Accumulated Other Comprehensive Loss (Summary of Changes in AOCI) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated Other Comprehensive Income (Loss), Net of Tax	$ (3,921)	$ (11,788)	$ (3,734)	$ (10,149)
Other comprehensive (loss) income before reclassifications, net - Foreign Currency Translation	(176)	(1,445)
Amounts reclassified, pretax - Foreign Currency Translation	0	0
Tax benefit (expense) - Foreign Currency Translation	(5)	9
Other comprehensive (loss) income before reclassifications, net - Pension	(10)	8
Amounts reclassified, pretax - Pension	64	102
Tax benefit (expense) - Pension	(12)	(31)
Other comprehensive (loss) income before reclassifications, net - Unrealized Hedging (Losses) Gains	(46)	(403)
Amounts reclassified, pretax - Unrealized Hedging (Losses) Gains	(14)	29
Tax benefit (expense) - Unrealized Hedging (Losses) Gains	12	92
Other comprehensive (loss) income before reclassifications, net	(232)	(1,840)
Amounts reclassified, pre-tax	50	131
Tax benefit (expense)	(5)	70
Foreign Currency Translation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated Other Comprehensive Income (Loss), Net of Tax	529	(4,647)	710	(3,211)
Defined Benefit Pension and Post-retirement Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated Other Comprehensive Income (Loss), Net of Tax	(4,441)	(6,693)	(4,483)	(6,772)
Unrealized hedging (losses) gains
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated Other Comprehensive Income (Loss), Net of Tax	$ (9)	$ (448)	$ 39	$ (166)