Postretirement Plans (Net Periodic Benefit Cost Tables) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 1
Interest cost	498	$ 614
Expected return on plan assets	(966)	(939)
Amortization of prior service credits	(20)	(20)
Recognized net actuarial loss	310	258
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement and Curtailment	1
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)	(176)	(87)
Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	22	21
Interest cost	23	36
Expected return on plan assets	(2)	(2)
Amortization of prior service credits	(9)	(9)
Recognized net actuarial loss	(17)	(12)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)	17	34
Operating Income (Loss) | Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit (income)/cost included in (Loss)/earnings before income taxes	(1)
Operating Income (Loss) | Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit (income)/cost included in (Loss)/earnings before income taxes	(22)	(21)
Other Income | Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit (income)/cost included in (Loss)/earnings before income taxes	(177)	(87)
Other Income | Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit (income)/cost included in (Loss)/earnings before income taxes	(5)	(13)
Operating Income (Loss) Before Taxes | Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit (income)/cost included in (Loss)/earnings before income taxes	(176)	(87)
Operating Income (Loss) Before Taxes | Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit (income)/cost included in (Loss)/earnings before income taxes	$ (17)	$ (34)