Income taxes - Income Tax Expense (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Income Tax Disclosure [Abstract]
Income tax (benefit) expense	$ (605)	$ 706	$ (227)	$ (2,076)
Effective Income Tax Rate Reconciliation, Percent	5.10%	(1.90%)	0.60%	1.80%