Other intangible assets - Rollforward of other intangible assets (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Carrying amount:
Balance at January 1, 2017			$ 17,738
Additions			6,100
Foreign currency translation adjustment			315
Balance at June 30, 2017	$ 24,153		24,153
Accumulated amortization:
Balance at January 1, 2017			3,861
Charge for the period	135	$ 148	268	$ 271
Foreign currency translation adjustment			105
Balance at June 30, 2017	4,234		4,234
Net book value:
Net book value	19,919		19,919		$ 13,877
Trade names
Carrying amount:
Balance at January 1, 2017			7,579
Additions			6,100
Foreign currency translation adjustment			124
Balance at June 30, 2017	13,803		13,803
Net book value:
Net book value	13,803		13,803		7,579
Favorable lease assets
Carrying amount:
Balance at January 1, 2017			8,501
Additions			0
Foreign currency translation adjustment			94
Balance at June 30, 2017	8,595		8,595
Accumulated amortization:
Balance at January 1, 2017			2,636
Charge for the period			184
Foreign currency translation adjustment			33
Balance at June 30, 2017	2,853		2,853
Net book value:
Net book value	5,742		5,742		5,865
Internet sites
Carrying amount:
Balance at January 1, 2017			1,658
Additions			0
Foreign currency translation adjustment			97
Balance at June 30, 2017	1,755		1,755
Accumulated amortization:
Balance at January 1, 2017			1,225
Charge for the period			84
Foreign currency translation adjustment			72
Balance at June 30, 2017	1,381		1,381
Net book value:
Net book value	$ 374		$ 374		$ 433