Employee Benefit Plans - Employer Contributions (Details) - 9 months ended Sep. 30, 2017 £ in Millions, $ in Millions	USD ($)	GBP (£)	GBP (£)
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, expected contributions in current fiscal year	$ 5
Employer contributions	5
Domestic Plan [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, expected contributions in current fiscal year	15
Employer contributions	9
UNITED KINGDOM | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, expected contributions in current fiscal year | £			£ 45
Employer contributions	$ 43	£ 34