v2.4.0.6
Commitments and contingencies (Details) (USD $)
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Commitments and contingencies      
Lease expense under operating leases $ 2,000,000 $ 1,000,000 $ 900,000
Future minimum lease commitments under operating leases      
2013 1,048,000    
2014 1,064,000    
2015 677,000    
2016 460,000    
2017 451,000    
Thereafter 4,805,000    
Future minimum lease commitments 8,505,000    
Estimated commitments under Transmission and Long-Term Service Agreements      
2013 3,178,000    
2014 3,225,000    
2015 4,744,000    
2016 4,754,000    
2017 4,615,000    
Thereafter 22,777,000    
Total 43,293,000    
Fuel Supply and Transportation Commitments      
2013 77,329,000    
2014 80,862,000    
2015 80,921,000    
2016 82,722,000    
2017 18,249,000    
Thereafter 114,054,000    
Total $ 454,137,000