Income taxes - Reconciliation of unrecognized tax benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reconciliation of unrecognized tax benefits
Balance at the beginning of the year	$ 3,678	$ 3,437	$ 2,988
Additional uncertain tax provision identified during the year	78	78	523
Increase to uncertain tax provision on prior year positions	4	236	49
Uncertain tax provisions settled during the year	(3,308)		(49)
Decreases as a result of expiration of the statute of limitations	(124)	(35)	(27)
Foreign exchange	(10)	(38)	(47)
Balance at the end of the year	318	3,678	3,437
Principal
Reconciliation of unrecognized tax benefits
Balance at the beginning of the year	2,967	2,966	2,935
Additional uncertain tax provision identified during the year	61	62	90
Increase to uncertain tax provision on prior year positions	0	0	49
Uncertain tax provisions settled during the year	(2,668)		(49)
Decreases as a result of expiration of the statute of limitations	(104)	(30)	(20)
Foreign exchange	(7)	(31)	(39)
Balance at the end of the year	249	2,967	2,966
Interest
Reconciliation of unrecognized tax benefits
Balance at the beginning of the year	656	416	7
Additional uncertain tax provision identified during the year	4	7	413
Increase to uncertain tax provision on prior year positions	4	236	0
Uncertain tax provisions settled during the year	(640)		0
Decreases as a result of expiration of the statute of limitations	(2)	(2)	(3)
Foreign exchange	(1)	(1)	(1)
Balance at the end of the year	21	656	416
Penalties
Reconciliation of unrecognized tax benefits
Balance at the beginning of the year	55	55	46
Additional uncertain tax provision identified during the year	13	9	20
Increase to uncertain tax provision on prior year positions	0	0	0
Uncertain tax provisions settled during the year	0		0
Decreases as a result of expiration of the statute of limitations	(18)	(3)	(4)
Foreign exchange	(2)	(6)	(7)
Balance at the end of the year	$ 48	$ 55	$ 55