Employee Benefit Plans - Employer Contributions (Details) - 3 months ended Mar. 31, 2019 £ 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	$ 1
Domestic Plan [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, expected contributions in current fiscal year	13
Employer contributions	3
UNITED KINGDOM | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, expected contributions in current fiscal year | £			£ 44
Employer contributions	$ 14	£ 11