OPERATING REVENUES - DISAGGREGATION OF OPERATING REVENES (Details) $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($) contract performance_obligations	Mar. 31, 2017 USD ($)
Disaggregation of Operating Revenues
Operating revenues	$ 941.5	$ 972.0
Other operating revenues
Disaggregation of Operating Revenues
Operating revenues	$ 1.1
Electric
Disaggregation of Operating Revenues
Number of contracts | contract	1
Number of performance obligations | performance_obligations	2
Number of days payment is due	30 days
Natural gas
Disaggregation of Operating Revenues
Number of days payment is due	30 days
Utility segment
Disaggregation of Operating Revenues
Operating revenues	$ 941.5	$ 972.0
Utility segment | Other operating revenues
Disaggregation of Operating Revenues
Operating revenues	1.1
Utility segment | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Operating revenues	940.4
Utility segment | Electric | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Operating revenues	779.6
Utility segment | Electric | Transferred over time | Revenues from contracts with customers | Residential
Disaggregation of Operating Revenues
Operating revenues	281.4
Utility segment | Electric | Transferred over time | Revenues from contracts with customers | Small commercial and industrial
Disaggregation of Operating Revenues
Operating revenues	237.7
Utility segment | Electric | Transferred over time | Revenues from contracts with customers | Large commercial and industrial
Disaggregation of Operating Revenues
Operating revenues	148.9
Utility segment | Electric | Transferred over time | Revenues from contracts with customers | Other
Disaggregation of Operating Revenues
Operating revenues	5.4
Utility segment | Electric | Transferred over time | Revenues from contracts with customers | Total retail
Disaggregation of Operating Revenues
Operating revenues	673.4
Utility segment | Electric | Transferred over time | Revenues from contracts with customers | Wholesale
Disaggregation of Operating Revenues
Operating revenues	28.5
Utility segment | Electric | Transferred over time | Revenues from contracts with customers | Resale
Disaggregation of Operating Revenues
Operating revenues	63.7
Utility segment | Electric | Transferred over time | Revenues from contracts with customers | Steam
Disaggregation of Operating Revenues
Operating revenues	9.7
Utility segment | Electric | Transferred over time | Revenues from contracts with customers | Other utility
Disaggregation of Operating Revenues
Operating revenues	4.3
Utility segment | Natural gas | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Operating revenues	160.8
Utility segment | Natural gas | Transferred over time | Revenues from contracts with customers | Residential
Disaggregation of Operating Revenues
Operating revenues	114.7
Utility segment | Natural gas | Transferred over time | Revenues from contracts with customers | Total retail
Disaggregation of Operating Revenues
Operating revenues	170.3
Utility segment | Natural gas | Transferred over time | Revenues from contracts with customers | Commercial and industrial
Disaggregation of Operating Revenues
Operating revenues	55.6
Utility segment | Natural gas | Transferred over time | Revenues from contracts with customers | Transport
Disaggregation of Operating Revenues
Operating revenues	4.4
Utility segment | Natural gas | Transferred over time | Revenues from contracts with customers | Other utility
Disaggregation of Operating Revenues
Operating revenues	$ (13.9)