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REGULATORY ASSETS AND LIABILITIES - REGULATORY ASSETS (Details) - USD ($)
$ in Millions
Sep. 30, 2019
Dec. 31, 2018
Regulatory assets    
Other current assets $ 36.3 $ 50.7
Regulatory assets 3,999.9 3,805.1
Total regulatory assets 4,036.2 3,855.8
Pension and OPEB costs    
Regulatory assets    
Total regulatory assets 1,126.5 1,193.5
Plant retirements    
Regulatory assets    
Total regulatory assets 1,032.1 832.3
Environmental remediation costs    
Regulatory assets    
Total regulatory assets 719.6 687.1
Income tax related items    
Regulatory assets    
Total regulatory assets 451.0 369.1
System support resource    
Regulatory assets    
Total regulatory assets 320.1 316.7
Asset retirement obligations    
Regulatory assets    
Total regulatory assets 222.5 185.4
Uncollectible expense    
Regulatory assets    
Total regulatory assets 39.7 38.7
We Power generation    
Regulatory assets    
Total regulatory assets 29.9 43.0
Electric transmission costs    
Regulatory assets    
Total regulatory assets 16.6 58.1
Energy efficiency programs    
Regulatory assets    
Total regulatory assets 5.1 14.0
Other, net    
Regulatory assets    
Total regulatory assets $ 73.1 $ 117.9