Employee Benefits - Cash Flows Narrative (Details) - Pension Plan $ in Millions	Dec. 31, 2017 USD ($)
U.S.
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Expected employer contributions during next fiscal year	$ 92
U.K.
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Expected employer contributions during next fiscal year	63
Other
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Expected employer contributions during next fiscal year	$ 22