Pension and Other Employee Benefits (Schedule of Components of Net Periodic Benefit Expense (Income) PRB Benefits) (Details) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Sep. 30, 2018
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Non-operating expense        
Loss recognized due to settlements $ 288      
Total non-service expense   $ 1,230 $ 913 $ 601
PRB [Member]        
Operating expense        
Service cost   5 6 6
Non-operating expense        
Interest cost   27 30 31
Expected return on plan assets   (21) (21) (25)
Amortization of prior service cost   0 (1) (1)
Amortization of net actuarial loss   11 10 3
Loss recognized due to settlements   1 1 2
Total non-service expense   18 19 10
Net periodic expense (income)   $ 23 $ 25 $ 16