Employee Benefit Plans - Benefit Obligations, Fair Value of Plan Assets and Funded Status (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension Benefits
Change in Benefit Obligation
Benefit obligation at beginning of period		$ 21,199	$ 19,809
Service cost		0	0	$ 0
Interest cost		700	833	781
Actuarial loss		2,051	1,678
Benefits paid, including lump sums and annuities		(1,233)	(1,107)
Participant contributions		0	0
Special termination benefits		0	0
Settlements		(91)	(14)
Benefit obligation at end of period		22,626	21,199	19,809
Change in Fair Value of Plan Assets
Fair value of plan assets at beginning of period		15,845	13,459
Actual gain on plan assets		1,973	2,485
Employer contributions		47	1,022
Participant contributions		0	0
Benefits paid, including lump sums and annuities		(1,233)	(1,107)
Settlements		(91)	(14)
Fair value of plan assets at end of period		16,541	15,845	13,459
Funded Status of Plan
Funded status at end of period		(6,085)	(5,354)
Other Postretirement and Postemployment Benefits
Change in Benefit Obligation
Benefit obligation at beginning of period		3,379	3,225
Service cost		96	83	85
Interest cost		120	137	126
Actuarial loss		247	226
Benefits paid, including lump sums and annuities		(356)	(315)
Participant contributions		20	23
Special termination benefits	$ 1,300	1,260	0
Settlements		0	0
Benefit obligation at end of period		4,766	3,379	3,225
Change in Fair Value of Plan Assets
Fair value of plan assets at beginning of period		607	637
Actual gain on plan assets		76	134
Employer contributions		189	159
Participant contributions		20	23
Benefits paid, including lump sums and annuities		(396)	(346)
Settlements		0	0
Fair value of plan assets at end of period		496	607	$ 637
Funded Status of Plan
Funded status at end of period		$ (4,270)	$ (2,772)