INFORMATION BY OPERATING SEGMENT - Schedule of Operating Segment Information (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017	Sep. 30, 2017
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	$ 813.4	$ 663.4	$ 1,375.2	$ 1,158.5
Total Operating Revenues	813.4	663.4	1,375.2	1,158.5
Gas Utility
Natural and propane gas	383.7	254.3	624.5	448.1
Operation and maintenance	143.6	98.4	241.5	197.8
Depreciation and amortization	41.1	37.9	81.4	75.6
Taxes, other than income taxes	58.0	48.3	94.7	81.7
Total Gas Utility Operating Expenses	626.4	438.9	1,042.1	803.2
Gas Marketing and other	45.2	44.1	86.2	85.8
Total Operating Expenses	671.6	483.0	1,128.3	889.0
Operating Income	141.8	180.4	246.9	269.5
Net Economic Earnings (Loss)	137.2	109.0	195.1	156.5
Total Assets	6,586.8	6,256.7	6,586.8	6,256.7	$ 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	1.5	0.1	1.8	1.2
Total Operating Revenues	4.4	1.8	7.2	3.6
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	6.4	2.3	10.9	4.3
Total Operating Expenses	6.4	2.3	10.9	4.3
Operating Income	(2.0)	(0.5)	(3.7)	(0.7)
Net Economic Earnings (Loss)	(4.7)	(3.2)	(9.9)	(8.9)
Total Assets	2,211.9	2,199.1	2,211.9	2,199.1	2,239.5
Other | Intersegment revenues
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	2.9	1.7	5.4	2.4
Eliminations
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	0.0	0.0	0.0	0.0
Total Operating Revenues	(3.1)	(4.3)	(5.7)	(9.4)
Gas Utility
Natural and propane gas	(19.5)	(21.3)	(42.1)	(42.0)
Operation and maintenance	(2.2)	(0.9)	(4.1)	(2.0)
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	(21.7)	(22.2)	(46.2)	(44.0)
Gas Marketing and other	18.6	17.9	40.5	34.6
Total Operating Expenses	(3.1)	(4.3)	(5.7)	(9.4)
Operating Income	0.0	0.0	0.0	0.0
Net Economic Earnings (Loss)	0.0	0.0	0.0	0.0
Total Assets	(1,318.0)	(1,451.4)	(1,318.0)	(1,451.4)	(1,490.2)
Eliminations | Intersegment revenues
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	(3.1)	(4.3)	(5.7)	(9.4)
Gas Utility | Operating Segments
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	790.6	641.1	1,332.5	1,113.4
Total Operating Revenues	790.8	643.7	1,332.8	1,120.4
Gas Utility
Natural and propane gas	403.2	275.6	666.6	490.1
Operation and maintenance	145.8	99.3	245.6	199.8
Depreciation and amortization	41.1	37.9	81.4	75.6
Taxes, other than income taxes	58.0	48.3	94.7	81.7
Total Gas Utility Operating Expenses	648.1	461.1	1,088.3	847.2
Gas Marketing and other	0.0	0.0	0.0	0.0
Total Operating Expenses	648.1	461.1	1,088.3	847.2
Operating Income	142.7	182.6	244.5	273.2
Net Economic Earnings (Loss)	131.7	112.2	191.2	164.0
Total Assets	5,484.3	5,290.8	5,484.3	5,290.8	5,551.2
Gas Utility | Operating Segments | Intersegment revenues
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	0.2	2.6	0.3	7.0
Gas Marketing | Operating Segments
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	21.3	22.2	40.9	43.9
Total Operating Revenues	21.3	22.2	40.9	43.9
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	20.2	23.9	34.8	46.9
Total Operating Expenses	20.2	23.9	34.8	46.9
Operating Income	1.1	(1.7)	6.1	(3.0)
Net Economic Earnings (Loss)	10.2	0.0	13.8	1.4
Total Assets	208.6	218.2	208.6	218.2	$ 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