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Acquisitions - Pro Forma Financial Information for Integrys Acquisition (Details) - Integrys - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Business Acquisition [Line Items]    
Operating Revenues $ 7,727.1 $ 9,135.4
Net income attributed to common shareholders $ 873.5 $ 869.9
Earnings per share (Basic) $ 2.77 $ 2.76
Earnings per share (Diluted) $ 2.75 $ 2.74