INFORMATION BY OPERATING SEGMENT - Schedule of Operating Segment Information (Details) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) reporting_unit	Jun. 30, 2016 USD ($)	Sep. 30, 2016 USD ($)
Segment Reporting [Abstract]
Number of operating segments | reporting_unit			2
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	$ 323.5	$ 249.3	$ 1,482.0	$ 1,258.0
Total Operating Revenues	323.5	249.3	1,482.0	1,258.0
Gas Utility
Natural and propane gas	76.7	54.1	524.8	463.7
Operation and maintenance	100.8	91.8	298.6	277.7
Depreciation and amortization	38.4	34.2	114.0	101.5
Taxes, other than income taxes	30.5	27.4	112.2	99.5
Total Gas Utility Operating Expenses	246.4	207.5	1,049.6	942.4
Gas Marketing and Other	26.8	6.5	112.6	25.6
Total Operating Expenses	273.2	214.0	1,162.2	968.0
Operating Income	50.3	35.3	319.8	290.0
Net Economic Earnings (Loss)	21.6	14.6	178.1	163.2
ASSETS
Total Assets	6,298.2	5,299.0	6,298.2	5,299.0	$ 6,064.4
Intersegment revenues
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	0.0	0.0	0.0	0.0
Other
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	0.5	0.4	1.1	1.2
Total Operating Revenues	2.1	0.9	5.7	2.6
Gas Utility
Natural and propane gas	0.0	0.0	0.0	0.0
Operation and maintenance	0.0	0.0	0.0	0.0
Depreciation and amortization	0.0	0.0	0.0	0.0
Taxes, other than income taxes	0.0	0.0	0.0	0.0
Total Gas Utility Operating Expenses	0.0	0.0	0.0	0.0
Gas Marketing and Other	4.8	2.7	9.1	7.4
Total Operating Expenses	4.8	2.7	9.1	7.4
Operating Income	(2.7)	(1.8)	(3.4)	(4.8)
Net Economic Earnings (Loss)	(4.0)	(5.2)	(12.9)	(11.8)
ASSETS
Total Assets	2,171.8	1,592.6	2,171.8	1,592.6	1,836.6
Other | Intersegment revenues
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	1.6	0.5	4.6	1.4
Eliminations
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	0.0	0.0	0.0	0.0
Total Operating Revenues	(3.1)	(7.2)	(12.5)	(33.2)
Gas Utility
Natural and propane gas	(12.0)	(7.0)	(54.0)	(32.3)
Operation and maintenance	(1.1)	(0.1)	(3.1)	(0.7)
Depreciation and amortization	0.0	0.0	0.0	0.0
Taxes, other than income taxes	0.0	0.0	0.0	0.0
Total Gas Utility Operating Expenses	(13.1)	(7.1)	(57.1)	(33.0)
Gas Marketing and Other	10.0	(0.1)	44.6	(0.2)
Total Operating Expenses	(3.1)	(7.2)	(12.5)	(33.2)
Operating Income	0.0	0.0	0.0	0.0
Net Economic Earnings (Loss)	0.0	0.0	0.0	0.0
ASSETS
Total Assets	(1,420.3)	(1,108.3)	(1,420.3)	(1,108.3)	(1,161.9)
Eliminations | Intersegment revenues
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	(3.1)	(7.2)	(12.5)	(33.2)
Gas Utility | Operating Segments
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	305.1	253.2	1,419.1	1,263.5
Total Operating Revenues	306.6	253.3	1,427.0	1,265.5
Gas Utility
Natural and propane gas	88.7	61.1	578.8	496.0
Operation and maintenance	101.9	91.9	301.7	278.4
Depreciation and amortization	38.4	34.2	114.0	101.5
Taxes, other than income taxes	30.5	27.4	112.2	99.5
Total Gas Utility Operating Expenses	259.5	214.6	1,106.7	975.4
Gas Marketing and Other	0.0	0.0	0.0	0.0
Total Operating Expenses	259.5	214.6	1,106.7	975.4
Operating Income	47.1	38.7	320.3	290.1
Net Economic Earnings (Loss)	23.3	18.0	187.3	170.5
ASSETS
Total Assets	5,323.1	4,646.9	5,323.1	4,646.9	5,184.7
Gas Utility | Operating Segments | Intersegment revenues
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	1.5	0.1	7.9	2.0
Gas Marketing | Operating Segments
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	17.9	(4.3)	61.8	(6.7)
Total Operating Revenues	17.9	2.3	61.8	23.1
Gas Utility
Natural and propane gas	0.0	0.0	0.0	0.0
Operation and maintenance	0.0	0.0	0.0	0.0
Depreciation and amortization	0.0	0.0	0.0	0.0
Taxes, other than income taxes	0.0	0.0	0.0	0.0
Total Gas Utility Operating Expenses	0.0	0.0	0.0	0.0
Gas Marketing and Other	12.0	3.9	58.9	18.4
Total Operating Expenses	12.0	3.9	58.9	18.4
Operating Income	5.9	(1.6)	2.9	4.7
Net Economic Earnings (Loss)	2.3	1.8	3.7	4.5
ASSETS
Total Assets	223.6	167.8	223.6	167.8	$ 205.0
Gas Marketing | Operating Segments | Intersegment revenues
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	$ 0.0	$ 6.6	$ 0.0	$ 29.8