INFORMATION BY OPERATING SEGMENT - Schedule of Operating Segment Information (Details) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2017 USD ($)	Mar. 31, 2016 USD ($)	Mar. 31, 2017 USD ($) reporting_unit	Mar. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)
Segment Reporting [Abstract]
Number of operating segments | reporting_unit			2
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	$ 663.4	$ 609.3	$ 1,158.5	$ 1,008.7
Total Operating Revenues	663.4	609.3	1,158.5	1,008.7
Gas Utility
Natural and propane gas	254.3	261.1	448.1	409.6
Operation and maintenance	98.4	94.3	197.8	185.9
Depreciation and amortization	37.9	33.8	75.6	67.3
Taxes, other than income taxes	48.3	43.9	81.7	72.1
Total Gas Utility Operating Expenses	438.9	433.1	803.2	734.9
Gas Marketing and Other	44.1	8.5	85.8	19.1
Total Operating Expenses	483.0	441.6	889.0	754.0
Operating Income	180.4	167.7	269.5	254.7
Net Economic Earnings (Loss)	109.0	103.5	156.5	148.6
ASSETS
Total Assets	6,256.7	5,307.5	6,256.7	5,307.5	$ 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	0.1	0.4	1.2	0.8
Total Operating Revenues	1.8	0.9	3.6	1.7
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	2.3	3.1	4.3	4.7
Total Operating Expenses	2.3	3.1	4.3	4.7
Operating Income	(0.5)	(2.2)	(0.7)	(3.0)
Net Economic Earnings (Loss)	(3.2)	(2.0)	(8.9)	(6.6)
ASSETS
Total Assets	2,199.1	1,516.5	2,199.1	1,516.5	1,836.6
Other | Intersegment revenues
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	1.7	0.5	2.4	0.9
Eliminations
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	0.0	0.0	0.0	0.0
Total Operating Revenues	(4.3)	(12.3)	(9.4)	(26.0)
Gas Utility
Natural and propane gas	(21.3)	(11.9)	(42.0)	(25.3)
Operation and maintenance	(0.9)	(0.3)	(2.0)	(0.6)
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	(22.2)	(12.2)	(44.0)	(25.9)
Gas Marketing and Other	17.9	(0.1)	34.6	(0.1)
Total Operating Expenses	(4.3)	(12.3)	(9.4)	(26.0)
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	(1,451.4)	(1,045.7)	(1,451.4)	(1,045.7)	(1,161.9)
Eliminations | Intersegment revenues
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	(4.3)	(12.3)	(9.4)	(26.0)
Gas Utility | Operating Segments
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	641.1	611.5	1,113.4	1,010.3
Total Operating Revenues	643.7	612.7	1,120.4	1,012.2
Gas Utility
Natural and propane gas	275.6	273.0	490.1	434.9
Operation and maintenance	99.3	94.6	199.8	186.5
Depreciation and amortization	37.9	33.8	75.6	67.3
Taxes, other than income taxes	48.3	43.9	81.7	72.1
Total Gas Utility Operating Expenses	461.1	445.3	847.2	760.8
Gas Marketing and Other	0.0	0.0	0.0	0.0
Total Operating Expenses	461.1	445.3	847.2	760.8
Operating Income	182.6	167.4	273.2	251.4
Net Economic Earnings (Loss)	112.2	102.5	164.0	152.5
ASSETS
Total Assets	5,290.8	4,680.2	5,290.8	4,680.2	5,184.7
Gas Utility | Operating Segments | Intersegment revenues
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	2.6	1.2	7.0	1.9
Gas Marketing | Operating Segments
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	22.2	(2.6)	43.9	(2.4)
Total Operating Revenues	22.2	8.0	43.9	20.8
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	23.9	5.5	46.9	14.5
Total Operating Expenses	23.9	5.5	46.9	14.5
Operating Income	(1.7)	2.5	(3.0)	6.3
Net Economic Earnings (Loss)	0.0	3.0	1.4	2.7
ASSETS
Total Assets	218.2	156.5	218.2	156.5	$ 205.0
Gas Marketing | Operating Segments | Intersegment revenues
Operating Segment Information [Abstract]
Total revenues including intersegment revenues	$ 0.0	$ 10.6	$ 0.0	$ 23.2