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Income Taxes - Statutory rate reconciliation (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Income Tax Disclosure [Abstract]      
Net impact of tax cuts and jobs act of 2017 $ 206.7    
Effective Income Tax Rate Reconciliation, Amount [Abstract]      
Expected tax at statutory federal tax rates, amount 555.5 $ 526.4 $ 375.5
State income taxes net of federal tax benefit, amount 100.8 72.8 73.1
Effective income tax rate reconciliation tax cuts and jobs act of 2017, amount (226.9) 0.0 0.0
Production tax credits, amount (16.8) (15.7) (17.4)
AFUDC - Equity, amount (4.0) (8.8) (7.1)
Investment tax credit restored, amount (2.7) (4.9) (1.7)
Other, net, Amount (22.4) (3.3) 11.4
Total income tax expense $ 383.5 $ 566.5 $ 433.8
Effective Income Tax Rate Reconciliation, Percent [Abstract]      
Expected tax at statutory federal tax rates, effective tax rate 35.00% 35.00% 35.00%
State income taxes net of federal tax benefit, effective tax rate 6.40% 4.80% 6.80%
Effective income tax rate reconciliation tax cuts and jobs act of 2017, effective tax rate (14.30%) (0.00%) (0.00%)
Production tax credits, effective tax rate (1.10%) (1.10%) (1.60%)
AFUDC - Equity, effective tax rate (0.30%) (0.60%) (0.70%)
Investment tax credit restored, effective tax rate (0.20%) (0.30%) (0.20%)
Other, net, effective tax rate (1.40%) (0.20%) 1.10%
Total income tax expense, effective tax rate 24.10% 37.60% 40.40%