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Summary of Significant Accounting Policies Equity investment and excess costs (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Schedule of Equity Method Investments [Line Items]    
Equity Method Investment, Difference Between Carrying Amount and Underlying Equity, Amortization Period 12 years  
Amortized [Member]    
Schedule of Equity Method Investments [Line Items]    
Equity Method of Accounting and Excess Investment Cost $ 470 $ 732
Not Subject To Amortization But Subject to Impairment [Member]    
Schedule of Equity Method Investments [Line Items]    
Equity Method of Accounting and Excess Investment Cost $ 1,967 $ 1,967