Commitments and contingencies: Current and potential litigation (Details) $ in Millions	1 Months Ended
	Oct. 31, 2017 USD ($)	Dec. 31, 2011 USD ($) employee	Dec. 31, 2017 USD ($)	Mar. 31, 2015 USD ($)
Commitments and contingencies
Estimate of possible loss in excess of accrual			$ 2.6
Number of former employees | employee		70
Settlement amount		$ 3.1
Settlement payment	$ 3.1
Spain
Commitments and contingencies
Estimate of possible loss				$ 9.0