INFORMATION BY OPERATING SEGMENT - Schedule of Operating Segment Information (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Sep. 30, 2014
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	$ 619.6	$ 468.6
Total Operating Revenues	619.6	468.6
Gas utility
Natural and propane gas	304.3	241.8
Other operation and maintenance expenses	97.1	62.3
Depreciation and amortization	32.0	20.0
Taxes, other than income taxes	38.0	28.6
Total Gas Utility Operating Expenses	471.4	352.7
Gas Marketing and other	58.6	51.8
Other	2.3	1.2
Total Operating Expenses	532.3	405.7
Operating Income	87.3	62.9
Net Economic Earnings (Loss)	45.7	36.3
ASSETS
Total Assets	5,288.5		5,074.0
Intersegment revenues
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	0	0
Gas Utility
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	581.4	435.2
Total Operating Revenues	582.4	435.3
Gas utility
Natural and propane gas	329.8	261.6
Other operation and maintenance expenses	97.4	62.5
Depreciation and amortization	32.0	20.0
Taxes, other than income taxes	38.0	28.6
Total Gas Utility Operating Expenses	497.2	372.7
Gas Marketing and other	0	0
Other	0	0
Total Operating Expenses	497.2	372.7
Operating Income	85.2	62.6
Net Economic Earnings (Loss)	49.8	35.8
ASSETS
Total Assets	4,735.6		4,520.0
Gas Utility | Intersegment revenues
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	1.0	0.1
Gas Marketing
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	37.8	33.2
Total Operating Revenues	62.2	52.7
Gas utility
Natural and propane gas	0	0
Other operation and maintenance expenses	0	0
Depreciation and amortization	0	0
Taxes, other than income taxes	0	0
Total Gas Utility Operating Expenses	0	0
Gas Marketing and other	58.6	51.8
Other	0	0
Total Operating Expenses	58.6	51.8
Operating Income	3.6	0.9
Net Economic Earnings (Loss)	0.4	0.8
ASSETS
Total Assets	152.7		156.7
Gas Marketing | Intersegment revenues
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	24.4	19.5
Other
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	0.4	0.2
Total Operating Revenues	0.9	0.6
Gas utility
Natural and propane gas	0	0
Other operation and maintenance expenses	0	0
Depreciation and amortization	0	0
Taxes, other than income taxes	0	0
Total Gas Utility Operating Expenses	0	0
Gas Marketing and other	0	0
Other	2.4	1.2
Total Operating Expenses	2.4	1.2
Operating Income	(1.5)	(0.6)
Net Economic Earnings (Loss)	(4.5)	(0.3)
ASSETS
Total Assets	1,576.1		1,575.7
Other | Intersegment revenues
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	0.5	0.4
Eliminations
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	0	0
Total Operating Revenues	(25.9)	(20.0)
Gas utility
Natural and propane gas	(25.5)	(19.8)
Other operation and maintenance expenses	(0.3)	(0.2)
Depreciation and amortization	0	0
Taxes, other than income taxes	0	0
Total Gas Utility Operating Expenses	(25.8)	(20.0)
Gas Marketing and other	0	0
Other	(0.1)	0
Total Operating Expenses	(25.9)	(20.0)
Operating Income	0	0
Net Economic Earnings (Loss)	0	0
ASSETS
Total Assets	(1,175.9)		(1,178.4)
Eliminations | Intersegment revenues
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	$ (25.9)	$ (20.0)