INFORMATION BY OPERATING SEGMENT - Schedule of Operating Segment Information (Details) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2016 USD ($)	Jun. 30, 2015 USD ($)	Jun. 30, 2016 USD ($) reporting_unit operating_segment	Jun. 30, 2015 USD ($)	Sep. 30, 2015 USD ($)
Segment Reporting [Abstract]
Number of operating segments | reporting_unit			2
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	$ 249.3	$ 275.2	$ 1,258.0	$ 1,772.2
Total Operating Revenues	249.3	275.2	1,258.0	1,772.2
Gas Utility
Natural and propane gas	54.1	57.7	463.7	844.8
Other operation and maintenance expenses	91.8	90.6	277.7	291.5
Depreciation and amortization	34.2	32.5	101.5	96.7
Taxes, other than income taxes	27.4	26.2	99.5	119.9
Total Gas Utility Operating Expenses	207.5	207.0	942.4	1,352.9
Gas Marketing and Other	6.5	32.2	25.6	138.3
Total Operating Expenses	214.0	239.2	968.0	1,491.2
Operating Income	35.3	36.0	290.0	281.0
Net Economic Earnings (Loss)	14.6	11.1	163.2	154.4
ASSETS
Total Assets	5,310.9	5,082.9	5,310.9	5,082.9	$ 5,290.2
Intersegment revenues
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	0.0	0.0	0.0	0.0
Eliminations
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	0.0	0.0	0.0	0.0
Total Operating Revenues	(7.2)	(15.9)	(33.2)	(58.4)
Gas Utility
Natural and propane gas	(7.0)	(15.5)	(32.3)	(57.3)
Other operation and maintenance expenses	(0.1)	(0.3)	(0.7)	(0.8)
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	(7.1)	(15.8)	(33.0)	(58.1)
Gas Marketing and Other	(0.1)	(0.1)	(0.2)	(0.3)
Total Operating Expenses	(7.2)	(15.9)	(33.2)	(58.4)
Operating Income	0.0	0.0	0.0	0.0
Net Economic Earnings (Loss)	0.0	0.0	0.0	0.0
ASSETS
Total Assets	(1,108.3)	(1,109.0)	(1,108.3)	(1,109.0)	(1,116.8)
Eliminations | Intersegment revenues
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	(7.2)	(15.9)	$ (33.2)	(58.4)
Gas Utility
ASSETS
Number of Operating Segments | operating_segment			3
Gas Utility | Operating Segments
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	253.2	260.2	$ 1,263.5	1,688.6
Total Operating Revenues	253.3	261.2	1,265.5	1,692.6
Gas Utility
Natural and propane gas	61.1	73.2	496.0	902.1
Other operation and maintenance expenses	91.9	90.9	278.4	292.3
Depreciation and amortization	34.2	32.5	101.5	96.7
Taxes, other than income taxes	27.4	26.2	99.5	119.9
Total Gas Utility Operating Expenses	214.6	222.8	975.4	1,411.0
Gas Marketing and Other	0.0	0.0	0.0	0.0
Total Operating Expenses	214.6	222.8	975.4	1,411.0
Operating Income	38.7	38.4	290.1	281.6
Net Economic Earnings (Loss)	18.0	16.5	170.5	162.8
ASSETS
Total Assets	4,653.7	4,480.1	4,653.7	4,480.1	4,686.2
Gas Utility | Operating Segments | Intersegment revenues
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	0.1	1.0	2.0	4.0
Gas Marketing | Operating Segments
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	(4.3)	14.5	(6.7)	82.3
Total Operating Revenues	2.3	28.9	23.1	135.2
Gas Utility
Natural and propane gas	0.0	0.0	0.0	0.0
Other operation and maintenance expenses	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	3.9	27.4	18.4	129.5
Total Operating Expenses	3.9	27.4	18.4	129.5
Operating Income	(1.6)	1.5	4.7	5.7
Net Economic Earnings (Loss)	1.8	0.5	4.5	3.0
ASSETS
Total Assets	167.8	151.2	167.8	151.2	160.6
Gas Marketing | Operating Segments | Intersegment revenues
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	6.6	14.4	29.8	52.9
Other | Operating Segments
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	0.4	0.5	1.2	1.3
Total Operating Revenues	0.9	1.0	2.6	2.8
Gas Utility
Natural and propane gas	0.0	0.0	0.0	0.0
Other operation and maintenance expenses	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	2.7	4.9	7.4	9.1
Total Operating Expenses	2.7	4.9	7.4	9.1
Operating Income	(1.8)	(3.9)	(4.8)	(6.3)
Net Economic Earnings (Loss)	(5.2)	(5.9)	(11.8)	(11.4)
ASSETS
Total Assets	1,597.7	1,560.6	1,597.7	1,560.6	$ 1,560.2
Other | Operating Segments | Intersegment revenues
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	$ 0.5	$ 0.5	$ 1.4	$ 1.5