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Fair value measurements (Details) - USD ($)
$ in Thousands
Sep. 30, 2017
Dec. 31, 2016
ASSETS:    
Derivatives $ 1,469  
Fair Value, Measurements, Recurring [Member]    
ASSETS:    
Cash equivalents 160,611 $ 79,014
Trust assets 2,192 3,147
Surety bonds 1,618 1,874
Derivatives 1,469 925
TOTAL 165,890 84,960
LIABILITIES:    
Contingent obligation - net 0 0
Derivatives 7,715 253
TOTAL 7,715 253
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]    
ASSETS:    
Cash equivalents 160,611 79,014
Trust assets 2,192 3,147
Surety bonds 1,618 1,874
Derivatives 0 0
TOTAL 164,421 84,035
LIABILITIES:    
Contingent obligation - net 0 0
Derivatives 0 0
TOTAL 0 0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]    
ASSETS:    
Cash equivalents 0 0
Trust assets 0 0
Surety bonds 0 0
Derivatives 0 0
TOTAL 0 0
LIABILITIES:    
Contingent obligation - net 0 0
Derivatives 6,234 0
TOTAL 6,234 0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]    
ASSETS:    
Cash equivalents 0 0
Trust assets 0 0
Surety bonds 0 0
Derivatives 1,469 925
TOTAL 1,469 925
LIABILITIES:    
Contingent obligation - net 0 0
Derivatives 1,481 253
TOTAL $ 1,481 $ 253