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Pension and Other Postretirement Benefits - Funded Status (Details) - United States [Member] - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Pension Benefits [Member]      
Change in benefit obligation [Abstract]      
Benefit obligation, January 1 $ 4,888 $ 4,934  
Service cost 3 2 $ 2
Interest cost 186 199 194
(Gain) loss from past experience 181 57  
Benefits paid from plan assets (277) (284)  
Benefits paid - other (12) (20)  
Benefit obligation, December 31 4,969 4,888 4,934
Change in plan assets [Roll Forward]      
Fair value of plan assets, January 1 3,977 3,981  
Actual return on plan assets 418 279  
Benefits paid (277) (284)  
Contributions 163 1  
Fair value of plan assets, December 31 4,281 3,977 3,981
Funded Status (688) (911)  
Pension Benefits [Member] | Qualified Plan [Member]      
Change in plan assets [Roll Forward]      
Expected total pension plan contributions for next fiscal year 0    
Pension Benefits [Member] | Non-qualifed Plan [Member]      
Change in plan assets [Roll Forward]      
Fair value of plan assets, December 31 0    
Other Postretirement Benefits [Member]      
Change in benefit obligation [Abstract]      
Benefit obligation, January 1 277 295  
Service cost 0 0 0
Interest cost 9 11 11
(Gain) loss from past experience 1 2  
Benefits paid from plan assets (3) (3)  
Benefits paid - other (26) (28)  
Benefit obligation, December 31 258 277 295
Change in plan assets [Roll Forward]      
Fair value of plan assets, January 1 5 8  
Actual return on plan assets 0 0  
Benefits paid (3) (3)  
Contributions 0 0  
Fair value of plan assets, December 31 2 5 $ 8
Funded Status (256) $ (272)  
Other Postretirement Benefits [Member] | Non-qualifed Plan [Member]      
Change in plan assets [Roll Forward]      
Fair value of plan assets, December 31 $ 0