Fair value measurements (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
ASSETS:
Derivative instruments	$ 1,724
LIABILITIES:
Derivative instruments	1,200
Recurring [Member]
ASSETS:
Cash equivalents	142,819	$ 79,014
Trust assets	1,757	3,147
Surety bonds	1,618	1,874
Derivative instruments	1,724	925
TOTAL	147,918	84,960
LIABILITIES:
Contingent obligation	0	0
Derivative instruments	1,200	253
TOTAL	1,200	253
Recurring [Member] | Level 1 [Member]
ASSETS:
Cash equivalents	142,819	79,014
Trust assets	1,757	3,147
Surety bonds	1,618	1,874
Derivative instruments	0	0
TOTAL	146,194	84,035
LIABILITIES:
Contingent obligation	0	0
Derivative instruments	0	0
TOTAL	0	0
Recurring [Member] | Level 2 [Member]
ASSETS:
Cash equivalents	0	0
Trust assets	0	0
Surety bonds		0
Derivative instruments	0	0
TOTAL	0	0
LIABILITIES:
Contingent obligation	0	0
Derivative instruments	0	0
TOTAL	0	0
Recurring [Member] | Level 3 [Member]
ASSETS:
Cash equivalents	0	0
Trust assets	0	0
Surety bonds	0	0
Derivative instruments	1,724	925
TOTAL	1,724	925
LIABILITIES:
Contingent obligation	0	0
Derivative instruments	1,200	253
TOTAL	$ 1,200	$ 253