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Summary of Significant Accounting Policies - Narrative (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2019
Sep. 30, 2019
Jan. 01, 2019
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]      
Weighted average remaining lease term for operating lease 6 years 10 months 24 days 6 years 10 months 24 days  
Weighted average discount rate (percent) 3.60% 3.60%  
Operating lease liabilities $ 339 $ 339 $ 357
Current operating lease liability 52 52 52
Non-current operating lease liability 287 287 305
Operating lease right of use assets 292 292 308
Rent And Occupancy Expense      
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]      
Lease rent expense 9 29  
Technology And Communication Expense      
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]      
Lease rent expense $ 5 $ 16  
Accounting Standards Update 2016-02      
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]      
Operating lease liabilities     357
Current operating lease liability     52
Non-current operating lease liability     305
Operating lease right of use assets     308
Deferred rent liabilities eliminated     49
Accounting Standards Update 2016-02 | Other Current Liabilities      
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]      
Deferred rent liabilities eliminated     2
Accounting Standards Update 2016-02 | Other Noncurrent Liabilities      
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]      
Deferred rent liabilities eliminated     $ 47