v2.4.0.6
Derivative instruments and hedging activities (Details 5) (Derivative instruments not designated as cash flow hedges, USD $)
In Millions, unless otherwise specified
 3 Months Ended
Mar. 31, 2013
Mar. 31, 2012
Unrealized gains and (losses)
    
Impact of derivative instruments on the consolidated statements of operations    
Gains and losses for derivative instruments $ (12.6) $ 57.2
Natural gas swaps | Unrealized gains and (losses)
    
Impact of derivative instruments on the consolidated statements of operations    
Gains and losses for derivative instruments (0.4) 0.9
Gas purchase agreements | Realized (gains) and losses
    
Impact of derivative instruments on the consolidated statements of operations    
Gains and losses for derivative instruments 16.3 10.8
Gas purchase agreements | Unrealized gains and (losses)
    
Impact of derivative instruments on the consolidated statements of operations    
Gains and losses for derivative instruments (8.1) 57.9
Interest rate swaps | Realized (gains) and losses
    
Impact of derivative instruments on the consolidated statements of operations    
Gains and losses for derivative instruments 2.6 1.1
Interest rate swaps | Unrealized gains and (losses)
    
Impact of derivative instruments on the consolidated statements of operations    
Gains and losses for derivative instruments (4.1) (1.6)
Foreign currency forward contracts | Realized (gains) and losses
    
Impact of derivative instruments on the consolidated statements of operations    
Gains and losses for derivative instruments (2.5) (11.9)
Foreign currency forward contracts | Unrealized gains and (losses)
    
Impact of derivative instruments on the consolidated statements of operations    
Gains and losses for derivative instruments $ 6.0 $ 9.2