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Income Taxes - Statutory rate reconciliation (Details) - USD ($) $ in Millions | 12 Months Ended
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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%

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