Employee Benefits - Pension Plans Narrative (Details) £ in Millions, $ in Millions	3 Months Ended		12 Months Ended
	Dec. 31, 2018 USD ($)	Dec. 31, 2018 GBP (£)	Dec. 31, 2020 USD ($)	Dec. 31, 2020 GBP (£)	Dec. 31, 2019 USD ($)	Dec. 31, 2019 GBP (£)	Dec. 31, 2018 USD ($)	Dec. 31, 2018 GBP (£)	Nov. 30, 2020 USD ($)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Percentage of the Company's projected benefit obligation			91.00%
U.S. | Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Plans with PBO in excess of plan assets, PBO			$ 3,300		$ 3,100
Plans with ABO in excess of the fair value of plan assets, ABO			3,300		3,100
Plans with ABO in excess of the fair value of plan assets, fair value of plan assets			2,200		2,000
Payment for settlements			0		0
Non-cash settlement charge			0		0		$ 0
U.K. | Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Estimated retirement benefits covered by insurance contract, impact amount									$ 94
Plans with PBO in excess of plan assets, PBO			54		54
Plans with ABO in excess of the fair value of plan assets, ABO			54		54
Plans with ABO in excess of the fair value of plan assets, fair value of plan assets			32
Plans with PBO in excess of plan assets, plan assets					34
Payment for settlements			7		22		176	£ 139
Reduction of projected benefit obligation	$ 154	£ 122
Non-cash settlement charge			2	£ 2	5	£ 4	37	£ 28
Other | Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Plans with PBO in excess of plan assets, PBO			1,600		1,400
Plans with ABO in excess of the fair value of plan assets, ABO			443		406
Plans with ABO in excess of the fair value of plan assets, fair value of plan assets			342		311
Plans with PBO in excess of plan assets, plan assets			1,400		1,300
Payment for settlements			0		0
Non-cash settlement charge			$ 0		$ 0		$ 0