Income Taxes - Additional Information (Details) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Dec. 31, 2018
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Income Tax Examination [Line Items]        
Utilization of net operating losses due to tax cuts & jobs act 2017   80.00%    
Tax Cuts and Jobs Act of 2017, Deferred income tax expense       $ 125
Tax Cuts and Jobs Act of 2017, decrease in deferred tax liability       1,390
Tax Cuts and Jobs Act of 2017, increase in regulatory liability       1,510
Cash flow hedges reclassification from accumulated other comprehensive loss of tax effects resulting from the Tax Cuts and Jobs Act $ 2 $ 0 $ 2 0
Pension reclassification from accumulated other comprehensive loss of tax effects resulting from the Tax Cuts and Jobs Act (22) 0 (22) $ 0
Unrecognized tax benefit excluding interest and penalties, that would affect the effective tax rate   12    
Federal        
Income Tax Examination [Line Items]        
Net operating loss carryforwards 813 673 813  
Capital loss carryforwards   66    
State        
Income Tax Examination [Line Items]        
Net operating loss carryforwards $ 387 $ 453 $ 387  
Net operating loss carryforwards expiration date   The state NOL carryforwards began to expire in 2019 through 2038