GOODWILL (Details) $ in Millions	6 Months Ended
Jun. 30, 2018 USD ($)
Changes to our goodwill balances by segment
Goodwill balance as of January 1, 2018	$ 3,053.5
Goodwill balance as of June 30, 2018	3,052.8
Accumulated impairment losses	0.0
Bluewater
Changes to our goodwill balances by segment
Adjustment to Bluewater purchase price allocation	(0.7)
Wisconsin
Changes to our goodwill balances by segment
Goodwill balance as of January 1, 2018	2,104.3
Goodwill balance as of June 30, 2018	2,104.3
Wisconsin \| Bluewater
Changes to our goodwill balances by segment
Adjustment to Bluewater purchase price allocation	0.0
Illinois
Changes to our goodwill balances by segment
Goodwill balance as of January 1, 2018	758.7
Goodwill balance as of June 30, 2018	758.7
Illinois \| Bluewater
Changes to our goodwill balances by segment
Adjustment to Bluewater purchase price allocation	0.0
Other States
Changes to our goodwill balances by segment
Goodwill balance as of January 1, 2018	183.2
Goodwill balance as of June 30, 2018	183.2
Other States \| Bluewater
Changes to our goodwill balances by segment
Adjustment to Bluewater purchase price allocation	0.0
Non-Utility Energy Infrastructure
Changes to our goodwill balances by segment
Goodwill balance as of January 1, 2018	7.3
Goodwill balance as of June 30, 2018	6.6
Non-Utility Energy Infrastructure \| Bluewater
Changes to our goodwill balances by segment
Adjustment to Bluewater purchase price allocation	$ (0.7)

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=77989000&loc=SL49117168-202975

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

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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=84229725&loc=d3e961-128460

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

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