INFORMATION BY OPERATING SEGMENT - Schedule of Operating Segment Information (Details) $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2018 USD ($) operating_segment	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)
Segment Reporting, Revenue Reconciling Item [Line Items]
Number of operating segments | operating_segment									2
Operating Segment Information [Abstract]
Total Operating Revenues	$ 239.2	$ 350.6	$ 813.4	$ 561.8	$ 258.7	$ 323.5	$ 663.4	$ 495.1	$ 1,965.0	$ 1,740.7	$ 1,537.3
Gas Utility
Natural and propane gas									770.1	570.5	492.2
Other operation and maintenance expenses									455.6	405.0	377.5
Depreciation and amortization									167.0	153.5	136.9
Taxes, other than income taxes									152.5	137.8	125.2
Total Gas Utility Operating Expenses									1,545.2	1,266.8	1,131.8
Gas Marketing and other									140.1	152.2	123.2
Total Operating Expenses									1,685.3	1,419.0	1,255.0
Operating Income	(19.6)	$ 52.4	$ 141.8	$ 105.1	1.9	$ 50.3	$ 180.4	$ 89.1	279.7	321.7	282.3
Net Economic Earnings									183.7	167.6	149.1
Capital Expenditures									499.4	438.1	293.3
ASSETS
Total Assets	6,843.6				6,546.7				6,843.6	6,546.7	6,064.4
Eliminations
Operating Segment Information [Abstract]
Total Operating Revenues									(11.5)	(14.2)	(5.4)
Gas Utility
Natural and propane gas									(72.5)	(75.4)	(47.5)
Other operation and maintenance expenses									(8.5)	(4.1)	(1.8)
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									(81.0)	(79.5)	(49.3)
Gas Marketing and other									69.5	65.3	43.9
Total Operating Expenses									(11.5)	(14.2)	(5.4)
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	(1,566.4)				(1,490.2)				(1,566.4)	(1,490.2)	(1,161.9)
Gas Utility
Operating Segment Information [Abstract]
Total Operating Revenues									1,888.0	1,660.0	1,457.2
Gas Utility | Operating Segments
Operating Segment Information [Abstract]
Total Operating Revenues									1,888.4	1,667.9	1,459.4
Gas Utility
Natural and propane gas									842.6	645.9	539.7
Other operation and maintenance expenses									464.1	409.1	379.3
Depreciation and amortization									167.0	153.5	136.9
Taxes, other than income taxes									152.5	137.8	125.2
Total Gas Utility Operating Expenses									1,626.2	1,346.3	1,181.1
Gas Marketing and other									0.0	0.0	0.0
Total Operating Expenses									1,626.2	1,346.3	1,181.1
Operating Income									262.2	321.6	278.3
Net Economic Earnings									183.1	181.5	160.3
Capital Expenditures									457.7	412.6	291.7
ASSETS
Total Assets	5,606.7				5,551.2				5,606.7	5,551.2	5,184.7
Gas Utility | Eliminations
Operating Segment Information [Abstract]
Total Operating Revenues									0.4	7.9	2.2
Gas Marketing
Operating Segment Information [Abstract]
Total Operating Revenues									71.6	79.3	78.5
Gas Marketing | Operating Segments
Operating Segment Information [Abstract]
Total Operating Revenues									71.6	79.3	78.5
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									37.8	74.1	66.7
Total Operating Expenses									37.8	74.1	66.7
Operating Income									33.8	5.2	11.8
Net Economic Earnings									22.9	6.8	6.4
Capital Expenditures									0.0	0.0	0.0
Depreciation and amortization expense									0.0	0.1	0.1
ASSETS
Total Assets	295.3				246.2				295.3	246.2	205.0
Gas Marketing | Eliminations
Operating Segment Information [Abstract]
Total Operating Revenues									0.0	0.0	0.0
Other
Operating Segment Information [Abstract]
Total Operating Revenues									5.4	1.4	1.6
Other | Operating Segments
Operating Segment Information [Abstract]
Total Operating Revenues									16.5	7.7	4.8
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									32.8	12.8	12.6
Total Operating Expenses									32.8	12.8	12.6
Operating Income									(16.3)	(5.1)	(7.8)
Net Economic Earnings									(22.3)	(20.7)	(17.6)
Capital Expenditures									41.7	25.5	1.6
Depreciation and amortization expense									1.4	0.5	0.5
ASSETS
Total Assets	$ 2,508.0				$ 2,239.5				2,508.0	2,239.5	1,836.6
Other | Eliminations
Operating Segment Information [Abstract]
Total Operating Revenues									$ 11.1	$ 6.3	$ 3.2