v2.4.0.6
Accounting for derivative instruments and hedging activities (Details 5) (Derivative instruments not designated as cash flow hedges, USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Unrealized gains and (losses)
      
Impact of derivative instruments on the consolidated statements of operations      
Gains and losses for derivative instruments $ (59,272) $ (14,594) $ 3,275
Natural gas swaps | Realized (gains) and losses
      
Impact of derivative instruments on the consolidated statements of operations      
Gains and losses for derivative instruments 43,470    
Natural gas swaps | Unrealized gains and (losses)
      
Impact of derivative instruments on the consolidated statements of operations      
Gains and losses for derivative instruments (1,241) (2,357) (148)
Gas purchase agreements | Unrealized gains and (losses)
      
Impact of derivative instruments on the consolidated statements of operations      
Gains and losses for derivative instruments (56,954)    
Interest rate swaps | Realized (gains) and losses
      
Impact of derivative instruments on the consolidated statements of operations      
Gains and losses for derivative instruments 4,584 4,166 1,664
Interest rate swaps | Unrealized gains and (losses)
      
Impact of derivative instruments on the consolidated statements of operations      
Gains and losses for derivative instruments (1,077) (12,237) 3,423
Foreign currency forward contracts | Realized (gains) and losses
      
Impact of derivative instruments on the consolidated statements of operations      
Gains and losses for derivative instruments (18,483) 5,201 (6,625)
Foreign currency forward contracts | Unrealized gains and (losses)
      
Impact of derivative instruments on the consolidated statements of operations      
Gains and losses for derivative instruments $ 11,956 $ 14,211 $ (3,542)