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REGULATORY ASSETS AND LIABILITIES - REGULATORY ASSETS (Details) - USD ($)
$ in Millions
Jun. 30, 2019
Dec. 31, 2018
Regulatory assets    
Current assets $ 28.3 $ 50.7
Regulatory assets 4,007.0 3,805.1
Total regulatory assets 4,035.3 3,855.8
Pension and OPEB costs    
Regulatory assets    
Total regulatory assets 1,153.3 1,193.5
Plant retirements    
Regulatory assets    
Total regulatory assets 1,031.7 832.3
Environmental remediation costs    
Regulatory assets    
Total regulatory assets 707.6 687.1
Income tax related items    
Regulatory assets    
Total regulatory assets 424.6 369.1
System support resource    
Regulatory assets    
Total regulatory assets 319.0 316.7
Asset retirement obligations    
Regulatory assets    
Total regulatory assets 212.2 185.4
Uncollectible expense    
Regulatory assets    
Total regulatory assets 43.6 38.7
We Power generation    
Regulatory assets    
Total regulatory assets 38.6 43.0
Electric transmission costs    
Regulatory assets    
Total regulatory assets 25.6 58.1
Energy efficiency programs    
Regulatory assets    
Total regulatory assets 8.9 14.0
Other, net    
Regulatory assets    
Total regulatory assets $ 70.2 $ 117.9