INFORMATION BY OPERATING SEGMENT - Schedule of Operating Segment Information (Details) $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($)	Jun. 30, 2014 USD ($)	Mar. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Sep. 30, 2015 USD ($) Operating_Segment	Sep. 30, 2014 USD ($)	Sep. 30, 2013 USD ($)
Segment Reporting, Revenue Reconciling Item [Line Items]
Number of operating segments | Operating_Segment									2
Operating Segment Information [Abstract]
Total revenues including intersegment revenues									$ 1,976.4	$ 1,627.2	$ 1,017.0
Total Operating Revenues	$ 204.2	$ 275.2	$ 877.4	$ 619.6	$ 222.3	$ 241.8	$ 694.5	$ 468.6	1,976.4	1,627.2	1,017.0
Gas Utility
Natural and propane gas									882.4	731.7	433.4
Other operation and maintenance									390.6	287.8	180.3
Depreciation and amortization									129.9	82.4	48.3
Taxes, other than income taxes									142.1	112.0	60.1
Total Gas Utility Operating Expenses									1,545.0	1,213.9	722.1
Gas Marketing and Other									158.9	246.9	198.4
Total Operating Expenses									1,703.9	1,460.8	920.5
Operating Income	(8.5)	$ 36.0	$ 157.7	$ 87.3	(8.4)	$ 24.7	$ 87.2	$ 62.9	272.5	166.4	96.5
Net Economic Earnings (Loss)									138.3	100.1	65.0
Capital Expenditures									289.8	171.0	130.8
ASSETS
Total Assets	5,290.2				5,074.0				5,290.2	5,074.0	3,125.4
Intersegment revenues
Operating Segment Information [Abstract]
Total revenues including intersegment revenues									0.0	0.0	0.0
Eliminations
Operating Segment Information [Abstract]
Total revenues including intersegment revenues									0.0	0.0	0.0
Total Operating Revenues									(76.5)	(91.0)	(36.4)
Gas Utility
Natural and propane gas									(75.2)	(90.1)	(35.7)
Other operation and maintenance									(1.0)	(0.9)	(0.4)
Depreciation and amortization									0.0	0.0	0.0
Taxes, other than income taxes									0.0	0.0	0.0
Total Gas Utility Operating Expenses									(76.2)	(91.0)	(36.1)
Gas Marketing and Other									(0.3)	0.0	(0.3)
Total Operating Expenses									(76.5)	(91.0)	(36.4)
Operating Income									0.0	0.0	0.0
Net Economic Earnings (Loss)									0.0	0.0	0.0
Capital Expenditures									0.0	0.0	0.0
ASSETS
Total Assets	(1,116.8)				(1,178.4)				(1,116.8)	(1,178.4)	(135.1)
Eliminations | Intersegment revenues
Operating Segment Information [Abstract]
Total revenues including intersegment revenues									(76.5)	(91.0)	(36.4)
Gas Utility | Operating Segments
Operating Segment Information [Abstract]
Total revenues including intersegment revenues									1,891.8	1,462.6	847.2
Total Operating Revenues									1,895.8	1,467.8	857.8
Gas Utility
Natural and propane gas									957.6	821.8	469.1
Other operation and maintenance									391.6	288.7	180.7
Depreciation and amortization									129.9	82.4	48.3
Taxes, other than income taxes									142.1	112.0	60.1
Total Gas Utility Operating Expenses									1,621.2	1,304.9	758.2
Gas Marketing and Other									0.0	0.0	0.0
Total Operating Expenses									1,621.2	1,304.9	758.2
Operating Income									274.6	162.9	99.6
Net Economic Earnings (Loss)									150.4	92.8	56.6
Capital Expenditures									284.4	168.6	128.5
ASSETS
Total Assets	4,686.2				4,520.0				4,686.2	4,520.0	2,981.0
Gas Utility | Operating Segments | Intersegment revenues
Operating Segment Information [Abstract]
Total revenues including intersegment revenues									4.0	5.2	10.6
Gas Marketing | Operating Segments
Operating Segment Information [Abstract]
Total revenues including intersegment revenues									82.9	162.6	165.1
Total Operating Revenues									153.4	246.6	189.4
Gas Utility
Natural and propane gas									0.0	0.0	0.0
Other operation and maintenance									0.0	0.0	0.0
Depreciation and amortization									0.0	0.0	0.0
Taxes, other than income taxes									0.0	0.0	0.0
Total Gas Utility Operating Expenses									0.0	0.0	0.0
Gas Marketing and Other									146.6	226.4	176.6
Total Operating Expenses									146.6	226.4	176.6
Operating Income									6.8	20.2	12.8
Net Economic Earnings (Loss)									4.2	10.2	8.9
Capital Expenditures									0.0	0.0	0.0
Depreciation and amortization expense									0.3	0.4	0.3
ASSETS
Total Assets	160.6				156.7				160.6	156.7	163.9
Gas Marketing | Operating Segments | Intersegment revenues
Operating Segment Information [Abstract]
Total revenues including intersegment revenues									70.5	84.0	24.3
Other Segments | Operating Segments
Operating Segment Information [Abstract]
Total revenues including intersegment revenues									1.7	2.0	4.7
Total Operating Revenues									3.7	3.8	6.2
Gas Utility
Natural and propane gas									0.0	0.0	0.0
Other operation and maintenance									0.0	0.0	0.0
Depreciation and amortization									0.0	0.0	0.0
Taxes, other than income taxes									0.0	0.0	0.0
Total Gas Utility Operating Expenses									0.0	0.0	0.0
Gas Marketing and Other									12.6	20.5	22.1
Total Operating Expenses									12.6	20.5	22.1
Operating Income									(8.9)	(16.7)	(15.9)
Net Economic Earnings (Loss)									(16.3)	(2.9)	(0.5)
Capital Expenditures									5.4	2.4	2.3
Depreciation and amortization expense									0.6	0.5	0.6
ASSETS
Total Assets	$ 1,560.2				$ 1,575.7				1,560.2	1,575.7	115.6
Other Segments | Operating Segments | Intersegment revenues
Operating Segment Information [Abstract]
Total revenues including intersegment revenues									$ 2.0	$ 1.8	$ 1.5