Employee Benefit Plans - Employer Contributions (Details) - 6 months ended Jun. 30, 2017 £ in Millions, $ in Millions	USD ($)	GBP (£)
Domestic Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, expected contributions in current fiscal year	$ 15
Employer contributions	7
Domestic Other Postretirement Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, expected contributions in current fiscal year	5
Employer contributions	4
UK Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, expected contributions in current fiscal year | £		£ 39
Employer contributions	$ 25	£ 20