INFORMATION BY OPERATING SEGMENT - Schedule of Operating Segment Information (Details) $ in Millions	3 Months Ended
	Dec. 31, 2016 USD ($) reporting_unit	Dec. 31, 2015 USD ($)	Sep. 30, 2016 USD ($)
Segment Reporting [Abstract]
Number of operating segments | reporting_unit	2
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	$ 495.1	$ 399.4
Total Operating Revenues	495.1	399.4
Gas Utility
Natural and propane gas	193.8	148.5
Other operation and maintenance expenses	99.4	91.6
Depreciation and amortization	37.7	33.5
Taxes, other than income taxes	33.4	28.2
Total Gas Utility Operating Expenses	364.3	301.8
Gas Marketing and Other	41.7	10.6
Total Operating Expenses	406.0	312.4
Operating Income	89.1	87.0
Net Economic Earnings (Loss)	47.5	45.1
ASSETS
Total Assets	6,310.1	5,398.8	$ 6,064.4
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	1.1	0.4
Total Operating Revenues	1.8	0.8
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	2.0	1.6
Total Operating Expenses	2.0	1.6
Operating Income	(0.2)	(0.8)
Net Economic Earnings (Loss)	(5.7)	(4.6)
ASSETS
Total Assets	1,848.7	1,545.5	1,836.6
Other | Intersegment revenues
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	0.7	0.4
Eliminations
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	0.0	0.0
Total Operating Revenues	(5.1)	(13.7)
Gas Utility
Natural and propane gas	(20.7)	(13.4)
Other operation and maintenance expenses	(1.1)	(0.3)
Depreciation and amortization	0.0	0.0
Taxes, other than income taxes	0.0	0.0
Total Gas Utility Operating Expenses	(21.8)	(13.7)
Gas Marketing and Other	16.7	0.0
Total Operating Expenses	(5.1)	(13.7)
Operating Income	0.0	0.0
Net Economic Earnings (Loss)	0.0	0.0
ASSETS
Total Assets	(1,139.2)	(1,109.5)	(1,161.9)
Eliminations | Intersegment revenues
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	(5.1)	(13.7)
Gas Utility | Operating Segments
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	472.3	398.8
Total Operating Revenues	476.7	399.5
Gas Utility
Natural and propane gas	214.5	161.9
Other operation and maintenance expenses	100.5	91.9
Depreciation and amortization	37.7	33.5
Taxes, other than income taxes	33.4	28.2
Total Gas Utility Operating Expenses	386.1	315.5
Gas Marketing and Other	0.0	0.0
Total Operating Expenses	386.1	315.5
Operating Income	90.6	84.0
Net Economic Earnings (Loss)	51.8	50.0
ASSETS
Total Assets	5,375.6	4,799.0	5,184.7
Gas Utility | Operating Segments | Intersegment revenues
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	4.4	0.7
Gas Marketing | Operating Segments
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	21.7	0.2
Total Operating Revenues	21.7	12.8
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	23.0	9.0
Total Operating Expenses	23.0	9.0
Operating Income	(1.3)	3.8
Net Economic Earnings (Loss)	1.4	(0.3)
ASSETS
Total Assets	225.0	163.8	$ 205.0
Gas Marketing | Operating Segments | Intersegment revenues
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	$ 0.0	$ 12.6