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Fair value measurements (Details) - Fair Value, Measurements, Recurring [Member] - USD ($)
$ in Thousands
Mar. 31, 2016
Dec. 31, 2015
ASSETS:    
Cash equivalents $ 120,061 $ 102,675
Trust assets 4,789 5,226
Surety bonds 1,870 1,870
TOTAL 126,720 109,771
LIABILITIES:    
E.ON contingent obligation - net 0 0
TOTAL 0 0
Fair Value, Inputs, Level 1 [Member]    
ASSETS:    
Cash equivalents 120,061 102,675
Trust assets 4,789 5,226
Surety bonds 1,870 1,870
TOTAL 126,720 109,771
LIABILITIES:    
E.ON contingent obligation - net 0 0
TOTAL 0 0
Fair Value, Inputs, Level 2 [Member]    
ASSETS:    
Cash equivalents 0 0
Trust assets 0 0
Surety bonds 0 0
TOTAL 0 0
LIABILITIES:    
E.ON contingent obligation - net 0 0
TOTAL 0 0
Fair Value, Inputs, Level 3 [Member]    
ASSETS:    
Cash equivalents 0 0
Trust assets 0 0
Surety bonds 0 0
TOTAL 0 0
LIABILITIES:    
E.ON contingent obligation - net 0 0
TOTAL $ 0 $ 0