Pension and Other Post-Retirement Benefits - Schedule of components of net periodic benefit cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	$ 634	$ 823	$ 1,367	$ 1,673
Interest cost	790	766	1,580	1,505
Expected return on plan assets	(627)	(643)	(1,253)	(1,308)
Amortization of net loss	448	410	895	735
Amortization of prior service (credit)/cost	(324)	(332)	(648)	(667)
Loss on settlement	0	15	0	545
Gain on curtailment			0	(296)
Net periodic benefit cost/(credit)	921	1,039	1,941	2,187
U.S. Plans | Defined Benefit
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	0	0	0	0
Interest cost	399	364	798	691
Expected return on plan assets	(451)	(408)	(902)	(836)
Amortization of net loss	245	300	490	600
Amortization of prior service (credit)/cost	0	0	0	0
Loss on settlement	0	15	0	545
Gain on curtailment			0	0
Net periodic benefit cost/(credit)	193	271	386	1,000
U.S. Plans | Retiree Healthcare
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	2	19	4	38
Interest cost	53	70	106	140
Expected return on plan assets	0	0	0	0
Amortization of net loss	11	0	22	0
Amortization of prior service (credit)/cost	(327)	(334)	(654)	(668)
Loss on settlement	0	0	0	0
Gain on curtailment			0	0
Net periodic benefit cost/(credit)	(261)	(245)	(522)	(490)
Non-U.S. Plans | Defined Benefit
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	632	804	1,363	1,635
Interest cost	338	332	676	674
Expected return on plan assets	(176)	(235)	(351)	(472)
Amortization of net loss	192	110	383	135
Amortization of prior service (credit)/cost	3	2	6	1
Loss on settlement	0	0	0	0
Gain on curtailment			0	(296)
Net periodic benefit cost/(credit)	$ 989	$ 1,013	$ 2,077	$ 1,677