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Employee Benefit Plans - Obligations and Funded Status, Additional Information (Details) - USD ($)
$ in Millions
 12 Months Ended
Jun. 30, 2017
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Weighted average interest credit rating for cash balance plan   3.30% 3.30% 3.30%
Fair value of plan assets had fair value of rabbi trusts been included   $ 3,483 $ 3,736  
Funded status of plan had fair value of rabbi trust assets been included   $ 517 $ (640)  
Pension Benefits        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Changes in assumptions, discount rate       4.22%
Discount rate   4.35% 3.73%  
Defined Benefit Plan, Benefit Obligation   $ 4,000 $ 4,376 $ 5,650
Changes in assumptions, mortality tables   281 350  
Transferred pension obligation $ 1,600      
Assets held in rabbi trusts and designated for the non-qualified pension plans   139 144  
Fair value of plan assets   3,344 3,592 $ 4,678
After-tax settlement charge $ 750   488  
Pension Benefits | Cash        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Defined Benefit Plan, Benefit Obligation   $ 412 $ 443