Other intangible assets - Rollforward of other intangible assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Carrying amount:
Balance at the beginning of the period	$ 17,738	$ 17,384
Additions	6,100	177
Disposals	(247)
Foreign currency translation adjustment	549	177
Balance at the end of the period	24,140	17,738	$ 17,384
Accumulated amortization:
Balance at the beginning of the period	3,861	3,533
Charge for the year	600	561
Disposals	(247)
Foreign currency translation adjustment	148	(233)
Balance at the end of the period	4,362	3,861	3,533
Net book value:
Net book value	19,778	13,877	13,851
Trade names
Carrying amount:
Balance at the beginning of the period	7,579	7,100
Additions	6,100	0
Foreign currency translation adjustment	322	479
Balance at the end of the period	14,001	7,579	7,100
Net book value:
Net book value	14,001	7,579	7,100
Favorable lease assets
Carrying amount:
Balance at the beginning of the period	8,501	8,493
Additions	0	0
Disposals	0
Foreign currency translation adjustment	59	8
Balance at the end of the period	8,560	8,501	8,493
Accumulated amortization:
Balance at the beginning of the period	2,636	2,273
Charge for the year	434	373
Disposals	0
Foreign currency translation adjustment	22	(10)
Balance at the end of the period	3,092	2,636	2,273
Net book value:
Net book value	5,468	5,865	6,220
Internet sites
Carrying amount:
Balance at the beginning of the period	1,658	1,791
Additions	0	177
Disposals	(247)
Foreign currency translation adjustment	168	(310)
Balance at the end of the period	1,579	1,658	1,791
Accumulated amortization:
Balance at the beginning of the period	1,225	1,260
Charge for the year	166	188
Disposals	(247)
Foreign currency translation adjustment	126	(223)
Balance at the end of the period	1,270	1,225	1,260
Net book value:
Net book value	$ 309	$ 433	$ 531