Employee Benefit Plans - Contributions and Benefit Payments (Details) - 12 months ended Dec. 31, 2016 £ in Millions, $ in Millions	USD ($)	GBP (£)
Domestic Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plans, Estimated Future Employer Contributions in Next Fiscal Year	$ 14
Defined Benefit Plan, Expected Future Benefit Payments, Next Twelve Months	219
Defined Benefit Plan, Expected Future Benefit Payments, Year Two	226
Defined Benefit Plan, Expected Future Benefit Payments, Year Three	224
Defined Benefit Plan, Expected Future Benefit Payments, Year Four	221
Defined Benefit Plan, Expected Future Benefit Payments, Year Five	214
Defined Benefit Plan, Expected Future Benefit Payments, Five Fiscal Years Thereafter	1,002
Domestic Other Postretirement Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plans, Estimated Future Employer Contributions in Next Fiscal Year	4
Defined Benefit Plan, Expected Future Benefit Payments, Next Twelve Months	56
Defined Benefit Plan, Expected Future Benefit Payments, Year Two	57
Defined Benefit Plan, Expected Future Benefit Payments, Year Three	57
Defined Benefit Plan, Expected Future Benefit Payments, Year Four	60
Defined Benefit Plan, Expected Future Benefit Payments, Year Five	57
Defined Benefit Plan, Expected Future Benefit Payments, Five Fiscal Years Thereafter	259
UK Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plans, Estimated Future Employer Contributions in Next Fiscal Year | £		£ 37
Defined Benefit Plan, Expected Future Benefit Payments, Next Twelve Months	75
Defined Benefit Plan, Expected Future Benefit Payments, Year Two	77
Defined Benefit Plan, Expected Future Benefit Payments, Year Three	79
Defined Benefit Plan, Expected Future Benefit Payments, Year Four	81
Defined Benefit Plan, Expected Future Benefit Payments, Year Five	83
Defined Benefit Plan, Expected Future Benefit Payments, Five Fiscal Years Thereafter	$ 448