v2.4.0.6
Accounting for derivative instruments and hedging activities (Details 4) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Jun. 30, 2009
Changes in the accumulated other comprehensive income (loss)        
Accumulated OCI balance at beginning of period $ (1,383) $ 255 $ (859)  
Change in fair value of cash flow hedges (949) (2,647) (360)  
Realized from OCI during the period 888 1,009 1,474  
Accumulated OCI balance at end of period (1,444) (1,383) 255  
Gains (losses) expected to be realized from OCI in the next 12 months, net of $ 593 tax 890      
Tax effect of gains (losses) expected to be realized from OCI in the next 12 months 593      
Loss on discontinuation of swap contracts (1,444) (1,383) 255  
Amortization of remaining loss on discontinuation of swap contracts 888 1,009 1,474  
Interest rate swaps
        
Changes in the accumulated other comprehensive income (loss)        
Accumulated OCI balance at beginning of period (1,704) (427) (538)  
Change in fair value of cash flow hedges (949) (2,647) (360)  
Realized from OCI during the period 1,120 1,370 471  
Accumulated OCI balance at end of period (1,533) (1,704) (427)  
Gains (losses) expected to be realized from OCI in the next 12 months, net of $ 593 tax 979      
Loss on discontinuation of swap contracts (1,533) (1,704) (427)  
Amortization of remaining loss on discontinuation of swap contracts 1,120 1,370 471  
Natural gas swaps
        
Changes in the accumulated other comprehensive income (loss)        
Accumulated OCI balance at beginning of period 321 682 (321)  
Realized from OCI during the period (232) (361) 1,003  
Accumulated OCI balance at end of period 89 321 682  
Gains (losses) expected to be realized from OCI in the next 12 months, net of $ 593 tax (89)      
Loss on discontinuation of swap contracts 89 321 682  
Amortization of remaining loss on discontinuation of swap contracts (232) (361) 1,003  
Discontinued cash flow hedge | Natural gas swaps
        
Changes in the accumulated other comprehensive income (loss)        
Accumulated OCI balance at beginning of period       5,100
Realized from OCI during the period (400) (600) 1,700  
Accumulated OCI balance at end of period       5,100
Loss on discontinuation of swap contracts       5,100
Amortization of remaining loss on discontinuation of swap contracts $ (400) $ (600) $ 1,700