Income taxes - Reconciliation of unrecognized tax benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reconciliation of unrecognized tax benefits
Balance at the beginning of the year	$ 532	$ 318	$ 3,678
Additional uncertain tax provision identified during the year	128	215	78
Increase to uncertain tax provision on prior year positions	21	27	4
Uncertain tax provisions settled during the year			(3,308)
Decreases as a result of expiration of the statute of limitations	(103)	(82)	(124)
Foreign exchange		54
Foreign exchange	(25)		(10)
Balance at the end of the year	553	532	318
Principal
Reconciliation of unrecognized tax benefits
Balance at the beginning of the year	438	249	2,967
Additional uncertain tax provision identified during the year	109	197	61
Increase to uncertain tax provision on prior year positions	9	17	0
Uncertain tax provisions settled during the year			(2,668)
Decreases as a result of expiration of the statute of limitations	(78)	(69)	(104)
Foreign exchange		44
Foreign exchange	(20)		(7)
Balance at the end of the year	458	438	249
Interest
Reconciliation of unrecognized tax benefits
Balance at the beginning of the year	26	21	656
Additional uncertain tax provision identified during the year	1	1	4
Increase to uncertain tax provision on prior year positions	12	7	4
Uncertain tax provisions settled during the year			(640)
Decreases as a result of expiration of the statute of limitations	(8)	(3)	(2)
Foreign exchange		0
Foreign exchange	(2)		(1)
Balance at the end of the year	29	26	21
Penalties
Reconciliation of unrecognized tax benefits
Balance at the beginning of the year	68	48	55
Additional uncertain tax provision identified during the year	18	17	13
Increase to uncertain tax provision on prior year positions	0	3	0
Uncertain tax provisions settled during the year			0
Decreases as a result of expiration of the statute of limitations	(17)	(10)	(18)
Foreign exchange		10
Foreign exchange	(3)		(2)
Balance at the end of the year	$ 66	$ 68	$ 48