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Income Taxes, Unrecognized Tax Benefits (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Summary of unrecognized income tax benefits      
Unrecognized tax benefit balance at beginning of year $ 43 $ 39 $ 36
Additions for tax positions of current year 6 12 7
Additions for tax positions of prior years 1 3 1
Reductions for tax positions of prior years (1) (1)  
Reductions for lapses in statutes of limitations (2) (10) (5)
Unrecognized tax benefit balance at end of year $ 47 $ 43 $ 39