Employee Benefit Plans - Funded Status (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Defined Benefit Plan, Benefit Obligation, Increase (Decrease) for Plan Amendment	$ 0	$ 8,000,000
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Defined Benefit Plan, Plan Assets, Amount	742,000,000	664,000,000	$ 736,000,000	$ 742,000,000	$ 664,000,000
Defined Benefit Plan, Benefit Obligation	673,000,000	672,000,000	721,000,000	673,000,000	672,000,000
Defined Benefit Plan, Funded (Unfunded) Status of Plan				69,000,000	(8,000,000)
Other assets				76,000,000	5,000,000
Other current liabilities				0	0
Liability, Defined Benefit Plan, Noncurrent				(7,000,000)	(13,000,000)
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position				69,000,000	(8,000,000)
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Beginning balance	664,000,000	736,000,000
Defined Benefit Plan, Plan Assets, Contributions by Employer	2,000,000	8,000,000
Defined Benefit Plan, Contributions by Plan Participants (Deprecated 2017-01-31)	9,000,000	8,000,000
Defined Benefit Plan, Plan Assets, Increase (Decrease) for Actual Return (Loss)	122,000,000	(38,000,000)
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)	(55,000,000)	(50,000,000)
Ending balance	742,000,000	664,000,000	736,000,000
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Beginning balance	672,000,000	721,000,000
Defined Benefit Plan, Service Cost	8,000,000	9,000,000	9,000,000
Defined Benefit Plan, Interest Cost	27,000,000	24,000,000	29,000,000
Defined Benefit Plan, Contributions by Plan Participants (Deprecated 2017-01-31)	9,000,000	8,000,000
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss)	12,000,000	(40,000,000)
Defined Benefit Plan, Benefit Obligation, Increase (Decrease) for Plan Amendment	0	0
Benefits Paid	(55,000,000)	(50,000,000)
Ending balance	673,000,000	672,000,000	721,000,000
Defined Benefit Plan, Benefit Obligation, Payment for Settlement	0	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				417,000,000	126,000,000
Defined Benefit Plan, Pension Plan with Projected Benefit Obligation in Excess of Plan Assets, Projected Benefit Obligation				429,000,000	143,000,000
Defined Benefit Plan, Plan with Accumulated Benefit Obligation in Excess of Plan Assets, Plan Assets				439,000,000	417,000,000
Defined Benefit Plan, Pension Plan with Accumulated Benefit Obligation in Excess of Plan Assets, Projected Benefit Obligation				446,000,000	429,000,000
Supplemental Employee Retirement Plan [Member]
Defined Benefit Plans, Supplemental Employee Retirement Plans [Abstract]
Life Insurance, Corporate or Bank Owned, Amount				252,000,000	256,000,000
UNITED STATES | Pension Plan [Member]
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Defined Benefit Plan, Plan Assets, Amount	2,656,000,000	2,396,000,000	2,761,000,000	2,656,000,000	2,396,000,000
Defined Benefit Plan, Benefit Obligation	2,878,000,000	2,718,000,000	3,006,000,000	2,878,000,000	2,718,000,000
Defined Benefit Plan, Funded (Unfunded) Status of Plan				(222,000,000)	(322,000,000)
Other assets				73,000,000	20,000,000
Other current liabilities				(13,000,000)	(13,000,000)
Liability, Defined Benefit Plan, Noncurrent				(282,000,000)	(329,000,000)
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position				(222,000,000)	(322,000,000)
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Beginning balance	2,396,000,000	2,761,000,000
Defined Benefit Plan, Plan Assets, Contributions by Employer	12,000,000	38,000,000
Defined Benefit Plan, Contributions by Plan Participants (Deprecated 2017-01-31)	0	0
Defined Benefit Plan, Plan Assets, Increase (Decrease) for Actual Return (Loss)	456,000,000	(147,000,000)
Defined Benefit Plan, Accumulated Benefit Obligation, (Increase) Decrease for Settlement and Curtailment	0	21,000,000	0
Defined Benefit Plan, Plan Assets, Payment for Settlement	(22,000,000)	(119,000,000)
Defined Benefit Plan, Benefits Paid (Deprecated 2017-01-31)	(186,000,000)	(137,000,000)
Ending balance	2,656,000,000	2,396,000,000	2,761,000,000
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Beginning balance	2,718,000,000	3,006,000,000
Defined Benefit Plan, Service Cost	16,000,000	21,000,000	24,000,000
Defined Benefit Plan, Interest Cost	111,000,000	105,000,000	116,000,000
Defined Benefit Plan, Contributions by Plan Participants (Deprecated 2017-01-31)	0	0
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss)	242,000,000	(160,000,000)
Defined Benefit Plan, Benefit Obligation, Increase (Decrease) for Plan Amendment	(1,000,000)	2,000,000
Benefits Paid	(186,000,000)	(137,000,000)
Ending balance	2,878,000,000	2,718,000,000	3,006,000,000
Defined Benefit Plan, Accumulated Benefit Obligation				2,867,000,000	2,709,000,000
Defined Benefit Plan, Benefit Obligation, Payment for Settlement	(22,000,000)	(119,000,000)
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				1,752,000,000	2,016,000,000
Defined Benefit Plan, Pension Plan with Projected Benefit Obligation in Excess of Plan Assets, Projected Benefit Obligation				2,091,000,000	2,327,000,000
Defined Benefit Plan, Pension Plan with Projected Benefit Obligation in Excess of Plan Assets, Accumulated Benefit Obligation				2,085,000,000	2,316,000,000
Defined Benefit Plan, Plan with Accumulated Benefit Obligation in Excess of Plan Assets, Plan Assets				1,939,000,000	1,752,000,000
Defined Benefit Plan, Pension Plan with Accumulated Benefit Obligation in Excess of Plan Assets, Projected Benefit Obligation				2,227,000,000	2,091,000,000
Defined Benefit Plan, Plan with Accumulated Benefit Obligation in Excess of Plan Assets, Accumulated Benefit Obligation				2,222,000,000	2,085,000,000
UNITED KINGDOM | Pension Plan [Member]
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Defined Benefit Plan, Plan Assets, Amount	2,151,000,000	1,989,000,000	2,368,000,000	2,151,000,000	1,989,000,000
Defined Benefit Plan, Benefit Obligation	2,019,000,000	1,833,000,000	2,201,000,000	2,019,000,000	1,833,000,000
Defined Benefit Plan, Funded (Unfunded) Status of Plan				132,000,000	156,000,000
Other assets				132,000,000	156,000,000
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Beginning balance	1,989,000,000	2,368,000,000
Defined Benefit Plan, Plan Assets, Contributions by Employer	56,000,000	60,000,000
Defined Benefit Plan, Contributions by Plan Participants (Deprecated 2017-01-31)	1,000,000	1,000,000
Defined Benefit Plan, Plan Assets, Increase (Decrease) for Actual Return (Loss)	194,000,000	(44,000,000)
Defined Benefit Plan, Plan Assets, Payment for Settlement	(99,000,000)	(205,000,000)
Defined Benefit Plan, Benefits Paid (Deprecated 2017-01-31)	(71,000,000)	(71,000,000)
Foreign Currency Exchange Rate Changes	(81,000,000)	(120,000,000)
Ending balance	2,151,000,000	1,989,000,000	2,368,000,000
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Beginning balance	1,833,000,000	2,201,000,000
Defined Benefit Plan, Service Cost	16,000,000	19,000,000	23,000,000
Defined Benefit Plan, Interest Cost	49,000,000	56,000,000	58,000,000
Defined Benefit Plan, Contributions by Plan Participants (Deprecated 2017-01-31)	1,000,000	1,000,000
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss)	175,000,000	(87,000,000)
Benefits Paid	(71,000,000)	(71,000,000)
Foreign currency exchange rate changes	115,000,000	(112,000,000)
Ending balance	2,019,000,000	1,833,000,000	$ 2,201,000,000
Defined Benefit Plan, Accumulated Benefit Obligation				$ 1,786,000,000	$ 1,637,000,000
Defined Benefit Plan, Benefit Obligation, Payment for Settlement	$ (99,000,000)	$ (182,000,000)