Nature of operations and summary of significant accounting policies (Stock-based compensation) (Details)	12 Months Ended
	Dec. 31, 2018 USD ($) tranche installment
Employees
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period (in years)	3 years
Time Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of vesting annual installments | installment	3
Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of tranches | tranche	3
Shares based awards expense recognized | $	$ 0
Award vesting period (in years)	3 years