Employee Benefit Plans - Contributions and Benefit Payments (Details) - Dec. 31, 2019 £ in Millions, $ in Millions	USD ($)	GBP (£)
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months	$ 57
Defined Benefit Plan, Expected Future Benefit Payment, Year Two	56
Defined Benefit Plan, Expected Future Benefit Payment, Year Three	55
Defined Benefit Plan, Expected Future Benefit Payment, Year Four	54
Defined Benefit Plan, Expected Future Benefit Payment, Year Five	51
Defined Benefit Plan, Expected Future Benefit Payment, Five Fiscal Years Thereafter	224
UNITED KINGDOM | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year | £		£ 43
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months	74
Defined Benefit Plan, Expected Future Benefit Payment, Year Two	75
Defined Benefit Plan, Expected Future Benefit Payment, Year Three	77
Defined Benefit Plan, Expected Future Benefit Payment, Year Four	79
Defined Benefit Plan, Expected Future Benefit Payment, Year Five	81
Defined Benefit Plan, Expected Future Benefit Payment, Five Fiscal Years Thereafter	436
UNITED STATES | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year	13
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months	233
Defined Benefit Plan, Expected Future Benefit Payment, Year Two	218
Defined Benefit Plan, Expected Future Benefit Payment, Year Three	213
Defined Benefit Plan, Expected Future Benefit Payment, Year Four	212
Defined Benefit Plan, Expected Future Benefit Payment, Year Five	205
Defined Benefit Plan, Expected Future Benefit Payment, Five Fiscal Years Thereafter	$ 927