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Income Taxes - Additional Information (Details) - USD ($)
$ in Millions
3 Months Ended 12 Months Ended
Dec. 31, 2017
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Income Tax Disclosure [Abstract]        
Federal corporate income tax rate, percent   35.00% 35.00% 35.00%
Increase to income tax rate provision as a result of Federal corporate tax rate change $ 865 $ 865 [1] $ 0 [1] $ 0 [1]
Unrecognized tax benefit, affect the effective tax rate 13 13 10 14
Accruals for payment of interest and penalties associated with uncertain tax positions 4 4 3 $ 4
Increase (decrease) in accruals for payment of interest and penalties associated with uncertain tax positions 1 1 (1)  
Net deferred tax asset $ 1,540 $ 1,540 $ 1,803  
[1] For 2017, the amount relates to the effects of revaluing our net deferred tax asset for new Tax Legislation that was enacted on December 22, 2017.