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Summary of Significant Accounting Policies - Recent Accounting Pronouncements (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2018
Jan. 01, 2018
Dec. 31, 2017
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, retrospective adjustment $ 62    
Cumulative effect on equity in period of adoption   62  
Accounting Standards Update 2016-01 [Member] | Accumulated Other Comprehensive Income [Member]      
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]      
Cumulative effect on equity, retrospective adjustment $ (4)    
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]      
Material effect on Company's financial statements Yes