152.5	137.8
Total Gas Utility Operating Expenses									1,567.4	1,611.8	1,327.1
Gas Marketing and other									0.0	0.0	0.0
Total Operating Expenses									1,567.4	1,611.8	1,327.1
Operating Income									293.4	276.6	340.8
Net Economic Earnings (Loss)									199.8	183.1	181.5
Capital Expenditures									565.4	457.7	412.6
Total Assets	6,094.6				5,606.7				6,094.6	5,606.7	5,551.2
Gas Marketing
Operating Segment Information [Abstract]
Total Operating Revenues									83.7	71.6	79.3
Gas Marketing | Eliminations
Operating Segment Information [Abstract]
Total Operating Revenues									0.0	0.0	0.0
Gas Marketing | Operating Segments
Operating Segment Information [Abstract]
Total Operating Revenues									83.7	71.6	79.3
Gas Utility
Natural and propane gas									0.0	0.0	0.0
Other operation and maintenance expenses									0.0	0.0	0.0
Depreciation and amortization									0.0	0.0	0.0
Taxes, other than income taxes									0.0	0.0	0.0
Total Gas Utility Operating Expenses									0.0	0.0	0.0
Gas Marketing and other									60.5	37.8	74.1
Total Operating Expenses									60.5	37.8	74.1
Operating Income									23.2	33.8	5.2
Net Economic Earnings (Loss)									19.4	22.9	6.8
Capital Expenditures									3.1	0.0	0.0
Total Assets	212.3				295.3				212.3	295.3	246.2
Corporate, Non-Segment
Operating Segment Information [Abstract]
Total Operating Revenues									9.5	5.4	1.4
Corporate, Non-Segment | Eliminations
Operating Segment Information [Abstract]
Total Operating Revenues									12.0	11.1	6.3
Corporate, Non-Segment | Operating Segments
Operating Segment Information [Abstract]
Total Operating Revenues									21.5	16.5	7.7
Gas Utility
Natural and propane gas									0.0	0.0	0.0
Other operation and maintenance expenses									0.0	0.0	0.0
Depreciation and amortization									0.0	0.0	0.0
Taxes, other than income taxes									0.0	0.0	0.0
Total Gas Utility Operating Expenses									0.0	0.0	0.0
Gas Marketing and other									35.8	32.8	12.8
Total Operating Expenses									35.8	32.8	12.8
Operating Income									(14.3)	(16.3)	(5.1)
Net Economic Earnings (Loss)									(24.1)	(22.3)	(20.7)
Capital Expenditures									254.8	41.7	25.5
Total Assets	$ 2,692.7				$ 2,508.0				$ 2,692.7	$ 2,508.0	$ 2,239.5