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Summary of Significant Accounting Policies - Maturities of Operating Lease Liabilities (Details) - USD ($)
$ in Millions
Dec. 31, 2019
Jan. 01, 2019
Leases [Abstract]    
2020 $ 62  
2021 65  
2022 63  
2023 45  
2024 41  
Thereafter 100  
Lease liability amounts repayable 376  
Interest costs 42  
Total operating lease liability $ 334 $ 368