Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 22
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=115929826&loc=d3e8736-108599

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.1)
-URI http://asc.fasb.org/extlink&oid=115205541&loc=SL114868664-224227

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 32
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=115929826&loc=d3e8933-108599

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 22
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=115929826&loc=d3e8736-108599

Name:	us-gaap_Revenues
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

OPERATING REVENUES (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
OPERATING REVENUES (Details) - USD ($) $ in Millions	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Disaggregation of Operating Revenues
Operating revenues	$ 379.2	$ 380.7	$ 1,122.9	$ 1,111.9
Utility segment
Disaggregation of Operating Revenues
Operating revenues	379.2		1,122.9
Utility segment \| Other operating revenues
Disaggregation of Operating Revenues
Operating revenues	0.8		2.4
Utility segment \| Transferred over time \| Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	378.4		1,120.5
Utility segment \| Electric \| Transferred over time \| Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	334.1		908.7
Utility segment \| Natural gas \| Transferred over time \| Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 44.3		$ 211.8