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Employee Benefit Plans (Components of Net Periodic Benefit Cost Included in General and Administrative Expenses) (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Pension Benefits    
Defined Benefit Plan Disclosure [Line Items]    
Service cost $ 83 $ 80
Interest cost 90 108
Expected return on plan assets (205) (201)
Amortization of prior service cost (1) (1)
Amortization of actuarial (gain) loss, net 62 65
Settlements 1 0
Special Termination Benefits 1 2
Net periodic (benefit) cost 31 53
Other Postretirement Benefits    
Defined Benefit Plan Disclosure [Line Items]    
Service cost 6 6
Interest cost 12 16
Expected return on plan assets (25) (25)
Amortization of prior service cost 2 2
Amortization of actuarial (gain) loss, net 4 4
Settlements 0 0
Special Termination Benefits 0 0
Net periodic (benefit) cost $ (1) $ 3