Fair value measurements (Details) - Recurring [Member] - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Jun. 30, 2015
ASSETS:
Cash equivalents	$ 79,014	$ 102,675
Trust assets	3,147	5,226
Surety bonds	1,874	1,870
Derivative instruments	925
TOTAL	84,960	109,771
LIABILITIES:
E.ON contingent obligation – net	0	0
Derivative instruments	253
TOTAL	253	0
Level 1 [Member]
ASSETS:
Cash equivalents	79,014	102,675
Trust assets	3,147	5,226
Surety bonds	1,874	1,870
Derivative instruments	0
TOTAL	84,035	109,771
LIABILITIES:
E.ON contingent obligation – net	0		$ 0
Derivative instruments	0
TOTAL	0	0
Level 2 [Member]
ASSETS:
Cash equivalents	0	0
Trust assets	0	0
Surety bonds	0	0
Derivative instruments	0
TOTAL	0	0
LIABILITIES:
E.ON contingent obligation – net	0	0
Derivative instruments	0
TOTAL	0	0
Level 3 [Member]
ASSETS:
Cash equivalents	0	0
Trust assets	0	0
Surety bonds	0	0
Derivative instruments	925
TOTAL	925	0
LIABILITIES:
E.ON contingent obligation – net	0	0
Derivative instruments	253
TOTAL	$ 253	$ 0