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Investment in Transmission Affiliates - ATC Summarized Financial Data (Details) - ATC - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Income statement data      
Revenues $ 690.5 $ 721.7 $ 650.8
Operating expenses 358.7 345.0 322.5
Other expense 108.3 104.1 95.5
Net income 223.5 272.6 $ 232.8
Balance sheet data      
Current assets 87.2 87.7  
Noncurrent assets 4,928.8 4,598.9  
Total assets 5,016.0 4,686.6  
Current liabilities 640.0 767.2  
Long-term debt 2,014.0 1,790.6  
Other noncurrent liabilities 295.3 240.3  
Shareholders' equity 2,066.7 1,888.5  
Total liabilities and shareholders' equity $ 5,016.0 $ 4,686.6