INFORMATION BY OPERATING SEGMENT - Schedule of Operating Segment Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Sep. 30, 2017
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	$ 350.6	$ 323.5	$ 1,725.8	$ 1,482.0
Total Operating Revenues	350.6	323.5	1,725.8	1,482.0
Gas Utility
Natural and propane gas	107.2	76.7	731.7	524.8
Operation and maintenance	105.6	100.8	347.1	298.6
Depreciation and amortization	40.5	38.4	121.9	114.0
Taxes, other than income taxes	33.5	30.5	128.2	112.2
Total Gas Utility Operating Expenses	286.8	246.4	1,328.9	1,049.6
Gas Marketing and other	11.4	26.8	97.6	112.6
Total Operating Expenses	298.2	273.2	1,426.5	1,162.2
Operating Income	52.4	50.3	299.3	319.8
Net Economic Earnings (Loss)	15.2	21.6	210.3	178.1
Total Assets	6,584.9	6,298.2	6,584.9	6,298.2	$ 6,546.7
Intersegment revenues
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	0.0	0.0	0.0	0.0
Other
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	1.4	0.5	3.2	1.1
Total Operating Revenues	4.3	2.1	11.5	5.7
Gas Utility
Natural and propane gas	0.0	0.0	0.0	0.0
Operation and maintenance	0.0	0.0	0.0	0.0
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 Gas Utility Operating Expenses	0.0	0.0	0.0	0.0
Gas Marketing and other	8.5	4.8	19.4	9.1
Total Operating Expenses	8.5	4.8	19.4	9.1
Operating Income	(4.2)	(2.7)	(7.9)	(3.4)
Net Economic Earnings (Loss)	(6.1)	(4.0)	(16.0)	(12.9)
Total Assets	2,135.9	2,171.8	2,135.9	2,171.8	2,239.5
Other | Intersegment revenues
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	2.9	1.6	8.3	4.6
Eliminations
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	0.0	0.0	0.0	0.0
Total Operating Revenues	(2.9)	(3.1)	(8.6)	(12.5)
Gas Utility
Natural and propane gas	(10.7)	(12.0)	(52.8)	(54.0)
Operation and maintenance	(2.3)	(1.1)	(6.4)	(3.1)
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 Gas Utility Operating Expenses	(13.0)	(13.1)	(59.2)	(57.1)
Gas Marketing and other	10.1	10.0	50.6	44.6
Total Operating Expenses	(2.9)	(3.1)	(8.6)	(12.5)
Operating Income	0.0	0.0	0.0	0.0
Net Economic Earnings (Loss)	0.0	0.0	0.0	0.0
Total Assets	(1,231.0)	(1,420.3)	(1,231.0)	(1,420.3)	(1,490.2)
Eliminations | Intersegment revenues
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	(2.9)	(3.1)	(8.6)	(12.5)
Gas Utility | Operating Segments
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	334.8	305.1	1,667.3	1,419.1
Total Operating Revenues	334.8	306.6	1,667.6	1,427.0
Gas Utility
Natural and propane gas	117.9	88.7	784.5	578.8
Operation and maintenance	107.9	101.9	353.5	301.7
Depreciation and amortization	40.5	38.4	121.9	114.0
Taxes, other than income taxes	33.5	30.5	128.2	112.2
Total Gas Utility Operating Expenses	299.8	259.5	1,388.1	1,106.7
Gas Marketing and other	0.0	0.0	0.0	0.0
Total Operating Expenses	299.8	259.5	1,388.1	1,106.7
Operating Income	35.0	47.1	279.5	320.3
Net Economic Earnings (Loss)	16.9	23.3	208.1	187.3
Total Assets	5,445.5	5,323.1	5,445.5	5,323.1	5,551.2
Gas Utility | Operating Segments | Intersegment revenues
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	0.0	1.5	0.3	7.9
Gas Marketing | Operating Segments
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	14.4	17.9	55.3	61.8
Total Operating Revenues	14.4	17.9	55.3	61.8
Gas Utility
Natural and propane gas	0.0	0.0	0.0	0.0
Operation and maintenance	0.0	0.0	0.0	0.0
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 Gas Utility Operating Expenses	0.0	0.0	0.0	0.0
Gas Marketing and other	(7.2)	12.0	27.6	58.9
Total Operating Expenses	(7.2)	12.0	27.6	58.9
Operating Income	21.6	5.9	27.7	2.9
Net Economic Earnings (Loss)	4.4	2.3	18.2	3.7
Total Assets	234.5	223.6	234.5	223.6	$ 246.2
Gas Marketing | Operating Segments | Intersegment revenues
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	$ 0.0	$ 0.0	$ 0.0	$ 0.0