INFORMATION BY OPERATING SEGMENT - Schedule of Operating Segment Information (Details) $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2017 USD ($) operating_segment	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)
Segment Reporting, Revenue Reconciling Item [Line Items]
Number of operating segments | operating_segment									2
Operating Segment Information [Abstract]
Total Operating Revenues	$ 258.7	$ 323.5	$ 663.4	$ 495.1	$ 279.3	$ 249.3	$ 609.3	$ 399.4	$ 1,740.7	$ 1,537.3	$ 1,976.4
Gas Utility
Natural and propane gas									570.5	492.2	882.4
Other operation and maintenance expenses									405.0	377.5	390.6
Depreciation and amortization									153.5	136.9	129.9
Taxes, other than income taxes									137.8	125.2	142.1
Total Gas Utility Operating Expenses									1,266.8	1,131.8	1,545.0
Gas Marketing and other									152.2	123.2	158.9
Total Operating Expenses									1,419.0	1,255.0	1,703.9
Operating Income	1.9	$ 50.3	$ 180.4	$ 89.1	(7.7)	$ 35.3	$ 167.7	$ 87.0	321.7	282.3	272.5
Net Economic Earnings									167.6	149.1	138.3
Capital Expenditures									438.1	293.3	289.8
ASSETS
Total Assets	6,546.7				6,064.4				6,546.7	6,064.4	5,277.6
Eliminations
Operating Segment Information [Abstract]
Total Operating Revenues									(14.2)	(5.4)	(76.5)
Gas Utility
Natural and propane gas									(75.4)	(47.5)	(75.2)
Other operation and maintenance expenses									(4.1)	(1.8)	(1.0)
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									(79.5)	(49.3)	(76.2)
Gas Marketing and other									65.3	43.9	(0.3)
Total Operating Expenses									(14.2)	(5.4)	(76.5)
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	(1,490.2)				(1,161.9)				(1,490.2)	(1,161.9)	(1,116.8)
Other
Operating Segment Information [Abstract]
Total Operating Revenues									1.4	1.6	1.7
Gas Utility
Depreciation and amortization expense									0.5	0.5	0.6
ASSETS
Total Assets	2,239.5				1,836.6				2,239.5	1,836.6	1,554.5
Corporate Eliminations
Operating Segment Information [Abstract]
Total Operating Revenues									6.3	3.2	2.0
Corporate And Eliminations
Operating Segment Information [Abstract]
Total Operating Revenues									7.7	4.8	3.7
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									12.8	12.6	12.6
Total Operating Expenses									12.8	12.6	12.6
Operating Income									(5.1)	(7.8)	(8.9)
Net Economic Earnings									(20.7)	(17.6)	(16.3)
Capital Expenditures									25.5	1.6	5.4
Gas Utility:
Operating Segment Information [Abstract]
Total Operating Revenues									1,660.0	1,457.2	1,891.8
Gas Utility: | Operating Segments
Operating Segment Information [Abstract]
Total Operating Revenues									1,667.9	1,459.4	1,895.8
Gas Utility
Natural and propane gas									645.9	539.7	957.6
Other operation and maintenance expenses									409.1	379.3	391.6
Depreciation and amortization									153.5	136.9	129.9
Taxes, other than income taxes									137.8	125.2	142.1
Total Gas Utility Operating Expenses									1,346.3	1,181.1	1,621.2
Gas Marketing and other									0.0	0.0	0.0
Total Operating Expenses									1,346.3	1,181.1	1,621.2
Operating Income									321.6	278.3	274.6
Net Economic Earnings									181.5	160.3	150.4
Capital Expenditures									412.6	291.7	284.4
ASSETS
Total Assets	5,551.2				5,184.7				5,551.2	5,184.7	4,679.3
Gas Utility: | Eliminations
Operating Segment Information [Abstract]
Total Operating Revenues									7.9	2.2	4.0
Gas Marketing:
Operating Segment Information [Abstract]
Total Operating Revenues									79.3	78.5	82.9
Gas Marketing: | Operating Segments
Operating Segment Information [Abstract]
Total Operating Revenues									79.3	78.5	153.4
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									74.1	66.7	146.6
Total Operating Expenses									74.1	66.7	146.6
Operating Income									5.2	11.8	6.8
Net Economic Earnings									6.8	6.4	4.2
Capital Expenditures									0.0	0.0	0.0
Depreciation and amortization expense									0.1	0.1	0.3
ASSETS
Total Assets	$ 246.2				$ 205.0				246.2	205.0	160.6
Gas Marketing: | Eliminations
Operating Segment Information [Abstract]
Total Operating Revenues									$ 0.0	$ 0.0	$ 70.5