INFORMATION BY OPERATING SEGMENT - Schedule of Operating Segment Information (Details) $ in Millions	3 Months Ended
	Dec. 31, 2017 USD ($) reporting_unit	Dec. 31, 2016 USD ($)	Sep. 30, 2017 USD ($)
Segment Reporting [Abstract]
Number of operating segments | reporting_unit	2
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	$ 561.8	$ 495.1
Total Operating Revenues	561.8	495.1
Gas Utility
Natural and propane gas	240.8	193.8
Operation and maintenance	97.9	99.4
Depreciation and amortization	40.3	37.7
Taxes, other than income taxes	36.7	33.4
Total Gas Utility Operating Expenses	415.7	364.3
Gas Marketing and other	41.0	41.7
Total Operating Expenses	456.7	406.0
Operating Income	105.1	89.1
Net Economic Earnings (Loss)	57.9	47.5
Assets	6,701.1	6,310.1	$ 6,546.7
Intersegment revenues
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	0.0	0.0
Other
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	0.3	1.1
Total Operating Revenues	2.8	1.8
Gas Utility
Natural and propane gas	0.0	0.0
Operation and maintenance	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	4.5	2.0
Total Operating Expenses	4.5	2.0
Operating Income	(1.7)	(0.2)
Net Economic Earnings (Loss)	(5.2)	(5.7)
Assets	2,427.7	1,848.7	2,239.5
Other | Intersegment revenues
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	2.5	0.7
Eliminations
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	0.0	0.0
Total Operating Revenues	(2.6)	(5.1)
Gas Utility
Natural and propane gas	(22.6)	(20.7)
Operation and maintenance	(1.9)	(1.1)
Depreciation and amortization	0.0	0.0
Taxes, other than income taxes	0.0	0.0
Total Gas Utility Operating Expenses	(24.5)	(21.8)
Gas Marketing and other	21.9	16.7
Total Operating Expenses	(2.6)	(5.1)
Operating Income	0.0	0.0
Net Economic Earnings (Loss)	0.0	0.0
Assets	(1,571.8)	(1,139.2)	(1,490.2)
Eliminations | Intersegment revenues
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	(2.6)	(5.1)
Gas Utility | Operating Segments
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	541.9	472.3
Total Operating Revenues	542.0	476.7
Gas Utility
Natural and propane gas	263.4	214.5
Operation and maintenance	99.8	100.5
Depreciation and amortization	40.3	37.7
Taxes, other than income taxes	36.7	33.4
Total Gas Utility Operating Expenses	440.2	386.1
Gas Marketing and other	0.0	0.0
Total Operating Expenses	440.2	386.1
Operating Income	101.8	90.6
Net Economic Earnings (Loss)	59.5	51.8
Assets	5,611.7	5,375.6	5,551.2
Gas Utility | Operating Segments | Intersegment revenues
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	0.1	4.4
Gas Marketing | Operating Segments
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	19.6	21.7
Total Operating Revenues	19.6	21.7
Gas Utility
Natural and propane gas	0.0	0.0
Operation and maintenance	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	14.6	23.0
Total Operating Expenses	14.6	23.0
Operating Income	5.0	(1.3)
Net Economic Earnings (Loss)	3.6	1.4
Assets	233.5	225.0	$ 246.2
Gas Marketing | Operating Segments | Intersegment revenues
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	$ 0.0	$ 0.0