Employee Benefit Plans - Funded Status (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2020	Dec. 31, 2019
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Defined Benefit Plan, Plan Assets, Amount	$ 744	$ 742	$ 664	$ 744	$ 742
Defined Benefit Plan, Benefit Obligation	673	673	672	758	673
Defined Benefit Plan, Funded (Unfunded) Status of Plan				(14)	69
Other assets				20	76
Other current liabilities				0	0
Liability, Defined Benefit Plan, Noncurrent				(34)	(7)
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position				(14)	69
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Beginning balance	742	664
Defined Benefit Plan, Plan Assets, Contributions by Employer	3	2
Defined Benefit Plan, Contributions by Plan Participants (Deprecated 2017-01-31)	8	9
Defined Benefit Plan, Plan Assets, Increase (Decrease) for Actual Return (Loss)	40	122
Defined Benefit Plan, Accumulated Benefit Obligation, (Increase) Decrease for Settlement and Curtailment	0	0	0
Defined Benefit Plan, Plan Assets, Payment for Settlement	0	0
Defined Benefit Plan, Benefits Paid (Deprecated 2017-01-31)	(49)	(55)
Ending balance	744	742	664
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Beginning balance	673	672
Defined Benefit Plan, Service Cost	7	8	9
Defined Benefit Plan, Interest Cost	21	27	24
Defined Benefit Plan, Contributions by Plan Participants (Deprecated 2017-01-31)	8	9
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss)	61	12
Defined Benefit Plan, Benefit Obligation, Increase (Decrease) for Plan Amendment	0	0
Benefits paid	(49)	(55)
Ending balance	758	673	672
Defined Benefit Plan, Benefit Obligation, Payment for Settlement	0	0
Defined Benefit Plan, Benefit Obligation, Business Combination	37	0
Defined Benefit Plan, Pension Plan with Project Benefit Obligation in Excess of Plan Assets [Abstract]
Defined Benefit Plan, Pension Plan with Projected Benefit Obligation in Excess of Plan Assets, Plan Assets				417	439
Defined Benefit Plan, Pension Plan with Projected Benefit Obligation in Excess of Plan Assets, Projected Benefit Obligation				451	446
Supplemental Employee Retirement Plan [Member]
Defined Benefit Plans, Supplemental Employee Retirement Plans [Abstract]
Life Insurance, Corporate or Bank Owned, Amount				303	252
UNITED STATES | Pension Plan [Member]
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Defined Benefit Plan, Plan Assets, Amount	2,656	2,656	2,396	2,824	2,656
Defined Benefit Plan, Benefit Obligation	2,878	2,878	2,718	3,077	2,878
Defined Benefit Plan, Funded (Unfunded) Status of Plan				(253)	(222)
Other assets				43	73
Other current liabilities				(13)	(13)
Liability, Defined Benefit Plan, Noncurrent				(283)	(282)
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position				(253)	(222)
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Beginning balance	2,656	2,396
Defined Benefit Plan, Plan Assets, Contributions by Employer	13	12
Defined Benefit Plan, Contributions by Plan Participants (Deprecated 2017-01-31)	0	0
Defined Benefit Plan, Plan Assets, Increase (Decrease) for Actual Return (Loss)	373	456
Defined Benefit Plan, Accumulated Benefit Obligation, (Increase) Decrease for Settlement and Curtailment	0	0	21
Defined Benefit Plan, Plan Assets, Payment for Settlement	0	(22)
Defined Benefit Plan, Benefits Paid (Deprecated 2017-01-31)	(218)	(186)
Ending balance	2,824	2,656	2,396
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Beginning balance	2,878	2,718
Defined Benefit Plan, Service Cost	17	16	21
Defined Benefit Plan, Interest Cost	93	111	105
Defined Benefit Plan, Contributions by Plan Participants (Deprecated 2017-01-31)	0	0
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss)	226	242
Defined Benefit Plan, Benefit Obligation, Increase (Decrease) for Plan Amendment	0	(1)
Benefits paid	(218)	(186)
Ending balance	3,077	2,878	2,718
Defined Benefit Plan, Accumulated Benefit Obligation				2,999	2,867
Defined Benefit Plan, Benefit Obligation, Payment for Settlement	0	(22)
Defined Benefit Plan, Benefit Obligation, Business Combination	81	0
Defined Benefit Plan, Pension Plan with Project Benefit Obligation in Excess of Plan Assets [Abstract]
Defined Benefit Plan, Pension Plan with Projected Benefit Obligation in Excess of Plan Assets, Plan Assets				1,782	1,939
Defined Benefit Plan, Pension Plan with Projected Benefit Obligation in Excess of Plan Assets, Projected Benefit Obligation				2,069	2,227
Defined Benefit Plan, Plan with Accumulated Benefit Obligation in Excess of Plan Assets, Plan Assets				1,064	1,939
Defined Benefit Plan, Plan with Accumulated Benefit Obligation in Excess of Plan Assets, Accumulated Benefit Obligation				1,341	2,222
UNITED KINGDOM | Pension Plan [Member]
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Defined Benefit Plan, Plan Assets, Amount	2,334	1,989	1,989	2,334	2,151
Defined Benefit Plan, Benefit Obligation	2,205	2,019	1,833	2,205	2,019
Defined Benefit Plan, Funded (Unfunded) Status of Plan				129	132
Other assets				129	132
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Beginning balance	2,151	1,989
Defined Benefit Plan, Plan Assets, Contributions by Employer	56	56
Defined Benefit Plan, Contributions by Plan Participants (Deprecated 2017-01-31)	1	1
Defined Benefit Plan, Plan Assets, Increase (Decrease) for Actual Return (Loss)	181	194
Defined Benefit Plan, Plan Assets, Payment for Settlement	(63)	(99)
Defined Benefit Plan, Benefits Paid (Deprecated 2017-01-31)	(67)	(71)
Foreign Currency Exchange Rate Changes	75	81
Ending balance	2,334	2,151	1,989
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Beginning balance	2,019	1,833
Defined Benefit Plan, Service Cost	16	16	19
Defined Benefit Plan, Interest Cost	40	49	56
Defined Benefit Plan, Contributions by Plan Participants (Deprecated 2017-01-31)	1	1
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss)	188	175
Benefits paid	(67)	(71)
Foreign currency exchange rate changes	71	115
Ending balance	2,205	2,019	$ 1,833
Defined Benefit Plan, Accumulated Benefit Obligation				$ 1,963	$ 1,786
Defined Benefit Plan, Benefit Obligation, Payment for Settlement	$ (63)	$ (99)