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Employee Postretirement Benefits (Components of Net Periodic Benefit Cost) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Defined Benefit Pension Plans      
Service cost $ 41 $ 42 $ 38
Interest cost 129 148 187
Expected return on plan assets [1] (156) (158) (215)
Recognized net actuarial loss 57 52 75
Settlements 7 1 1,357
Other (9) (9) (10)
Net periodic benefit cost 69 76 1,432
Other Postretirement Benefit Plans      
Service cost 12 11 12
Interest cost 32 33 32
Expected return on plan assets [1] 0 0 0
Recognized net actuarial loss 1 0 0
Settlements 0 0 0
Other (2) (1) (1)
Net periodic benefit cost $ 43 $ 43 $ 43
[1] The expected return on plan assets is determined by multiplying the fair value of plan assets at the remeasurement date, typically the prior year-end adjusted for estimated current year cash benefit payments and contributions, by the expected long-term rate of return.