Employee Benefits - Weighted-average assumptions used to determine the net periodic benefit cost (Details)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
U.K.
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate	3.96%	3.70%	4.55%
Expected return on plan assets, net of administration expenses	4.55%	5.09%	6.00%
U.K. | Minimum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Rate of compensation increase	3.63%	3.55%	3.70%
U.K. | Maximum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Rate of compensation increase	4.13%	4.05%	4.40%
U.S.
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Expected return on plan assets, net of administration expenses	7.81%	7.96%	8.80%
U.S. | Minimum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate	3.69%	3.37%	3.97%
U.S. | Maximum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate	4.43%	4.08%	4.87%
Other | Minimum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate	2.43%	2.03%	3.60%
Expected return on plan assets, net of administration expenses	3.47%	3.99%	4.70%
Rate of compensation increase	2.00%	2.25%	2.25%
Other | Maximum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate	3.96%	3.91%	4.71%
Expected return on plan assets, net of administration expenses	4.95%	5.21%	6.50%
Rate of compensation increase	3.50%	3.50%	3.50%