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, 2019	Dec. 31, 2018	Dec. 31, 2017
Non-operating expense
Loss recognized due to settlements	$ 288
Total non-service expense		$ 688	$ 1,230	$ 913
PRB [Member]
Operating expense
Service cost		3	5	6
Non-operating expense
Interest cost		28	27	30
Expected return on plan assets		(18)	(21)	(21)
Amortization of prior service cost		0	0	(1)
Amortization of net actuarial loss		10	11	10
Loss recognized due to settlements		2	1	1
Total non-service expense		22	18	19
Net periodic expense (income)		$ 25	$ 23	$ 25