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Summary of Significant Accounting Policies - Recent Accounting Pronouncements (Details) - USD ($)
$ in Millions
6 Months Ended
Jun. 30, 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 [1]   $ 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]        
Material effect on Company's financial statements Yes      
[1]
See Note 2 for further information about adjustments resulting from the Company's adoption of new accounting standards in 2018.