INFORMATION BY OPERATING SEGMENT - Schedule of Operating Segment Information (Details) $ in Millions	3 Months Ended
	Dec. 31, 2015 USD ($) operating_segment	Dec. 31, 2014 USD ($)	Sep. 30, 2015 USD ($)
Segment Reporting [Abstract]
Number of operating segments | operating_segment	2
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	$ 399.4	$ 619.6
Total Operating Revenues	399.4	619.6
Gas Utility
Natural and propane gas	148.5	304.3
Other operation and maintenance expenses	91.6	97.1
Depreciation and amortization	33.5	32.0
Taxes, other than income taxes	28.2	38.0
Total Gas Utility Operating Expenses	301.8	471.4
Gas Marketing and Other	10.6	60.9
Total Operating Expenses	312.4	532.3
Operating Income	87.0	87.3
Net Economic Earnings (Loss)	45.1	45.7
ASSETS
Total Assets	5,411.4	5,288.5	$ 5,290.2
Intersegment revenues
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	0.0	0.0
Eliminations
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	0.0	0.0
Total Operating Revenues	(13.7)	(25.9)
Gas Utility
Natural and propane gas	(13.4)	(25.5)
Other operation and maintenance expenses	(0.3)	(0.3)
Depreciation and amortization	0.0	0.0
Taxes, other than income taxes	0.0	0.0
Total Gas Utility Operating Expenses	(13.7)	(25.8)
Gas Marketing and Other	0.0	(0.1)
Total Operating Expenses	(13.7)	(25.9)
Operating Income	0.0	0.0
Net Economic Earnings (Loss)	0.0	0.0
ASSETS
Total Assets	(1,109.5)	(1,175.9)	(1,116.8)
Eliminations | Intersegment revenues
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	(13.7)	(25.9)
Gas Utility | Operating Segments
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	398.8	581.4
Total Operating Revenues	399.5	582.4
Gas Utility
Natural and propane gas	161.9	329.8
Other operation and maintenance expenses	91.9	97.4
Depreciation and amortization	33.5	32.0
Taxes, other than income taxes	28.2	38.0
Total Gas Utility Operating Expenses	315.5	497.2
Gas Marketing and Other	0.0	0.0
Total Operating Expenses	315.5	497.2
Operating Income	84.0	85.2
Net Economic Earnings (Loss)	50.0	49.8
ASSETS
Total Assets	4,806.0	4,735.6	4,686.2
Gas Utility | Operating Segments | Intersegment revenues
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	0.7	1.0
Gas Marketing | Operating Segments
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	0.2	37.8
Total Operating Revenues	12.8	62.2
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	9.0	58.6
Total Operating Expenses	9.0	58.6
Operating Income	3.8	3.6
Net Economic Earnings (Loss)	(0.3)	0.4
ASSETS
Total Assets	163.8	152.7	160.6
Gas Marketing | Operating Segments | Intersegment revenues
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	12.6	24.4
Other | Operating Segments
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	0.4	0.4
Total Operating Revenues	0.8	0.9
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	1.6	2.4
Total Operating Expenses	1.6	2.4
Operating Income	(0.8)	(1.5)
Net Economic Earnings (Loss)	(4.6)	(4.5)
ASSETS
Total Assets	1,551.1	1,576.1	$ 1,560.2
Other | Operating Segments | Intersegment revenues
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	$ 0.4	$ 0.5