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Income Taxes - Quarter information (Detail) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Income Taxes          
Effective tax rate of income (loss) before income taxes 18.10% 36.60% 20.20% 34.40%  
Tax audit settlement $ (33)   $ (33)    
Income tax expense at U.S. federal statutory rate     21.00%   35.00%
Deferred income tax benefits included in income tax expense 1 $ 2 $ 12 $ 34  
Equity in net losses of unconsolidated entities 13 13 20 45  
Investments Qualifying for Federal Tax Credits [Member]          
Income Taxes          
Equity in net losses of unconsolidated entities 6 9 12 15  
Income tax (expense) benefit, including tax credits, from equity method investment $ 12 $ 14 $ 22 $ 25