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Income Taxes (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Income Tax Disclosure [Abstract]    
Effective income tax rate (as a percent) 11.00% 25.00%
Production tax credits $ 14 $ 23
Deferred income tax benefit associated with convertible investment tax credits 40 36
Income tax benefits related to the amortization of deferred regulatory credits $ 33 29
Tax Cuts and Jobs Act of 2017, income tax expense (benefit)   $ 125
Production tax credit (PTC), roll off period 10 years