Employee Benefit Plans - Contributions and Benefit Payments (Details) - Dec. 31, 2017 £ in Millions, $ in Millions	USD ($)	GBP (£)
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year	$ 3
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months	54
Defined Benefit Plan, Expected Future Benefit Payment, Year Two	55
Defined Benefit Plan, Expected Future Benefit Payment, Year Three	57
Defined Benefit Plan, Expected Future Benefit Payment, Year Four	55
Defined Benefit Plan, Expected Future Benefit Payment, Year Five	54
Defined Benefit Plan, Expected Future Benefit Payment, Five Fiscal Years Thereafter	243
Domestic Plan [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year	39
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months	226
Defined Benefit Plan, Expected Future Benefit Payment, Year Two	224
Defined Benefit Plan, Expected Future Benefit Payment, Year Three	224
Defined Benefit Plan, Expected Future Benefit Payment, Year Four	222
Defined Benefit Plan, Expected Future Benefit Payment, Year Five	214
Defined Benefit Plan, Expected Future Benefit Payment, Five Fiscal Years Thereafter	979
UNITED KINGDOM | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year | £		£ 45
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months	72
Defined Benefit Plan, Expected Future Benefit Payment, Year Two	74
Defined Benefit Plan, Expected Future Benefit Payment, Year Three	75
Defined Benefit Plan, Expected Future Benefit Payment, Year Four	77
Defined Benefit Plan, Expected Future Benefit Payment, Year Five	79
Defined Benefit Plan, Expected Future Benefit Payment, Five Fiscal Years Thereafter	$ 427