Retirement Benefit Plans (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans
Plan contributions	$ 37		$ 151
Pension Plans | Domestic Plan
Defined Benefit Plans and Other Postretirement Benefit Plans
Service cost	0	$ 0	0	$ 0
Interest cost	11	390	22	780
Expected return on plan assets	0	(586)	0	(1,171)
Amortization of net actuarial loss	11	11	24	21
Amortization of prior service cost	0	0	0	0
Foreign currency	0	0	0	0
Net periodic cost (credit)	22	(185)	46	(370)
Settlement charge	0	205	50	205
Net benefit cost (credit)	22	20	96	(165)
Pension Plans | Foreign Plan
Defined Benefit Plans and Other Postretirement Benefit Plans
Service cost	35	24	72	48
Interest cost	87	129	158	219
Expected return on plan assets	(82)	(101)	(191)	(197)
Amortization of net actuarial loss	0	0	0	0
Amortization of prior service cost	32	49	106	96
Foreign currency	(23)	234	(94)	229
Net periodic cost (credit)	49	335	51	395
Settlement charge	0	0	0	0
Net benefit cost (credit)	49	335	51	395
Postretirement Medical Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans
Plan contributions	287		556
Service cost	14	20	28	40
Interest cost	75	90	150	181
Expected return on plan assets	0	0	0	0
Amortization of net actuarial loss	0	0	0	0
Amortization of prior service cost	0	0	0	0
Foreign currency	0	0	0	0
Net periodic cost (credit)	89	110	178	221
Settlement charge	0	0	0	0
Net benefit cost (credit)	$ 89	$ 110	$ 178	$ 221