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INVESTMENT IN AMERICAN TRANSMISSION COMPANY - SUMMARIZED FINANCIAL DATA (Details) - ATC - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Dec. 31, 2016
Income statement data          
Revenues $ 176.6 $ 154.3 $ 351.3 $ 318.5  
Operating expenses 82.7 81.7 165.1 160.8  
Other expense 25.7 23.7 52.1 47.7  
Net income 68.2 $ 48.9 134.1 $ 110.0  
Balance sheet data          
Current assets 86.2   86.2   $ 75.8
Noncurrent assets 4,489.3   4,489.3   4,312.9
Total assets 4,575.5   4,575.5   4,388.7
Current liabilities 640.3   640.3   495.1
Long-term debt 1,740.6   1,740.6   1,865.3
Other noncurrent liabilities 291.4   291.4   271.5
Shareholders' equity 1,903.2   1,903.2   1,756.8
Total liabilities and shareholders' equity $ 4,575.5   $ 4,575.5   $ 4,388.7