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Financial instruments and risk management - Derivatives (Details) (CAD)
In Thousands, unless otherwise specified
12 Months Ended 0 Months Ended 12 Months Ended
Mar. 31, 2013
Mar. 31, 2012
Mar. 31, 2011
Apr. 08, 2010
8.75% senior notes
Senior notes
Mar. 31, 2013
8.75% senior notes
Senior notes
Nov. 26, 2003
8.75% senior notes
Senior notes
Mar. 31, 2013
Not designated as hedging instrument
Mar. 31, 2012
Not designated as hedging instrument
Mar. 31, 2011
Not designated as hedging instrument
Mar. 31, 2013
Not designated as hedging instrument
Cross-currency and interest rate swaps
Mar. 31, 2012
Not designated as hedging instrument
Cross-currency and interest rate swaps
Mar. 31, 2011
Not designated as hedging instrument
Cross-currency and interest rate swaps
Mar. 31, 2013
Not designated as hedging instrument
Supplier contracts
Embedded price escalation features
Mar. 31, 2012
Not designated as hedging instrument
Supplier contracts
Embedded price escalation features
Mar. 31, 2011
Not designated as hedging instrument
Supplier contracts
Embedded price escalation features
Mar. 31, 2013
Not designated as hedging instrument
Construction contracts
Embedded price escalation features
Mar. 31, 2012
Not designated as hedging instrument
Construction contracts
Embedded price escalation features
Mar. 31, 2011
Not designated as hedging instrument
Construction contracts
Embedded price escalation features
Mar. 31, 2013
Not designated as hedging instrument
Level 2
Measured on recurring basis
Supplier contracts
Embedded price escalation features
Mar. 31, 2012
Not designated as hedging instrument
Level 2
Measured on recurring basis
Supplier contracts
Embedded price escalation features
Derivative Liabilities [Abstract]                                        
Derivative Liabilities                                     6,441 9,146
Derivative Liabilities, Current (4,261) (3,220)                                 (4,261) (3,220)
Derivative Liabilities, Noncurrent 2,180 5,926                                 2,180 5,926
Settlement amount of the cross-currency and interest rate swaps 0 0 91,125 91,125                                
Stated interest rate on long term debt         8.75% 8.75%                            
Unrealized Gain (Loss) on Derivatives and Commodity Contracts [Abstract]                                        
Unrealized Gain (Loss) on Derivatives (2,705) (2,382) (2,305)       2,705 2,382 2,305 0 0 2,111 (2,705) 3,495 (3,812) 0 (5,877) (604)