Income taxes (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Effective tax rates reconciliation
Annual income tax rate (as a percent)	47.70%	35.90%
Statutory federal income tax rate (as a percent)	21.00%		35.00%
Annual effective rate relates to the required valuation allowance (as a percent)	25.30%
Income tax expense	$ (323)	$ 1,684
U.S.
Effective tax rates reconciliation
Annual income tax rate (as a percent)	22.30%