Pensions - Reconciliations of fair value measurements using significant unobservable inputs (Level 3) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Rollforward of fair values
Fair value of plan assets at beginning of year	$ 24,202	$ 24,968	$ 23,162
Foreign exchange	(4,501)	(1,298)	(1,572)
Actual return on plan assets:
Fair value of plan assets at end of year	24,018	24,202	24,968
Annuities
Rollforward of fair values
Fair value of plan assets at beginning of year	2,026
Actual return on plan assets:
Fair value of plan assets at end of year	1,942	2,026
Level 3
Rollforward of fair values
Fair value of plan assets at beginning of year	2,026
Actual return on plan assets:
Fair value of plan assets at end of year	1,942	2,026
Level 3 | Annuities
Rollforward of fair values
Fair value of plan assets at beginning of year	2,026	2,186
Foreign exchange	(342)	(112)
Actual return on plan assets:
Assets still held at the reporting date	321	25
Purchases, sales and settlements, net	(63)	(73)
Fair value of plan assets at end of year	$ 1,942	$ 2,026	$ 2,186