INFORMATION BY OPERATING SEGMENT - Schedule of Operating Segment Information (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2014		Sep. 30, 2013		Sep. 30, 2012
Operating Segment Information [Abstract]
Total revenues including intersegment revenues									$ 1,627.2		$ 1,017.0		$ 1,125.5
Total Operating Revenues	222.3	241.8	694.5	468.6	147.1	165.3	397.6	307.0	1,627.2		1,017.0		1,125.5
Gas Utility
Natural and propane gas									731.7		433.4		397.3
Other operation and maintenance expenses									287.8		180.3		167.4
Depreciation & amortization									82.4		48.3		40.7
Taxes, other than income taxes									112.0		60.1		53.7
Total Gas Utility Operating Expenses									1,213.9		722.1		659.1
Gas Marketing and other									226.4		176.6		353.3
Other									20.5		21.8		2.5
Total Operating Expenses									1,460.8		920.5		1,014.9
Operating Income	(8.4)	24.7	87.2	62.9	(9.7)	12.3	51.8	42.1	166.4		96.5		110.6
Net Economic Earnings (Non-GAAP)									100.1		65.0		62.6
Capital Expenditures									171.0		130.8		108.8
ASSETS
Total Assets	5,074.0				3,125.4				5,074.0		3,125.4		1,880.3
Intersegment revenues
Operating Segment Information [Abstract]
Total revenues including intersegment revenues									0		0		0
Gas Utility
Operating Segment Information [Abstract]
Total revenues including intersegment revenues									1,462.6		847.2		763.5
Total Operating Revenues									1,467.8		857.8		764.7
Gas Utility
Natural and propane gas									821.8		469.1		414.8
Other operation and maintenance expenses									288.7		180.7		167.4
Depreciation & amortization									82.4		48.3		40.7
Taxes, other than income taxes									112.0		60.1		53.7
Total Gas Utility Operating Expenses									1,304.9		758.2		676.6
Gas Marketing and other									0		0
Other									0		0
Total Operating Expenses									1,304.9		758.2		676.6
Operating Income									162.9		99.6		88.1
Net Economic Earnings (Non-GAAP)									92.8		56.6		48.1
Capital Expenditures									168.6		128.5		106.7
ASSETS
Total Assets	4,520.0				2,981.0				4,520.0		2,981.0		1,759.0
Gas Utility | Intersegment revenues
Operating Segment Information [Abstract]
Total revenues including intersegment revenues									5.2		10.6		1.2
Gas Marketing
Operating Segment Information [Abstract]
Total revenues including intersegment revenues									162.6		165.1		358.1
Total Operating Revenues									246.6		189.4		373.5
Gas Utility
Natural and propane gas									0		0		0
Other operation and maintenance expenses									0		0		0
Depreciation & amortization									0		0		0
Taxes, other than income taxes									0		0		0
Total Gas Utility Operating Expenses									0		0		0
Gas Marketing and other									226.4	[1]	176.6	[1]	353.3	[1]
Other									0		0
Total Operating Expenses									226.4		176.6		353.3
Operating Income									20.2		12.8		20.2
Net Economic Earnings (Non-GAAP)									10.2		8.9		12.3
Capital Expenditures									0		0		0.1
Depreciation and amortization expense									0.4		0.3		0.3
ASSETS
Total Assets	156.7				163.9				156.7		163.9		190.7
Gas Marketing | Intersegment revenues
Operating Segment Information [Abstract]
Total revenues including intersegment revenues									84.0		24.3		15.4
Other
Operating Segment Information [Abstract]
Total revenues including intersegment revenues									2.0		4.7		3.9
Total Operating Revenues									3.8		6.2		4.9
Gas Utility
Natural and propane gas									0		0		0
Other operation and maintenance expenses									0		0		0
Depreciation & amortization									0		0		0
Taxes, other than income taxes									0		0		0
Total Gas Utility Operating Expenses									0		0		0
Gas Marketing and other											0
Other									20.5	[2]	22.1	[2]	2.6	[2]
Total Operating Expenses									20.5		22.1		2.6
Operating Income									(16.7)		(15.9)		2.3
Net Economic Earnings (Non-GAAP)									(2.9)		(0.5)		2.2
Capital Expenditures									2.4		2.3		2.0
Depreciation and amortization expense									0.5		0.6		0.3
ASSETS
Total Assets	1,575.7				115.6				1,575.7		115.6		102.2
Other | Intersegment revenues
Operating Segment Information [Abstract]
Total revenues including intersegment revenues									1.8		1.5		1.0
Eliminations
Operating Segment Information [Abstract]
Total revenues including intersegment revenues									0		0		0
Total Operating Revenues									(91.0)		(36.4)		(17.6)
Gas Utility
Natural and propane gas									(90.1)		(35.7)		(17.5)
Other operation and maintenance expenses									(0.9)		(0.4)		0
Depreciation & amortization									0		0		0
Taxes, other than income taxes									0		0		0
Total Gas Utility Operating Expenses									(91.0)		(36.1)		(17.5)
Gas Marketing and other									0		0
Other									0		(0.3)		(0.1)
Total Operating Expenses									(91.0)		(36.4)		(17.6)
Operating Income									0		0		0
Net Economic Earnings (Non-GAAP)									0		0		0
Capital Expenditures									0		0		0
ASSETS
Total Assets	(1,178.4)				(135.1)				(1,178.4)		(135.1)		(171.6)
Eliminations | Intersegment revenues
Operating Segment Information [Abstract]
Total revenues including intersegment revenues									$ (91.0)		$ (36.4)		$ (17.6)

[1] Depreciation and amortization for Gas Marketing are included in Gas Marketing Expenses on the Statements of Consolidated Income ($0.4 for fiscal year 2014, $0.3 for fiscal year 2013, and $0.3 for fiscal year 2012).

[2] Depreciation, amortization, and accretion for Other is included in the Other Operating Expenses on the Statements of Consolidated Income ($0.5 for fiscal year 2014, $0.6 for fiscal year 2013 and $0.3 for fiscal year 2012).