SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Changes in Goodwill (Details) - USD ($) $ in Millions	12 Months Ended
Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Goodwill [Roll Forward]
Beginning balance	$ 946.0	$ 937.8	$ 247.1
Adjustments to finalize acquisition		8.2	(36.9)
Acquisition	218.9		727.6
Ending balance	1,164.9	946.0	937.8
Operating Segments \| Gas Utility
Goodwill [Roll Forward]
Beginning balance	210.2	210.2	247.1
Adjustments to finalize acquisition		0.0	(36.9)
Acquisition	0.0		0.0
Ending balance	210.2	210.2	210.2
Operating Segments \| Gas Marketing
Goodwill [Roll Forward]
Beginning balance	0.0	0.0	0.0
Adjustments to finalize acquisition		0.0	0.0
Acquisition	0.0		0.0
Ending balance	0.0	0.0	0.0
Other
Goodwill [Roll Forward]
Beginning balance	735.8	727.6	0.0
Adjustments to finalize acquisition		8.2	0.0
Acquisition	218.9		727.6
Ending balance	$ 954.7	$ 735.8	$ 727.6

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6388280&loc=d3e13770-109266

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=64834752&loc=SL49117168-202975

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Amount of increase in asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized resulting from a business combination.

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Amount of increase (decrease) from adjustments after acquisition date under purchase accounting of an asset representing the future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 10
-Section 25
-Paragraph 16
-URI http://asc.fasb.org/extlink&oid=73531572&loc=d3e961-128460

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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