Postretirement Plans (Net Periodic Benefit Cost Tables) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost		$ 1
Interest cost	614	731
Expected return on plan assets	(939)	(965)
Amortization of prior service credits	(20)	(20)
Recognized net actuarial loss	258	161
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)	(87)	(92)
Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	21	19
Interest cost	36	49
Expected return on plan assets	(2)	(2)
Amortization of prior service credits	(9)	(9)
Recognized net actuarial loss	(12)	(11)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)	34	46
Operating Income (Loss) | Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit (income)/cost included in (Loss)/earnings before income taxes		78
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	21	22
Other Income | Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit (income)/cost included in (Loss)/earnings before income taxes	(87)	(93)
Other Income | Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit (income)/cost included in (Loss)/earnings before income taxes	13	27
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	(87)	(15)
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	$ 34	$ 49