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Fair value measurements (Details) - USD ($)
$ in Thousands
Mar. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Jun. 30, 2015
ASSETS:        
Derivative instruments $ 1,610      
Recurring [Member]        
ASSETS:        
Cash equivalents 103,286 $ 79,014 $ 102,675  
Trust assets 3,138 3,147 5,226  
Surety bonds 1,618 1,874 1,870  
Derivative instruments 1,610 925    
TOTAL 109,652 84,960 109,771  
LIABILITIES:        
E.ON contingent obligation – net 0 0 0  
Derivative instruments 14,915 253    
TOTAL 14,915 253 0  
Recurring [Member] | Level 1 [Member]        
ASSETS:        
Cash equivalents 103,286 79,014 102,675  
Trust assets 3,138 3,147 5,226  
Surety bonds 1,618 1,874 1,870  
Derivative instruments 0 0    
TOTAL 108,042 84,035 109,771  
LIABILITIES:        
E.ON contingent obligation – net 0 0   $ 0
Derivative instruments 0 0    
TOTAL 0 0 0  
Recurring [Member] | Level 2 [Member]        
ASSETS:        
Cash equivalents 0 0 0  
Trust assets 0 0 0  
Surety bonds 0 0 0  
Derivative instruments 0 0    
TOTAL 0 0 0  
LIABILITIES:        
E.ON contingent obligation – net 0 0 0  
Derivative instruments 13,015 0    
TOTAL 13,015 0 0  
Recurring [Member] | Level 3 [Member]        
ASSETS:        
Cash equivalents 0 0 0  
Trust assets 0 0 0  
Surety bonds 0 0 0  
Derivative instruments 1,610 925    
TOTAL 1,610 925 0  
LIABILITIES:        
E.ON contingent obligation – net 0 0 0  
Derivative instruments 1,900 253    
TOTAL $ 1,900 $ 253 $ 0