Other intangible assets - Rollforward of other intangible assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Carrying amount:
Balance at the beginning of the period	$ 17,384	$ 17,574
Additions	177	714
Foreign currency translation adjustment	177	(904)
Balance at the end of the period	17,738	17,384	$ 17,574
Accumulated amortization:
Balance at the beginning of the period	3,533	3,616
Charge for the year	561	531
Foreign currency translation adjustment	(233)	(614)
Balance at the end of the period	3,861	3,533	3,616
Net book value:
Net book value	13,877	13,851	13,958
Trade names
Carrying amount:
Balance at the beginning of the period	7,100	7,100
Additions	0	0
Foreign currency translation adjustment	479	0
Balance at the end of the period	7,579	7,100	7,100
Net book value:
Net book value	7,579	7,100	7,100
Favorable lease assets
Carrying amount:
Balance at the beginning of the period	8,493	8,586
Additions	0	714
Foreign currency translation adjustment	8	(807)
Balance at the end of the period	8,501	8,493	8,586
Accumulated amortization:
Balance at the beginning of the period	2,273	2,486
Charge for the year	373	341
Foreign currency translation adjustment	(10)	(554)
Balance at the end of the period	2,636	2,273	2,486
Net book value:
Net book value	5,865	6,220	6,100
Internet sites
Carrying amount:
Balance at the beginning of the period	1,791	1,888
Additions	177	0
Foreign currency translation adjustment	(310)	(97)
Balance at the end of the period	1,658	1,791	1,888
Accumulated amortization:
Balance at the beginning of the period	1,260	1,130
Charge for the year	188	190
Foreign currency translation adjustment	(223)	(60)
Balance at the end of the period	1,225	1,260	1,130
Net book value:
Net book value	$ 433	$ 531	$ 758