Pension and Postretirement Benefit Plans - Change in Benefit Obligation and Plan Assets and Amounts Recognized in Balance Sheet and AOCI (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Amounts recognized in the Consolidated Balance Sheet
Non-current assets	$ 230	$ 630	$ 230
Accrued benefit cost
Current liabilities	(67)	(71)	(67)
Non-current liabilities	(3,911)	(4,405)	(3,911)
Funded | Qualified and Non-qualified Pension Benefits
Change in plan assets
Company contributions		153	207
Funded | Postretirement Benefits
Change in benefit obligation
Benefit obligation at beginning of year		2,242	2,175
Service cost		43	43	$ 52
Interest cost		62	82	79
Foreign exchange rate changes		(14)
Plan amendments			(171)
Actuarial (gain) loss		176	225
Benefit payments		(107)	(112)
Settlements, curtailments, special termination benefits and other		(5)
Benefit obligation at end of year	2,242	2,397	2,242	2,175
Change in plan assets
Fair value, beginning balance		1,338	1,260
Actual return on plan assets		147	187
Company contributions		3	3
Benefit payments		(107)	(112)
Settlements, curtailments, special termination benefits and other		(5)
Fair value, ending balance	1,338	1,376	1,338	1,260
Funded status at end of year	(904)	(1,021)	(904)
Accrued benefit cost
Current liabilities	(4)	(4)	(4)
Non-current liabilities	(900)	(1,017)	(900)
Ending balance	(904)	(1,021)	(904)
Amounts recognized in accumulated other comprehensive income
Net actuarial loss (gain)	663	713	663
Prior service cost (credit)	(262)	(230)	(262)
Ending balance	401	483	401
Funded | United States | Qualified and Non-qualified Pension Benefits
Change in benefit obligation
Benefit obligation at beginning of year		17,935	15,948
Service cost		261	251	288
Interest cost		499	620	563
Actuarial (gain) loss		1,785	2,209
Benefit payments		(1,104)	(1,128)
Settlements, curtailments, special termination benefits and other	32		35
Benefit obligation at end of year	17,935	19,376	17,935	15,948
Change in plan assets
Fair value, beginning balance		16,099	14,803
Actual return on plan assets		2,071	2,323
Company contributions		61	101
Benefit payments		(1,104)	(1,128)
Fair value, ending balance	16,099	17,127	16,099	14,803
Funded status at end of year	(1,836)	(2,249)	(1,836)
Accrued benefit cost
Current liabilities	(48)	(52)	(48)
Non-current liabilities	(1,788)	(2,197)	(1,788)
Ending balance	(1,836)	(2,249)	(1,836)
Amounts recognized in accumulated other comprehensive income
Net actuarial loss (gain)	5,899	6,080	5,899
Prior service cost (credit)	(128)	(104)	(128)
Ending balance	5,771	5,976	5,771
Accumulated benefit obligations in excess of plan assets
Projected benefit obligation	17,935	19,376	17,935
Accumulated benefit obligation	17,125	18,441	17,125
Fair value of plan assets	16,099	17,127	16,099
Total Accumulated Benefit Obligation	17,125	18,441	17,125
Funded | United States | Postretirement Benefits
Change in plan assets
Fair value, beginning balance		1,338
Fair value, ending balance	1,338	1,376	1,338
Funded | International | Qualified and Non-qualified Pension Benefits
Change in benefit obligation
Benefit obligation at beginning of year		7,931	6,965
Acquisitions/Transfers		1	9
Service cost		152	131	143
Interest cost		117	156	157
Participant contributions		9	7
Foreign exchange rate changes		427	55
Plan amendments			3
Actuarial (gain) loss		464	906
Benefit payments		(274)	(302)
Settlements, curtailments, special termination benefits and other		(57)	1
Benefit obligation at end of year	7,931	8,770	7,931	6,965
Change in plan assets
Fair value, beginning balance		6,923	6,170
Acquisitions/Transfers			4
Actual return on plan assets		1,102	858
Company contributions		92	106
Participant contributions		9	7
Foreign exchange rate changes		376	80
Benefit payments		(274)	(302)
Settlements, curtailments, special termination benefits and other		(34)
Fair value, ending balance	6,923	8,194	6,923	$ 6,170
Funded status at end of year	(1,008)	(576)	(1,008)
Amounts recognized in the Consolidated Balance Sheet
Non-current assets	230	630	230
Accrued benefit cost
Current liabilities	(15)	(15)	(15)
Non-current liabilities	(1,223)	(1,191)	(1,223)
Ending balance	(1,008)	(576)	(1,008)
Amounts recognized in accumulated other comprehensive income
Net transition obligation (asset)	10	9	10
Net actuarial loss (gain)	1,967	1,557	1,967
Prior service cost (credit)	(5)	(2)	(5)
Ending balance	1,972	1,564	1,972
Accumulated benefit obligations in excess of plan assets
Projected benefit obligation	2,986	3,385	2,986
Accumulated benefit obligation	2,752	3,119	2,752
Fair value of plan assets	1,778	2,199	1,778
Total Accumulated Benefit Obligation	$ 7,355	$ 8,181	$ 7,355