Employee Benefit Plans - Plan Amendments (Details) £ in Millions, $ in Millions	1 Months Ended		12 Months Ended
	Oct. 31, 2020 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2020 GBP (£)	Dec. 31, 2020 USD ($)	Dec. 31, 2018 GBP (£)	Dec. 31, 2018 USD ($)
Pension Plan | United States | United States of America, Dollars
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Cost of equalization					$ 4		$ 42
Pension Plan | U.K.
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Cost of equalization, percent of defined benefit plan obligation (percent)				1.20%	1.20%
Pension Plan | U.K. | United Kingdom, Pounds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Cost of equalization | £				£ 3		£ 33
Other Postretirement Benefits Plan | United States
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Accumulated benefit obligation, increase (decrease) for plan amendment	$ (11)		$ 283
Benefit obligation, increase (decrease) for plan amendment			216
Actuarial gain, increase (decrease) for plan amendment			$ 67
Other Postretirement Benefits Plan | Canada
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Accumulated benefit obligation, increase (decrease) for plan amendment		$ (19)
United States | U.K.
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Annual amortization to prior service cost					$ 2