Employee Benefits - Reconciliation of the changes in the benefit obligations and fair value of assets and a statement of the funded status (Details) - Pension Plan £ in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2018 GBP (£)
U.S.
Change in projected benefit obligation
Balance at the beginning of the Period	$ 3,192	$ 2,877
Service cost	0	0	$ 0
Interest cost	85	108	99
Plan amendment	0	0
Settlements	0	0
Actuarial loss (gain)	274	373
Benefit payments	(171)	(166)
Foreign currency impact	0	0
Balance at the end of the period	3,380	3,192	2,877
Accumulated benefit obligation at end of year	3,380	3,192
Change in fair value of plan assets
Balance at the beginning of the period	2,066	1,796
Actual return on plan assets	289	398
Employer contributions	92	38
Settlements	0	0
Benefit payments	(171)	(166)
Foreign currency impact	0	0
Balance at the end of the period	2,276	2,066	1,796
Market related value at end of year	2,076	1,969
Funded status	(1,104)	(1,126)
Unrecognized prior-service cost	0	1
Unrecognized loss	1,812	1,762
Net amount recognized	708	637
U.K.
Change in projected benefit obligation
Balance at the beginning of the Period	4,779	4,129
Service cost	0	0	0
Interest cost	88	109	109
Plan amendment	3	10
Settlements	(7)	(22)
Actuarial loss (gain)	520	594
Benefit payments	(209)	(168)
Foreign currency impact	232	127
Balance at the end of the period	5,406	4,779	4,129
Accumulated benefit obligation at end of year	5,406	4,779
Change in fair value of plan assets
Balance at the beginning of the period	5,959	5,225
Actual return on plan assets	618	687
Employer contributions	8	78
Settlements	(7)	(22)	(176)	£ (139)
Benefit payments	(209)	(168)
Foreign currency impact	283	159
Balance at the end of the period	6,652	5,959	5,225
Market related value at end of year	6,652	5,959
Funded status	1,246	1,180
Unrecognized prior-service cost	43	40
Unrecognized loss	1,286	1,204
Net amount recognized	2,575	2,424
Other
Change in projected benefit obligation
Balance at the beginning of the Period	1,425	1,271
Service cost	0	0	0
Interest cost	19	27	27
Plan amendment	0	0
Settlements	0	0
Actuarial loss (gain)	112	177
Benefit payments	(44)	(42)
Foreign currency impact	113	(8)
Balance at the end of the period	1,625	1,425	1,271
Accumulated benefit obligation at end of year	1,592	1,391
Change in fair value of plan assets
Balance at the beginning of the period	1,303	1,155
Actual return on plan assets	109	182
Employer contributions	20	19
Settlements	0	0
Benefit payments	(44)	(42)
Foreign currency impact	109	(11)
Balance at the end of the period	1,497	1,303	$ 1,155
Market related value at end of year	1,497	1,303
Funded status	(128)	(122)
Unrecognized prior-service cost	(7)	(6)
Unrecognized loss	521	460
Net amount recognized	$ 386	$ 332