INFORMATION BY OPERATING SEGMENT - Schedule of Operating Segment Information (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2015	Mar. 31, 2014	Sep. 30, 2014
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	$ 877.4	$ 694.5	$ 1,497.0	$ 1,163.1
Total Operating Revenues	877.4	694.5	1,497.0	1,163.1
Gas utility
Natural and propane gas	482.8	405.3	787.1	647.1
Other operation and maintenance expenses	103.8	72.0	200.9	134.3
Depreciation and amortization	32.2	20.1	64.2	40.1
Taxes, other than income taxes	55.7	41.8	93.7	70.4
Total Gas Utility Operating Expenses	674.5	539.2	1,145.9	891.9
Gas Marketing	43.5	65.0	102.1	116.8
Other	1.7	3.1	4.0	4.3
Total Operating Expenses	719.7	607.3	1,252.0	1,013.0
Operating Income	157.7	87.2	245.0	150.1
Net Economic Earnings (Loss)	97.6	51.7	143.3	88.0
ASSETS
Total Assets	5,180.7		5,180.7		5,074.0
Intersegment revenues
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	0	0	0	0
Gas Utility
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	847.0	634.4	1,428.4	1,069.6
Total Operating Revenues	849.0	638.6	1,431.4	1,073.9
Gas utility
Natural and propane gas	499.1	430.5	828.9	692.1
Other operation and maintenance expenses	104.0	72.2	201.4	134.7
Depreciation and amortization	32.2	20.1	64.2	40.1
Taxes, other than income taxes	55.7	41.8	93.7	70.4
Total Gas Utility Operating Expenses	691.0	564.6	1,188.2	937.3
Gas Marketing	0	0	0	0
Other	0	0	0	0
Total Operating Expenses	691.0	564.6	1,188.2	937.3
Operating Income	158.0	74.0	243.2	136.6
Net Economic Earnings (Loss)	96.5	44.7	146.3	80.5
ASSETS
Total Assets	4,615.6		4,615.6		4,520.0
Gas Utility | Intersegment revenues
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	2.0	4.2	3.0	4.3
Gas Marketing
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	30.0	59.9	67.8	93.1
Total Operating Revenues	44.1	80.5	106.3	133.2
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	43.5	65.0	102.1	116.8
Other	0	0	0	0
Total Operating Expenses	43.5	65.0	102.1	116.8
Operating Income	0.6	15.5	4.2	16.4
Net Economic Earnings (Loss)	2.1	7.1	2.5	7.9
ASSETS
Total Assets	131.7		131.7		156.7
Gas Marketing | Intersegment revenues
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	14.1	20.6	38.5	40.1
Other
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	0.4	0.2	0.8	0.4
Total Operating Revenues	0.9	0.8	1.8	1.4
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	0	0	0	0
Other	1.8	3.1	4.2	4.3
Total Operating Expenses	1.8	3.1	4.2	4.3
Operating Income	(0.9)	(2.3)	(2.4)	(2.9)
Net Economic Earnings (Loss)	(1.0)	(0.1)	(5.5)	(0.4)
ASSETS
Total Assets	1,590.8		1,590.8		1,575.7
Other | Intersegment revenues
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	0.5	0.6	1.0	1.0
Eliminations
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	0	0	0	0
Total Operating Revenues	(16.6)	(25.4)	(42.5)	(45.4)
Gas utility
Natural and propane gas	(16.3)	(25.2)	(41.8)	(45.0)
Other operation and maintenance expenses	(0.2)	(0.2)	(0.5)	(0.4)
Depreciation and amortization	0	0	0	0
Taxes, other than income taxes	0	0	0	0
Total Gas Utility Operating Expenses	(16.5)	(25.4)	(42.3)	(45.4)
Gas Marketing	0	0	0	0
Other	(0.1)	0	(0.2)	0
Total Operating Expenses	(16.6)	(25.4)	(42.5)	(45.4)
Operating Income	0	0	0	0
Net Economic Earnings (Loss)	0	0	0	0
ASSETS
Total Assets	(1,157.4)		(1,157.4)		(1,178.4)
Eliminations | Intersegment revenues
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	$ (16.6)	$ (25.4)	$ (42.5)	$ (45.4)