OPERATING REVENUES - DISAGGREGATION OF NATURAL GAS UTILITY OPERATING REVENUES BY CUSTOMER CLASS (Details) - Utility - Revenues from contracts with customers - Transferred over time $ in Millions	12 Months Ended
Dec. 31, 2018 USD ($)
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 1,497.7
Natural gas
Disaggregation of Operating Revenues
Revenues from contracts with customers	305.5
Natural gas \| Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	285.3
Natural gas \| Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	177.7
Natural gas \| Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	107.6
Natural gas \| Transport
Disaggregation of Operating Revenues
Revenues from contracts with customers	19.7
Natural gas \| Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 0.5

Amount, excluding tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value added and excise.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130543-203045

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130539-203045

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130545-203045

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