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Investment in American Transmission Company - Summarized Financial Data (Details) - ATC - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2016
Mar. 31, 2015
Dec. 31, 2015
Income statement data      
Revenues $ 164.2 $ 152.4  
Operating expenses 79.1 80.0  
Other expense 24.0 24.4  
Net income 61.1 $ 48.0  
Balance sheet data      
Current assets 88.7   $ 80.5
Noncurrent assets 4,022.1   3,948.3
Total assets 4,110.8   4,028.8
Current liabilities 337.8   330.3
Long-term debt 1,790.9   1,790.7
Other noncurrent liabilities 265.8   245.0
Shareholders' equity 1,716.3   1,662.8
Total liabilities and shareholders' equity $ 4,110.8   $ 4,028.8