INFORMATION BY OPERATING SEGMENT - Schedule of Operating Segment Information (Details) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2016 USD ($)	Mar. 31, 2015 USD ($)	Mar. 31, 2016 USD ($) reporting_unit operating_segment	Mar. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)
Segment Reporting [Abstract]
Number of operating segments | reporting_unit			2
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	$ 609.3	$ 877.4	$ 1,008.7	$ 1,497.0
Total Operating Revenues	609.3	877.4	1,008.7	1,497.0
Gas Utility
Natural and propane gas	261.1	482.8	409.6	787.1
Other operation and maintenance expenses	94.3	103.8	185.9	200.9
Depreciation and amortization	33.8	32.2	67.3	64.2
Taxes, other than income taxes	43.9	55.7	72.1	93.7
Total Gas Utility Operating Expenses	433.1	674.5	734.9	1,145.9
Gas Marketing and Other	8.5	45.2	19.1	106.1
Total Operating Expenses	441.6	719.7	754.0	1,252.0
Operating Income	167.7	157.7	254.7	245.0
Net Economic Earnings (Loss)	103.5	97.6	148.6	143.3
ASSETS
Total Assets	5,319.8	5,180.7	5,319.8	5,180.7	$ 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	(12.3)	(16.6)	(26.0)	(42.5)
Gas Utility
Natural and propane gas	(11.9)	(16.3)	(25.3)	(41.8)
Other operation and maintenance expenses	(0.3)	(0.2)	(0.6)	(0.5)
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	(12.2)	(16.5)	(25.9)	(42.3)
Gas Marketing and Other	(0.1)	(0.1)	(0.1)	(0.2)
Total Operating Expenses	(12.3)	(16.6)	(26.0)	(42.5)
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	(1,045.7)	(1,157.4)	(1,045.7)	(1,157.4)	(1,116.8)
Eliminations | Intersegment revenues
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	(12.3)	(16.6)	$ (26.0)	(42.5)
Gas Utility
ASSETS
Number of Operating Segments | operating_segment			3
Gas Utility | Operating Segments
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	611.5	847.0	$ 1,010.3	1,428.4
Total Operating Revenues	612.7	849.0	1,012.2	1,431.4
Gas Utility
Natural and propane gas	273.0	499.1	434.9	828.9
Other operation and maintenance expenses	94.6	104.0	186.5	201.4
Depreciation and amortization	33.8	32.2	67.3	64.2
Taxes, other than income taxes	43.9	55.7	72.1	93.7
Total Gas Utility Operating Expenses	445.3	691.0	760.8	1,188.2
Gas Marketing and Other	0.0	0.0	0.0	0.0
Total Operating Expenses	445.3	691.0	760.8	1,188.2
Operating Income	167.4	158.0	251.4	243.2
Net Economic Earnings (Loss)	102.5	96.5	152.5	146.3
ASSETS
Total Assets	4,687.1	4,615.6	4,687.1	4,615.6	4,686.2
Gas Utility | Operating Segments | Intersegment revenues
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	1.2	2.0	1.9	3.0
Gas Marketing | Operating Segments
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	(2.6)	30.0	(2.4)	67.8
Total Operating Revenues	8.0	44.1	20.8	106.3
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	5.5	43.5	14.5	102.1
Total Operating Expenses	5.5	43.5	14.5	102.1
Operating Income	2.5	0.6	6.3	4.2
Net Economic Earnings (Loss)	3.0	2.1	2.7	2.5
ASSETS
Total Assets	156.5	131.7	156.5	131.7	160.6
Gas Marketing | Operating Segments | Intersegment revenues
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	10.6	14.1	23.2	38.5
Other | Operating Segments
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	0.4	0.4	0.8	0.8
Total Operating Revenues	0.9	0.9	1.7	1.8
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	3.1	1.8	4.7	4.2
Total Operating Expenses	3.1	1.8	4.7	4.2
Operating Income	(2.2)	(0.9)	(3.0)	(2.4)
Net Economic Earnings (Loss)	(2.0)	(1.0)	(6.6)	(5.5)
ASSETS
Total Assets	1,521.9	1,590.8	1,521.9	1,590.8	$ 1,560.2
Other | Operating Segments | Intersegment revenues
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	$ 0.5	$ 0.5	$ 0.9	$ 1.0