Employee Benefits Plans - Valuation (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Valuation
Maximum percentage of unrecognized gains (losses) for amortization	10.00%
Amortization period for gains (losses) of defined benefit plans	15 years
Weighted-average assumptions used to determine net periodic benefit cost:
Expected asset return	3.87%
U.K. Pension Plan
Weighted-average assumptions used to determine benefit obligations:
Discount rate	2.80%	2.50%
Weighted-average assumptions used to determine net periodic benefit cost:
Discount rate	2.10%	2.70%
Expected asset return	3.87%	4.34%
U.K. Pension Plan | Minimum
Valuation
Historical performance period	15 years	10 years
U.K. Pension Plan | Maximum
Valuation
Historical performance period	20 years	20 years
Asterion Pension Plan
Weighted-average assumptions used to determine benefit obligations:
Discount rate	1.80%
Rate of compensation increase	2.50%
Weighted-average assumptions used to determine net periodic benefit cost:
Discount rate	1.80%
Expected asset return	1.80%
Rate of compensation increase	2.50%
Germany Pension Plan
Weighted-average assumptions used to determine benefit obligations:
Discount rate	1.90%	1.70%
Weighted-average assumptions used to determine net periodic benefit cost:
Discount rate	1.90%
Expected asset return	3.68%
Norway Pension Plan
Weighted-average assumptions used to determine benefit obligations:
Discount rate	2.60%	2.40%
Rate of compensation increase	1.75%	1.50%
Weighted-average assumptions used to determine net periodic benefit cost:
Discount rate	2.60%	2.40%
Expected asset return	4.30%	4.10%
Rate of compensation increase	1.75%	1.50%
Norway Pension Plan | Minimum
Valuation
Historical performance period	10 years
Norway Pension Plan | Maximum
Valuation
Historical performance period	20 years