Employee Benefit Plans (Net Periodic Benefit Costs and Other Changes Recognized in OCI) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Other changes in plan assets and benefit obligations recognized in OCI:
Total recognized in OCI	$ (181)	$ (30)	$ (263)
Pension Benefits
Net periodic benefit costs [Abstract]
Service costs	226	214	223
Interest costs	363	425	391
Settlement and curtailment costs	10	0	(1)
Expected return on plan assets	(528)	(489)	(533)
Amortization of net actuarial (gains) losses	189	201	182
Amortization of prior service costs (credit)	(14)	(15)	(3)
Total net periodic benefit costs (credit)	246	336	259
Other changes in plan assets and benefit obligations recognized in OCI:
Net actuarial (gains) losses	(35)	231	244
Prior service costs (credit)	0	3	(110)
Amortization of net actuarial (gains) losses	(189)	(201)	(182)
Amortization of prior service (costs) credit	14	15	3
Other Comprehensive Income (Loss), Defined Benefit Plan, Settlement and Curtailment Gain (Loss), before Tax	(10)	0	0
Total recognized in OCI	(215)	48	(45)
Total recognized in net periodic benefit costs and OCI	31	384	214
Pension Benefits | Accumulated Other Comprehensive Income (Loss) Foreign Currency Translation Adjustments
Other changes in plan assets and benefit obligations recognized in OCI:
Total recognized in OCI	5	0	0
Other Postretirement Benefits
Net periodic benefit costs [Abstract]
Service costs	5	5	6
Interest costs	42	53	55
Settlement and curtailment costs	0	2	0
Expected return on plan assets	(62)	(67)	(71)
Amortization of net actuarial (gains) losses	(74)	(48)	(34)
Amortization of prior service costs (credit)	(3)	(12)	(20)
Total net periodic benefit costs (credit)	(92)	(67)	(64)
Other changes in plan assets and benefit obligations recognized in OCI:
Net actuarial (gains) losses	(42)	(138)	(248)
Prior service costs (credit)	0	0	(7)
Amortization of net actuarial (gains) losses	74	48	34
Amortization of prior service (costs) credit	3	12	20
Other Comprehensive Income (Loss), Defined Benefit Plan, Settlement and Curtailment Gain (Loss), before Tax	0	0	0
Total recognized in OCI	35	(78)	(201)
Total recognized in net periodic benefit costs and OCI	(57)	(145)	(265)
Other Postretirement Benefits | Accumulated Other Comprehensive Income (Loss) Foreign Currency Translation Adjustments
Other changes in plan assets and benefit obligations recognized in OCI:
Total recognized in OCI	$ 0	$ 0	$ 0