|
Derivative instruments and hedging activities (Details 2)
In Millions, unless otherwise specified |
Jun. 30, 2013
Natural gas swaps
MMBTU
|
Dec. 31, 2012
Natural gas swaps
MMBTU
|
Jun. 30, 2013
Gas purchase agreements
gigajoule
|
Dec. 31, 2012
Gas purchase agreements
gigajoule
|
Jun. 30, 2013
Interest rate swaps
USD ($)
|
Dec. 31, 2012
Interest rate swaps
USD ($)
|
Jun. 30, 2013
Currency forward
CAD
|
Dec. 31, 2012
Currency forward
CAD
|
|---|---|---|---|---|---|---|---|---|
| Derivative instruments | ||||||||
| Volume of forecasted transactions | 5,600,000 | 10,600,000 | 46,100,000 | 49,800,000 | ||||
| Volume of forecasted transactions, (in dollars) | $ 166.6 | $ 172.0 | 40.9 | 176.6 |