Other intangible assets - Rollforward of other intangible assets (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013
Carrying amount:
Balance at January 1, 2014	$ 17,483
Foreign currency translation adjustment	228
Balance at March 31, 2014	17,711
Accumulated amortization:
Balance at January 1, 2014	3,331
Charge for the period	68	90
Foreign currency translation adjustment	61
Balance at March 31, 2014	3,460
Net book value:
Net book value	14,251		14,152
Trade names
Carrying amount:
Balance at January 1, 2014	7,100
Foreign currency translation adjustment	0
Balance at March 31, 2014	7,100
Net book value:
Net book value	7,100		7,100
Favorable lease assets
Carrying amount:
Balance at January 1, 2014	8,660
Foreign currency translation adjustment	218
Balance at March 31, 2014	8,878
Accumulated amortization:
Balance at January 1, 2014	2,268
Charge for the period	33
Foreign currency translation adjustment	55
Balance at March 31, 2014	2,356
Net book value:
Net book value	6,522		6,392
Internet sites
Carrying amount:
Balance at January 1, 2014	1,723
Foreign currency translation adjustment	10
Balance at March 31, 2014	1,733
Accumulated amortization:
Balance at January 1, 2014	1,063
Charge for the period	35
Foreign currency translation adjustment	6
Balance at March 31, 2014	1,104
Net book value:
Net book value	$ 629		$ 660