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REGULATORY ENVIRONMENT (Details)
$ in Millions
 1 Months Ended 6 Months Ended
Apr. 30, 2017
USD ($)
Mar. 31, 2017
Jan. 31, 2017
Oct. 31, 2016
USD ($)
Aug. 31, 2016
USD ($)
MW
Apr. 30, 2015
USD ($)
Jun. 30, 2017
USD ($)
WE, WG, and WPS | Public Service Commission of Wisconsin (PSCW) | Natural gas storage facilities in Michigan              
Regulatory environment              
Percentage of natural gas storage needs provided by the facilities     33.33%        
WE, WG, and WPS | Public Service Commission of Wisconsin (PSCW) | 2018 and 2019 rates              
Regulatory environment              
Authorized revenue requirement for the ReACT project $ 275.0            
AFUDC             $ 51.0
Estimated cost of the ReACT project, excluding AFUDC             $ 342.0
Number of utilities with earnings sharing mechanism 3            
Percentage of first 50 basis points of additional utility earnings shared with customers 50.00%            
ROE in excess of authorized amount (as a percent) 0.50%            
PGL | Illinois Commerce Commission (ICC)              
Regulatory environment              
Number of years code of conduct reports must be provided to the ICC   4 years          
Number of years quality assurance reports must be provided to the ICC   4 years          
Number of appeals filed             0
Amount of assurance that PGL's QIP rider costs will be recoverable             0
MERC | Minnesota Public Utilities Commission (MPUC) | 2016 rates | Natural gas rates              
Regulatory environment              
Approved rate increase       $ 6.8      
Approved rate increase (as a percent)       3.00%      
Approved return on equity (as a percent)       9.11%      
Approved common equity component average (as a percent)       50.32%      
Number of years decoupling mechanism authorized for use       3 years      
Interim rate refund             $ 4.1
UMERC              
Regulatory environment              
Term of electric power purchase agreement (in years)         20 years    
Capacity of natural gas-fired generation facility (in megawatts) | MW         180    
Estimated cost of constructing a power plant         $ 265.0    
Estimated cost of constructing a power plant, including AFUDC         $ 275.0    
Portion of the power plant costs recoverable from Tilden Mining Company (as a percent)         50.00%    
Portion of the power plant costs recoverable from utility customers (as a percent)         50.00%    
UMERC | Michigan Public Service Commission (MPSC) | 2015 rates | Electric rates              
Regulatory environment              
Approved rate increase           $ 4.0  
Approved return on equity (as a percent)           10.20%  
Approved common equity component average (as a percent)           50.48%  
Period of rate implementation (in years)           3 years