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Goodwill (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Goodwill    
Accumulated impairment losses $ 0.0  
Goodwill impairment loss 0.0  
Changes to goodwill balances by segment    
Goodwill balance as of January 1 3,046.2 $ 3,023.5
Adjustment to Integrys purchase price allocation 0.0 22.7
Acquisition of Bluewater 7.3 0.0
Goodwill balance as of December 31 3,053.5 3,046.2
Wisconsin    
Changes to goodwill balances by segment    
Goodwill balance as of January 1 2,104.3 2,109.5
Adjustment to Integrys purchase price allocation 0.0 (5.2)
Acquisition of Bluewater 0.0 0.0
Goodwill balance as of December 31 2,104.3 2,104.3
Illinois    
Changes to goodwill balances by segment    
Goodwill balance as of January 1 758.7 731.2
Adjustment to Integrys purchase price allocation 0.0 27.5
Acquisition of Bluewater 0.0 0.0
Goodwill balance as of December 31 758.7 758.7
Other States    
Changes to goodwill balances by segment    
Goodwill balance as of January 1 183.2 182.8
Adjustment to Integrys purchase price allocation 0.0 0.4
Acquisition of Bluewater 0.0 0.0
Goodwill balance as of December 31 183.2 183.2
Non-Utility Energy Infrastructure    
Changes to goodwill balances by segment    
Goodwill balance as of January 1 0.0 0.0
Adjustment to Integrys purchase price allocation 0.0 0.0
Acquisition of Bluewater 7.3 0.0
Goodwill balance as of December 31 $ 7.3 $ 0.0