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Fair value measurements (Details) - Fair Value, Measurements, Recurring [Member] - USD ($)
$ in Thousands
Sep. 30, 2016
Dec. 31, 2015
ASSETS:    
Cash equivalents $ 104,730 $ 102,675
Trust assets 3,664 5,226
Surety bonds 1,874 1,870
Derivative contracts 904  
TOTAL 111,172 109,771
LIABILITIES:    
E.ON contingent obligation - net 0 0
Derivative contracts 574  
TOTAL 574 0
Fair Value, Inputs, Level 1 [Member]    
ASSETS:    
Cash equivalents 104,730 102,675
Trust assets 3,664 5,226
Surety bonds 1,874 1,870
Derivative contracts 0  
TOTAL 110,268 109,771
LIABILITIES:    
E.ON contingent obligation - net 0 0
Derivative contracts 0  
TOTAL 0 0
Fair Value, Inputs, Level 2 [Member]    
ASSETS:    
Cash equivalents 0 0
Trust assets 0 0
Surety bonds 0 0
Derivative contracts 330  
TOTAL 330 0
LIABILITIES:    
E.ON contingent obligation - net 0 0
Derivative contracts 0  
TOTAL 0 0
Fair Value, Inputs, Level 3 [Member]    
ASSETS:    
Cash equivalents 0 0
Trust assets 0 0
Surety bonds 0 0
Derivative contracts 574  
TOTAL 574 0
LIABILITIES:    
E.ON contingent obligation - net 0 0
Derivative contracts 574  
TOTAL $ 574 $ 0