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Income Taxes - Additional Information (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2019
Dec. 31, 2017
US federal statutory income tax rate     21.00% 35.00%   35.00%
Income tax expense (benefit) $ (4) $ 31 $ 44 $ 61    
Effective income tax rate     16.00% 30.00%    
Effective income tax rate reconciliation, other adjustments, state law changes and tax planning strategies 46          
Effective income tax rate reconciliation, foreign tax matters $ 7          
Effective income tax rate, excluding effects of special items     31.00%      
Maximum [Member] | Scenario, Forecast [Member]            
Possible release of valuation allowance         $ 8