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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 $ 75    
Balance at end of year 90 $ 75  
Deferred tax asset valuation allowance      
Deferred tax valuation allowance, rollfoward      
Balance at beginning of year 75 77 $ 65
Charged to income tax expense 15 5 12
Charged to Retained earnings 0 (7) 0
Balance at end of year $ 90 $ 75 $ 77