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Income Taxes - Components of deferred tax assets and liabilities (Details) - USD ($)
$ in Millions
 12 Months Ended
Jan. 01, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Income taxes        
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent   35.00% 35.00% 35.00%
Non-current        
Tax gross up - regulatory items   $ 585.8 $ 0.0  
Future tax benefits   303.9 430.4  
Employee benefits and compensation   164.2 222.0  
Deferred revenues   128.8 207.2  
Property-related   24.4 54.5  
Other   185.0 230.6  
Total deferred tax assets   1,392.1 1,144.7  
Valuation allowance   (15.7) (15.0)  
Net deferred tax assets   1,376.4 1,129.7  
Non-current        
Property-related   3,464.6 4,979.3  
Investment in transmission affiliate   321.2 476.9  
Employee benefits and compensation   285.8 401.6  
Deferred transmission costs   60.1 93.1  
Other   244.5 325.4  
Total deferred tax liabilities   4,376.2 6,276.3  
Deferred tax liability, net   2,999.8 $ 5,146.6  
Utility operations        
Income taxes        
Estimated tax benefit related to the remeasurement of deferred taxes from tax legislation   2,657.0    
Non-utility operations        
Income taxes        
Estimated tax benefit related to the remeasurement of deferred taxes from tax legislation   $ 206.7    
Federal tax rate change due to the tax legislation        
Income taxes        
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent 21.00%