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Employee Benefit Plans - Components of Net Periodic Benefit Cost (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Pension Benefits        
Defined Benefit Plan Disclosure [Line Items]        
Service cost $ 57 $ 48 $ 171 $ 146
Interest cost 54 55 162 165
Expected return on plan assets (73) (61) (219) (182)
Amortization of unrecognized (gain) loss and prior service cost (credit) 32 32 97 95
Settlement loss 0 3 0 5
Total 70 77 211 229
Other Postretirement Benefits        
Defined Benefit Plan Disclosure [Line Items]        
Service cost 3 4 9 10
Interest cost 16 16 46 50
Expected return on plan assets (1) 0 (1) (1)
Amortization of unrecognized (gain) loss and prior service cost (credit) (17) (18) (52) (54)
Settlement loss 0 0 0 0
Total $ 1 $ 2 $ 2 $ 5