INFORMATION BY OPERATING SEGMENT - Schedule of Operating Segment Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Sep. 30, 2014
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	$ 275.2	$ 241.8	$ 1,772.2	$ 1,404.9
Total Operating Revenues	275.2	241.8	1,772.2	1,404.9
Gas utility
Natural and propane gas	57.7	49.3	844.8	696.4
Other operation and maintenance expenses	90.6	73.0	291.5	207.3
Depreciation and amortization	32.5	18.4	96.7	58.5
Taxes, other than income taxes	26.2	22.2	119.9	92.6
Total Gas Utility Operating Expenses	207.0	162.9	1,352.9	1,054.8
Gas Marketing	27.4	50.0	129.5	166.8
Other	4.8	4.2	8.8	8.5
Total Operating Expenses	239.2	217.1	1,491.2	1,230.1
Operating Income	36.0	24.7	281.0	174.8
Net Economic Earnings (Loss)	11.1	14.5	154.4	102.5
ASSETS
Total Assets	5,082.9		5,082.9		$ 5,074.0
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	(15.9)	(28.6)	(58.4)	(74.0)
Gas utility
Natural and propane gas	(15.5)	(28.3)	(57.3)	(73.3)
Other operation and maintenance expenses	(0.3)	(0.3)	(0.8)	(0.7)
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	(15.8)	(28.6)	(58.1)	(74.0)
Gas Marketing	0.0	0.0	0.0	0.0
Other	(0.1)	0.0	(0.3)	0.0
Total Operating Expenses	(15.9)	(28.6)	(58.4)	(74.0)
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	(1,109.0)		(1,109.0)		(1,178.4)
Eliminations | Intersegment revenues
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	(15.9)	(28.6)	(58.4)	(74.0)
Gas Utility | Operating Segments
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	260.2	214.0	1,688.6	1,283.6
Total Operating Revenues	261.2	214.2	1,692.6	1,288.1
Gas utility
Natural and propane gas	73.2	77.6	902.1	769.7
Other operation and maintenance expenses	90.9	73.3	292.3	208.0
Depreciation and amortization	32.5	18.4	96.7	58.5
Taxes, other than income taxes	26.2	22.2	119.9	92.6
Total Gas Utility Operating Expenses	222.8	191.5	1,411.0	1,128.8
Gas Marketing	0.0	0.0	0.0	0.0
Other	0.0	0.0	0.0	0.0
Total Operating Expenses	222.8	191.5	1,411.0	1,128.8
Operating Income	38.4	22.7	281.6	159.3
Net Economic Earnings (Loss)	16.5	13.3	162.8	93.8
ASSETS
Total Assets	4,480.1		4,480.1		4,520.0
Gas Utility | Operating Segments | Intersegment revenues
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	1.0	0.2	4.0	4.5
Gas Marketing | Operating Segments
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	14.5	27.1	82.3	120.2
Total Operating Revenues	28.9	55.0	135.2	188.2
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	27.4	50.0	129.5	166.8
Other	0.0	0.0	0.0	0.0
Total Operating Expenses	27.4	50.0	129.5	166.8
Operating Income	1.5	5.0	5.7	21.4
Net Economic Earnings (Loss)	0.5	1.9	3.0	9.8
ASSETS
Total Assets	151.2		151.2		156.7
Gas Marketing | Operating Segments | Intersegment revenues
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	14.4	27.9	52.9	68.0
Other | Operating Segments
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	0.5	0.7	1.3	1.1
Total Operating Revenues	1.0	1.2	2.8	2.6
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	0.0	0.0	0.0	0.0
Other	4.9	4.2	9.1	8.5
Total Operating Expenses	4.9	4.2	9.1	8.5
Operating Income	(3.9)	(3.0)	(6.3)	(5.9)
Net Economic Earnings (Loss)	(5.9)	(0.7)	(11.4)	(1.1)
ASSETS
Total Assets	1,560.6		1,560.6		$ 1,575.7
Other | Operating Segments | Intersegment revenues
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	$ 0.5	$ 0.5	$ 1.5	$ 1.5