Employee Benefits - Weighted-average assumptions used to determine future benefit obligations (Details)	Dec. 31, 2016	Dec. 31, 2015
U.K.
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate	2.77%	3.96%
Underlying price inflation	1.83%	1.88%
U.K. | Minimum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Rate of compensation increase	3.70%	3.63%
U.K. | Maximum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Rate of compensation increase	4.20%	4.13%
U.S. | Minimum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate	3.53%	3.69%
U.S. | Maximum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate	4.11%	4.43%
Other | Minimum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate	1.85%	2.43%
Rate of compensation increase	1.00%	2.00%
Underlying price inflation	2.00%	2.00%
Other | Maximum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate	3.81%	3.96%
Rate of compensation increase	3.50%	3.50%
Underlying price inflation	2.50%	2.50%