Pension and Other Postretirement Benefits (Funded Status and Amounts Recognized in Accumulated Other Comprehensive Income (Loss) on the Consolidated Balance Sheets) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Pension benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial (loss) gain	$ (2,144)	$ (2,153)
Net prior service cost	(1)	(3)
Share of equity investee	(2)	(2)
Net amount recognized in accumulated other comprehensive income (loss) before tax	(2,147)	(2,158)
Deferred income taxes	560	563
Net amount recognized in accumulated other comprehensive income (loss)	(1,587)	(1,595)
Accumulated benefit obligation for all defined benefit pension plans	5,160	4,870
Postretirement benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial (loss) gain	140	147
Net prior service cost	0	(1)
Share of equity investee	0	0
Net amount recognized in accumulated other comprehensive income (loss) before tax	140	146
Deferred income taxes	(13)	(14)
Net amount recognized in accumulated other comprehensive income (loss)	$ 127	$ 132