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Summary of Significant Accounting Policies Asset Retirement Obligation(Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Entity Information [Line Items]      
Asset Retirement Obligation $ 11 $ 12 $ 11
Accretion Expense, Including Asset Retirement Obligations 1 1  
Asset Retirement Obligation, Liabilities Incurred (2) 2  
Increase (Decrease) in Asset Retirement Obligations 0 (1)  
Advertising Expense $ 28 $ 21 $ 19