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Investment in Transmission Affiliates - ATC Summarized Financial Data (Details) - ATC - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Income statement data      
Revenues $ 721.7 $ 650.8 $ 615.8
Operating expenses 345.0 322.5 319.3
Other expense 104.1 95.5 96.1
Net income 272.6 232.8 $ 200.4
Balance sheet data      
Current assets 87.7 75.8  
Noncurrent assets 4,598.9 4,312.9  
Total assets 4,686.6 4,388.7  
Current liabilities 767.2 495.1  
Long-term debt 1,790.6 1,865.3  
Other noncurrent liabilities 240.3 271.5  
Shareholders' equity 1,888.5 1,756.8  
Total liabilities and shareholders' equity $ 4,686.6 $ 4,388.7