Employee Benefit Plans - Employer Contributions (Details) - 6 months ended Jun. 30, 2018 £ 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	$ 7
Domestic Plan [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, expected contributions in current fiscal year	37
Employer contributions	6
UNITED KINGDOM | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, expected contributions in current fiscal year | £			£ 46
Employer contributions	$ 32	£ 23