v3.8.0.1
Employee Benefit Plans (Components of Net Periodic Benefit Cost Included in General and Administrative Expenses) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Pension Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Service cost $ 284 $ 253 $ 244
Interest cost 476 498 469
Expected return on plan assets (781) (754) (775)
Amortization of transition obligation 0 0 0
Amortization of prior service cost (3) (6) (8)
Amortization of actuarial (gain) loss, net 191 181 168
Settlements 13 7 5
Special termination benefits 4 2 4
Net periodic (benefit) cost 184 181 107
Other Postretirement Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Service cost 20 19 20
Interest cost 82 91 86
Expected return on plan assets (102) (105) (115)
Amortization of transition obligation 0 0 0
Amortization of prior service cost 0 (2) (5)
Amortization of actuarial (gain) loss, net 36 41 38
Settlements 0 0 0
Special termination benefits 0 0 0
Net periodic (benefit) cost $ 36 $ 44 $ 24