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Summary of Significant Accounting Policies Recently issued but not yet adopted ASU (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
New Accounting Pronouncements or Change in Accounting Principle [Line Items]        
Operating Leases, Future Minimum Payments Due $ 2,652      
Tax Cuts and Jobs Act of 2017, reclassification from AOCI to retained earnings, tax effect 170      
Securities held to maturity, at carrying value 28,984 $ 25,712    
Accumulated other comprehensive loss (926) $ (949) $ (616) $ (430)
Accounting Standards Update 2017-12        
New Accounting Pronouncements or Change in Accounting Principle [Line Items]        
Securities held to maturity, at carrying value 9,000      
Accumulated other comprehensive loss $ 107