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Income Taxes (Details) - USD ($)
$ in Millions
3 Months Ended 12 Months Ended
Dec. 31, 2017
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Current income tax expense (benefit):          
Federal   $ 0 $ 0 $ (5)  
State   (11) 3 (5)  
Foreign   0 2 5  
Total current   (11) 5 (5)  
Deferred income tax expense (benefit):          
Federal   399 285 (145)  
State   51 (10) (31)  
Total deferred   450 275 (176)  
Non-current tax expense (benefit)   11 6 4  
Income tax expense (benefit)   $ 450 286 (177)  
Federal statutory tax rate related to federal tax reform law   21.00%      
Additional tax expense related to the federal tax reform law $ 58 $ 58      
Unrecognized Tax Benefits $ 25 $ 25 $ 28 $ 29 $ 330