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Summary of Significant Accounting Policies - Amortization (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Summary of Significant Accounting Standards and Policies      
Amortization expense $ 30 $ 30 $ 25
Expected intangible amortization expense for subsequent years      
2019 29    
2020 27    
2021 19    
2022 18    
2023 18    
Balance thereafter $ 171