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Income Tax (Components of Tax Expense) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Current income tax expense:      
Federal $ 0 $ 0 $ 4
State 0 2 1
Total current 0 2 5
Deferred income tax expense (benefit):      
Federal 0 0 (16)
State 8 (1) (1)
Total deferred 8 (1) (17)
Provision/(benefit) for income taxes $ 8 $ 1 $ (12)