Pension and Other Postretirement Employee Benefit Plans - Net Periodic Benefit Costs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Amortization:
Other defined benefit pension and OPEB plans	$ (81)	$ (72)	$ (72)
Total net periodic defined benefit plan cost/(credit)	(285)	144	(27)
Total defined contribution plans	1,332	952	872
Total pension and OPEB cost included in noninterest expense	1,047	1,096	845
Changes recognized in other comprehensive income
Total recognized in other comprehensive income	(214)	(1,157)	450
Total recognized in net periodic defined benefit plan cost/(credit) and other comprehensive income	(499)	(1,013)	423
Pension plans | U.S.
Components of net periodic benefit cost, U.S. defined benefit pension plans
Benefits earned during the year	2	327	323
Interest cost on benefit obligations	422	518	478
Expected return on plan assets	(634)	(776)	(836)
Amortization:
Net (gain)/loss	6	147	80
Prior service (credit)/cost	0	0	(21)
Curtailment (gain)/loss	0	0	21
Net periodic defined benefit plan cost/(credit), U.S. defined benefit pension plans	(204)	216	45
Changes recognized in other comprehensive income
Prior service (credit)/cost arising during the year	0	5	0
Net (gain)/loss arising during the year	(181)	(745)	453
Amortization of net (loss)/gain	(6)	(147)	(80)
Amortization of prior service (cost)/credit	0	0	21
Curtailment (loss)/gain	0	0	(21)
Total recognized in other comprehensive income	$ (187)	$ (887)	$ 373
Weighted-average assumptions used to determine net periodic benefit costs
Discount rate	3.30%	4.30%
Expected long-term rate of return on plan assets	4.00%	5.50%	5.50%
Rate of compensation increase		2.30%	2.30%
Interest crediting rate	4.65%	4.90%	4.90%
Pension plans | Minimum | U.S.
Weighted-average assumptions used to determine net periodic benefit costs
Discount rate			3.70%
Pension plans | Maximum | U.S.
Weighted-average assumptions used to determine net periodic benefit costs
Discount rate			4.50%
OPEB plans
Changes recognized in other comprehensive income
Total recognized in other comprehensive income	$ (27)	$ (270)	$ 77