INFORMATION BY OPERATING SEGMENT - Schedule of Operating Segment Information (Details) $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2016 USD ($) operating_segment	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)
Segment Reporting, Revenue Reconciling Item [Line Items]
Number of operating segments | operating_segment									2
Operating Segment Information [Abstract]
Total Operating Revenues	$ 279.3	$ 249.3	$ 609.3	$ 399.4	$ 204.2	$ 275.2	$ 877.4	$ 619.6	$ 1,537.3	$ 1,976.4	$ 1,627.2
Gas Utility
Natural and propane gas									492.2	882.4	731.7
Other operation and maintenance expenses									377.5	390.6	287.8
Depreciation and amortization									136.9	129.9	82.4
Taxes, other than income taxes									125.2	142.1	112.0
Total Gas Utility Operating Expenses									1,131.8	1,545.0	1,213.9
Gas Marketing and other									123.2	158.9	246.9
Total Operating Expenses									1,255.0	1,703.9	1,460.8
Operating Income	(7.7)	$ 35.3	$ 167.7	$ 87.0	(8.5)	$ 36.0	$ 157.7	$ 87.3	282.3	272.5	166.4
Net Economic Earnings									149.1	138.3	100.1
Capital Expenditures									293.3	289.8	171.0
ASSETS
Total Assets	6,077.4				5,290.2				6,077.4	5,290.2	5,074.0
Eliminations
Operating Segment Information [Abstract]
Total Operating Revenues									(5.4)	(76.5)	(91.0)
Gas Utility
Natural and propane gas									(47.5)	(75.2)	(90.1)
Other operation and maintenance expenses									(1.8)	(1.0)	(0.9)
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									(49.3)	(76.2)	(91.0)
Gas Marketing and other									43.9	(0.3)	0.0
Total Operating Expenses									(5.4)	(76.5)	(91.0)
Operating Income									0.0	0.0	0.0
Net Economic Earnings									0.0	0.0	0.0
Capital Expenditures									0.0	0.0	0.0
ASSETS
Total Assets	(1,161.9)				(1,116.8)				(1,161.9)	(1,116.8)	(1,178.4)
Other
Operating Segment Information [Abstract]
Total Operating Revenues									1.6	1.7	2.0
Gas Utility
Depreciation and amortization expense									0.5	0.6	0.5
ASSETS
Total Assets	1,842.4				1,560.2				1,842.4	1,560.2	1,575.7
Corporate Eliminations
Operating Segment Information [Abstract]
Total Operating Revenues									3.2	2.0	1.8
Corporate And Eliminations
Operating Segment Information [Abstract]
Total Operating Revenues									4.8	3.7	3.8
Gas Utility
Natural and propane gas									0.0	0.0	0.0
Other operation and maintenance expenses									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	12.6	20.5
Total Operating Expenses									12.6	12.6	20.5
Operating Income									(7.8)	(8.9)	(16.7)
Net Economic Earnings									(17.6)	(16.3)	(2.9)
Capital Expenditures									1.6	5.4	2.4
Gas Utility
Operating Segment Information [Abstract]
Total Operating Revenues									1,457.2	1,891.8	1,462.6
Gas Utility | Operating Segments
Operating Segment Information [Abstract]
Total Operating Revenues									1,459.4	1,895.8	1,467.8
Gas Utility
Natural and propane gas									539.7	957.6	821.8
Other operation and maintenance expenses									379.3	391.6	288.7
Depreciation and amortization									136.9	129.9	82.4
Taxes, other than income taxes									125.2	142.1	112.0
Total Gas Utility Operating Expenses									1,181.1	1,621.2	1,304.9
Gas Marketing and other									0.0	0.0	0.0
Total Operating Expenses									1,181.1	1,621.2	1,304.9
Operating Income									278.3	274.6	162.9
Net Economic Earnings									160.3	150.4	92.8
Capital Expenditures									291.7	284.4	168.6
ASSETS
Total Assets	5,191.9				4,686.2				5,191.9	4,686.2	4,520.0
Gas Utility | Eliminations
Operating Segment Information [Abstract]
Total Operating Revenues									2.2	4.0	5.2
Gas Marketing
Operating Segment Information [Abstract]
Total Operating Revenues									78.5	82.9	162.6
Gas Marketing | Operating Segments
Operating Segment Information [Abstract]
Total Operating Revenues									78.5	153.4	246.6
Gas Utility
Natural and propane gas									0.0	0.0	0.0
Other operation and maintenance expenses									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									66.7	146.6	226.4
Total Operating Expenses									66.7	146.6	226.4
Operating Income									11.8	6.8	20.2
Net Economic Earnings									6.4	4.2	10.2
Capital Expenditures									0.0	0.0	0.0
Depreciation and amortization expense									0.1	0.3	0.4
ASSETS
Total Assets	$ 205.0				$ 160.6				205.0	160.6	156.7
Gas Marketing | Eliminations
Operating Segment Information [Abstract]
Total Operating Revenues									$ 0.0	$ 70.5	$ 84.0