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Income Taxes Changes in Unrecognized Tax Benefits (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Changes in Unrecognized Tax Benefits [Abstract]      
Balance at beginning of year $ 610 $ 440 $ 429
Additions attributable to tax positions taken in the current year 8 178 20
Additions attributable to tax positions taken in prior years 18 0 0
Reduction attributable to tax positions taken in prior years (20) 0 (1)
Settlements 0 0 5
Lapse of statute 1 8 3
Balance at end of year $ 615 $ 610 $ 440