Information by Operating Segment - Schedule of Operating Segment Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Sep. 30, 2018
Operating Segment Information [Abstract]
Total Operating Revenues	$ 321.3	$ 350.6	$ 1,726.8	$ 1,725.8
Gas Utility
Natural and propane gas	75.5	107.2	664.6	731.7
Other operation and maintenance expenses	111.2	99.1	323.2	333.4
Depreciation and amortization	45.1	40.5	133.2	121.9
Taxes, other than income taxes	29.7	33.5	126.3	128.2
Total Gas Utility Operating Expenses	261.5	280.3	1,247.3	1,315.2
Gas Marketing and other	46.5	11.4	151.6	97.6
Total Operating Expenses	308.0	291.7	1,398.9	1,412.8
Operating Income	13.3	58.9	327.9	313.0
Net Economic Earnings (Loss)	5.0	15.2	218.8	210.3
Total Assets	7,332.0	6,584.9	7,332.0	6,584.9	$ 6,843.6
Eliminations
Operating Segment Information [Abstract]
Total Operating Revenues	(3.2)	(2.9)	(10.7)	(8.6)
Gas Utility
Natural and propane gas	(12.6)	(10.7)	(82.0)	(52.8)
Other operation and maintenance expenses	(2.2)	(2.3)	(7.1)	(6.4)
Depreciation and amortization	0.0	0.0
Taxes, other than income taxes	0.0	0.0
Total Gas Utility Operating Expenses	(14.8)	(13.0)	(89.1)	(59.2)
Gas Marketing and other	11.6	10.1	78.4	50.6
Total Operating Expenses	(3.2)	(2.9)	(10.7)	(8.6)
Operating Income	0.0	0.0
Net Economic Earnings (Loss)	0.0	0.0
Total Assets	(1,267.8)	(1,231.0)	(1,267.8)	(1,231.0)	(1,566.4)
Gas Utility
Operating Segment Information [Abstract]
Total Operating Revenues	301.4	334.8	1,651.9	1,667.3
Gas Utility | Eliminations
Operating Segment Information [Abstract]
Total Operating Revenues	0.2	0.0	1.7	0.3
Gas Utility | Operating Segments
Operating Segment Information [Abstract]
Total Operating Revenues	301.6	334.8	1,653.6	1,667.6
Gas Utility
Natural and propane gas	88.1	117.9	746.6	784.5
Other operation and maintenance expenses	113.4	101.4	330.3	339.8
Depreciation and amortization	45.1	40.5	133.2	121.9
Taxes, other than income taxes	29.7	33.5	126.3	128.2
Total Gas Utility Operating Expenses	276.3	293.3	1,336.4	1,374.4
Gas Marketing and other	0.0	0.0
Total Operating Expenses	276.3	293.3	1,336.4	1,374.4
Operating Income	25.3	41.5	317.2	293.2
Net Economic Earnings (Loss)	7.6	16.9	220.7	208.1
Total Assets	5,830.1	5,445.5	5,830.1	5,445.5	5,606.7
Gas Marketing
Operating Segment Information [Abstract]
Total Operating Revenues	18.4	14.4	69.6	55.3
Gas Marketing | Eliminations
Operating Segment Information [Abstract]
Total Operating Revenues	(0.1)	0.0
Gas Marketing | Operating Segments
Operating Segment Information [Abstract]
Total Operating Revenues	18.3	14.4	69.6	55.3
Gas Utility
Natural and propane gas	0.0	0.0
Other operation and maintenance expenses	0.0	0.0
Depreciation and amortization	0.0	0.0
Taxes, other than income taxes	0.0	0.0
Total Gas Utility Operating Expenses	0.0	0.0
Gas Marketing and other	25.3	(7.2)	47.3	27.6
Total Operating Expenses	25.3	(7.2)	47.3	27.6
Operating Income	(7.0)	21.6	22.3	27.7
Net Economic Earnings (Loss)	3.3	4.4	17.8	18.2
Total Assets	232.3	234.5	232.3	234.5	295.3
Corporate, Non-Segment
Operating Segment Information [Abstract]
Total Operating Revenues	1.5	1.4	5.3	3.2
Corporate, Non-Segment | Eliminations
Operating Segment Information [Abstract]
Total Operating Revenues	3.1	2.9	9.0	8.3
Corporate, Non-Segment | Operating Segments
Operating Segment Information [Abstract]
Total Operating Revenues	4.6	4.3	14.3	11.5
Gas Utility
Natural and propane gas	0.0	0.0
Other operation and maintenance expenses	0.0	0.0
Depreciation and amortization	0.0	0.0
Taxes, other than income taxes	0.0	0.0
Total Gas Utility Operating Expenses	0.0	0.0
Gas Marketing and other	9.6	8.5	25.9	19.4
Total Operating Expenses	9.6	8.5	25.9	19.4
Operating Income	(5.0)	(4.2)	(11.6)	(7.9)
Net Economic Earnings (Loss)	(5.9)	(6.1)	(19.7)	(16.0)
Total Assets	$ 2,537.4	$ 2,135.9	$ 2,537.4	$ 2,135.9	$ 2,508.0