Employee Benefit Plans (Schedule of PRB Amounts Recognized in Other Comprehensive Income (Loss)) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Actuarial loss arising during the period	$ 202	$ 543	$ 1,819
Amortization of actuarial gain	373	170	344
Current year prior service cost (credit)	(2,095)	6	22
Amortization of prior service cost	(373)	(228)	$ (344)
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Actuarial loss arising during the period	47	(10)
Amortization of actuarial gain	12	12
Current year prior service cost (credit)	(7)	0
Amortization of prior service cost	3	42
Net settlement and curtailment	(1)	0
Other	5	2
Total recognized in other comprehensive (income) loss	59	46
Net recognized in net periodic benefit (income) cost and other comprehensive loss	$ 75	$ 24