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Fair value measurements (Details) - USD ($)
$ in Thousands
Jun. 30, 2017
Dec. 31, 2016
ASSETS:    
Derivatives $ 354  
Fair Value, Measurements, Recurring [Member]    
ASSETS:    
Cash equivalents 82,567 $ 79,014
Trust assets 2,703 3,147
Surety bonds 1,625 1,874
Derivatives 354 925
TOTAL 87,249 84,960
LIABILITIES:    
E.ON contingent obligation - net 0 0
Derivatives 7,539 253
TOTAL 7,539 253
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]    
ASSETS:    
Cash equivalents 82,567 79,014
Trust assets 2,703 3,147
Surety bonds 1,625 1,874
Derivatives 0 0
TOTAL 86,895 84,035
LIABILITIES:    
E.ON 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:    
E.ON contingent obligation - net 0 0
Derivatives 7,208 0
TOTAL 7,208 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 354 925
TOTAL 354 925
LIABILITIES:    
E.ON contingent obligation - net 0 0
Derivatives 331 253
TOTAL $ 331 $ 253