Intangible assets - Schedule of Changes in Net Book Value of Intangible Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Finite-lived Intangible Assets [Roll Forward]
Beginning balance	$ 16,470	$ 12,821
Additions to intangible assets	8,525	7,371
Amortization expense	(8,418)	(10,235)
Change in consolidation scope	87,021	7,801
Currency translation adjustment	(654)	(1,288)
Transfer into service	0	0
Ending balance	102,944	16,470
Gross book value at end of period	129,992	35,550
Accumulated depreciation and impairment at end of period	(27,048)	(19,080)
Construction in Progress
Indefinite-lived Intangible Assets [Roll Forward]
Beginning balance	1,149	343
Additions to intangible assets	0	1,172
Change in consolidation scope	788	0
Currency translation adjustment	(14)	(52)
Transfer into service	(1,029)	(314)
Ending balance	894	1,149
Software
Finite-lived Intangible Assets [Roll Forward]
Beginning balance	6,881	5,122
Additions to intangible assets	7,972	5,645
Amortization expense	(3,449)	(3,631)
Change in consolidation scope	3	1
Currency translation adjustment	(455)	(570)
Transfer into service	435	314
Ending balance	11,387	6,881
Gross book value at end of period	22,770	15,863
Accumulated depreciation and impairment at end of period	(11,383)	(8,982)
Technology and customer relationships
Finite-lived Intangible Assets [Roll Forward]
Beginning balance	8,440	7,356
Additions to intangible assets	553	554
Amortization expense	(4,969)	(6,604)
Change in consolidation scope	86,230	7,800
Currency translation adjustment	(185)	(666)
Transfer into service	594	0
Ending balance	90,663	8,440
Gross book value at end of period	106,328	18,538
Accumulated depreciation and impairment at end of period	$ (15,665)	$ (10,098)