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Income Tax Expense Income Tax Expense (Components of Tax Expense) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Income Taxes [Line Items]      
Federal $ 0 $ (65) $ (3)
State 2 1 1
Total current income tax expense 2 (64) (2)
Federal (302) (12) 12
State (6) 4 19
Total deferred tax expense (benefit) (308) (8) 31
Net income tax expense (benefit) $ (306) $ (72) $ 29