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Income Taxes - Deferred Tax Valuation Allowance (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Deferred tax valuation allowance, rollfoward      
Balance at beginning of year $ 135    
Balance at end of year 152 $ 135  
Deferred tax asset valuation allowance      
Deferred tax valuation allowance, rollfoward      
Balance at beginning of year 135 147 $ 122
Charged to income tax expense 17 (5) 25
Charged to Retained earnings 0 (7) 0
Balance at end of year $ 152 $ 135 $ 147