Employee Benefits - Weighted-average assumptions used to determine the net periodic benefit cost (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
U.K. | Minimum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Rate of compensation increase	3.70%	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%	4.20%	4.13%
U.K. | Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate	2.63%	2.77%	3.96%
Expected return on plan assets, net of administration expenses	3.34%	3.36%	4.55%
U.S. | Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Expected return on plan assets, net of administration expenses	7.71%	7.88%	7.81%
U.S. | Pension Plan | Minimum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate	3.27%	3.53%	3.69%
U.S. | Pension Plan | Maximum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate	3.61%	4.11%	4.43%
Other | Minimum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Rate of compensation increase	1.00%	1.00%	2.00%
Other | Maximum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Rate of compensation increase	3.00%	3.50%	3.50%
Other | Pension Plan | Minimum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate	1.78%	1.85%	2.43%
Expected return on plan assets, net of administration expenses	1.70%	2.68%	3.47%
Other | Pension Plan | Maximum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate	3.39%	3.81%	3.96%
Expected return on plan assets, net of administration expenses	4.85%	5.15%	4.95%