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Fair value measurements (Details) - Recurring [Member] - USD ($)
$ in Thousands
Dec. 31, 2015
Dec. 31, 2014
ASSETS:    
Cash equivalents $ 102,675 $ 137,712
Trust assets 5,226 8,067
Surety bonds 1,870 1,987
TOTAL 109,771 147,766
LIABILITIES:    
E.ON contingent obligation – net 0 0
TOTAL 0 0
Level 1 [Member]    
ASSETS:    
Cash equivalents 102,675 137,712
Trust assets 5,226 8,067
Surety bonds 1,870 1,987
TOTAL 109,771 147,766
LIABILITIES:    
E.ON contingent obligation – net 0 0
TOTAL 0 0
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
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