Other intangible assets - Rollforward of other intangible assets (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Carrying amount:
Balance at the beginning of the period	$ 24,140,000	$ 17,738,000
Additions	10,764,000
Additions		6,100,000
Impairment	(468,000)	0	$ 0
Disposals	(754,000)	(247,000)
Foreign currency translation adjustment	(2,380,000)	549,000
Balance at the end of the period	31,302,000	24,140,000	17,738,000
Accumulated amortization:
Balance at the beginning of the period	4,362,000	3,861,000
Charge for the year	1,086,000	600,000
Disposals	(754,000)	(247,000)
Impairment	(312,000)
Foreign currency translation adjustment	(221,000)	148,000
Balance at the end of the period	4,161,000	4,362,000	3,861,000
Net book value:
Net book value	27,141,000	19,778,000	13,877,000
Trade names
Carrying amount:
Balance at the beginning of the period	14,001,000	7,579,000
Additions	2,676,000	6,100,000
Impairment	0
Foreign currency translation adjustment	(1,660,000)	322,000
Balance at the end of the period	15,017,000	14,001,000	7,579,000
Net book value:
Net book value	15,017,000	14,001,000	7,579,000
Favorable lease assets
Carrying amount:
Balance at the beginning of the period	8,560,000	8,501,000
Additions	0	0
Impairment	(468,000)
Disposals	0	0
Foreign currency translation adjustment	(264,000)	59,000
Balance at the end of the period	7,828,000	8,560,000	8,501,000
Accumulated amortization:
Balance at the beginning of the period	3,092,000	2,636,000
Charge for the year	347,000	434,000
Disposals	0	0
Impairment	(312,000)
Foreign currency translation adjustment	(115,000)	22,000
Balance at the end of the period	3,012,000	3,092,000	2,636,000
Net book value:
Net book value	4,816,000	5,468,000	5,865,000
Internet sites
Carrying amount:
Balance at the beginning of the period	1,579,000	1,658,000
Additions	1,858,000	0
Impairment	0
Disposals	(754,000)	(247,000)
Foreign currency translation adjustment	(175,000)	168,000
Balance at the end of the period	2,508,000	1,579,000	1,658,000
Accumulated amortization:
Balance at the beginning of the period	1,270,000	1,225,000
Charge for the year	258,000	166,000
Disposals	(754,000)	(247,000)
Foreign currency translation adjustment	(84,000)	126,000
Balance at the end of the period	690,000	1,270,000	1,225,000
Net book value:
Net book value	1,818,000	309,000	433,000
Software
Carrying amount:
Balance at the beginning of the period	0	0
Additions	6,230,000	0
Foreign currency translation adjustment	(281,000)
Balance at the end of the period	5,949,000	0	0
Accumulated amortization:
Charge for the year	481,000
Foreign currency translation adjustment	(22,000)
Balance at the end of the period	459,000
Net book value:
Net book value	$ 5,490,000	$ 0	$ 0