Employee Benefit Plans - Contributions and Benefit Payments (Details) - 12 months ended Dec. 31, 2015 £ 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	$ 34
Defined Benefit Plan, Expected Future Benefit Payments, Next Twelve Months	221
Defined Benefit Plan, Expected Future Benefit Payments, Year Two	224
Defined Benefit Plan, Expected Future Benefit Payments, Year Three	226
Defined Benefit Plan, Expected Future Benefit Payments, Year Four	224
Defined Benefit Plan, Expected Future Benefit Payments, Year Five	225
Defined Benefit Plan, Expected Future Benefit Payments, Five Fiscal Years Thereafter	1,054
Domestic Other Postretirement Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plans, Estimated Future Employer Contributions in Next Fiscal Year	1
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	58
Defined Benefit Plan, Expected Future Benefit Payments, Year Four	58
Defined Benefit Plan, Expected Future Benefit Payments, Year Five	61
Defined Benefit Plan, Expected Future Benefit Payments, Five Fiscal Years Thereafter	272
UK Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plans, Estimated Future Employer Contributions in Next Fiscal Year | £		£ 40
Defined Benefit Plan, Expected Future Benefit Payments, Next Twelve Months	88
Defined Benefit Plan, Expected Future Benefit Payments, Year Two	90
Defined Benefit Plan, Expected Future Benefit Payments, Year Three	92
Defined Benefit Plan, Expected Future Benefit Payments, Year Four	95
Defined Benefit Plan, Expected Future Benefit Payments, Year Five	97
Defined Benefit Plan, Expected Future Benefit Payments, Five Fiscal Years Thereafter	$ 522