Pensions - Reconciliations of fair value measurements using significant unobservable inputs (Level 3) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Rollforward of fair values
Fair value of plan assets at beginning of year	$ 28,232	$ 24,018	$ 24,202
Foreign exchange	(1,626)	2,426	(4,501)
Actual return on plan assets:
Fair value of plan assets at end of year	25,564	28,232	24,018
Annuities
Rollforward of fair values
Fair value of plan assets at beginning of year	2,179
Actual return on plan assets:
Fair value of plan assets at end of year	1,892	2,179
Level 3
Rollforward of fair values
Fair value of plan assets at beginning of year	2,179
Actual return on plan assets:
Fair value of plan assets at end of year	1,892	2,179
Level 3 | Annuities
Rollforward of fair values
Fair value of plan assets at beginning of year	2,179	1,942
Foreign exchange	(129)	190
Actual return on plan assets:
Assets still held at the reporting date	(88)	120
Purchases, sales and settlements, net	(70)	(73)
Fair value of plan assets at end of year	$ 1,892	$ 2,179	$ 1,942