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Fair value measurements (Details) - USD ($)
$ in Thousands
Mar. 31, 2017
Dec. 31, 2016
ASSETS:    
Derivatives $ 1,610  
Fair Value, Measurements, Recurring [Member]    
ASSETS:    
Cash equivalents 103,286 $ 79,014
Trust assets 3,138 3,147
Surety bonds 1,618 1,874
Derivatives 1,610 925
TOTAL 109,652 84,960
LIABILITIES:    
E.ON contingent obligation - net 0 0
Derivatives 14,915 253
TOTAL 14,915 253
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]    
ASSETS:    
Cash equivalents 103,286 79,014
Trust assets 3,138 3,147
Surety bonds 1,618 1,874
Derivatives 0 0
TOTAL 108,042 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 13,015 0
TOTAL 13,015 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 1,610 925
TOTAL 1,610 925
LIABILITIES:    
E.ON contingent obligation - net 0 0
Derivatives 1,900 253
TOTAL $ 1,900 $ 253