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Income Taxes Changes in Unrecognized Tax Benefits (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Changes in Unrecognized Tax Benefits [Abstract]      
Balance at beginning of year $ 615 $ 610 $ 440
Additions attributable to tax positions taken in the current year 0 8 178
Additions attributable to tax positions taken in prior years 28 18 0
Reduction attributable to tax positions taken in prior years (25) (20) 0
Lapse of statute 9 1 8
Balance at end of year $ 609 $ 615 $ 610