v2.4.0.6

Derivative instruments and hedging activities (Details 2) In Millions, unless otherwise specified | Mar. 31, 2013 Gas purchase agreements gigajoule | Dec. 31, 2012 Gas purchase agreements gigajoule | Mar. 31, 2013 Interest rate swaps USD ($) | Dec. 31, 2012 Interest rate swaps USD ($) | Mar. 31, 2013 Currency forward CAD | Dec. 31, 2012 Currency forward CAD
--- | --- | --- | --- | --- | --- | ---
Derivative instruments |  |  |  |  |  |
Volume of forecasted transactions | 46,800,000 | 49,800,000 |  |  |  |
Volume of forecasted transactions, (in dollars) |  |  | $ 170.6 | $ 172.0 | 153.7 | 176.6

X
- Definition Aggregate notional amount of derivative expressed in nonmonetary units. For example, the number of barrels specified in a fuel oil forward purchase contract. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 12 -Section 13 -Sentence Column B + Details
Name: | invest_DerivativeNonmonetaryNotionalAmount
Namespace Prefix: | invest_
Data Type: | xbrli:decimalItemType
Balance Type: | na
Period Type: | instant

X
- Definition Aggregate notional amount specified by the derivative(s). Expressed as an absolute value. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 12 -Section 13 -Sentence Column B + Details
Name: | invest_DerivativeNotionalAmount
Namespace Prefix: | invest_
Data Type: | xbrli:monetaryItemType
Balance Type: | na
Period Type: | instant

X
- Details
Name: | us-gaap_DerivativeLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration