Income taxes - Significant components of Income tax expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Current:
U.S. federal current expense (benefit)	$ (231)	$ 0	$ (112)
State current expense (benefit)	(130)	(706)	2,252
Foreign current expense	5,726	13,473	15,098
Total current expense	5,365	12,767	17,238
Deferred:
U.S. federal deferred benefit	(1,564)	(1,564)	(1,696)
State deferred benefit	0	0	(12)
Foreign deferred tax expense	(977)	(1,927)	2,778
Total deferred benefit	(2,541)	(3,491)	1,070
Total income tax expense	$ 2,824	$ 9,276	$ 18,308