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Summary of Significant Accounting Policies - Recent Accounting Pronouncements (Details) - USD ($)
$ in Millions
Dec. 31, 2018
Jan. 01, 2018
Dec. 31, 2017
Dec. 31, 2016
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]        
Cumulative effect on equity in period of adoption   $ 58    
Accounting Standards Update 2014-09 [Member] | Retained Earnings [Member]        
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]        
Cumulative effect on equity, retrospective adjustment       $ 24
Accounting Standards Update 2016-01 [Member]        
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]        
Carrying value of certain limited partnership interests   275 $ 200  
Accounting Standards Update 2016-01 [Member] | Retained Earnings [Member]        
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]        
Cumulative effect on equity in period of adoption   62    
Accounting Standards Update 2017-12 [Member] | Retained Earnings [Member]        
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]        
Cumulative effect on equity in period of adoption      
Accounting Standards Update 2017-12 [Member] | Accumulated Other Comprehensive Income [Member]        
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]        
Cumulative effect on equity in period of adoption      
Accounting Standards Update 2018-02 [Member] | Retained Earnings [Member]        
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]        
Cumulative effect on equity in period of adoption   229    
Accounting Standards Update 2018-02 [Member] | Accumulated Other Comprehensive Income [Member]        
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]        
Cumulative effect on equity in period of adoption   $ (229)    
Accounting Standards Update 2016-02 [Member]        
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]        
Impact of accounting standard on Total Assets $ 700      
Impact of accounting standard on Total Liabilities $ 700