Financial instruments and risk management - Trade receivables (Details) (CAD) In Thousands, unless otherwise specified	12 Months Ended
Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2011
Fair Value Disclosures [Abstract]
Not past due	76,646	166,362
Past due 1-30 days	14,203	27,617
Past due 31-60 days	957	8,476
More than 61 days	1,645	10,596
Total	93,451	213,051
Percentage of allowance for doubtful accounts that are more than 61 days past due	100.00%
Allowance for Doubtful Accounts Receivable [Roll Forward]
Opening balance	210	30	1,691
Payments received on provided balances	(1)	0	(682)
Current year allowance	365	180	518
Write-offs	(574)	0	(1,497)
Ending balance	0	210	30

Accounts Receivable, 1 to 30 Days Past Due

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Accounts Receivable, 31 to 60 Days Past Due

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Accounts Receivable, Current

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- Definition

Accounts Receivable, Greater than 60 Days Past Due

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Allowance for Doubtful Accounts Receivable, Percentage of Allowance Over Sixty-one Days Past Due

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- Definition

Amount due from customers or clients, within one year of the balance sheet date (or the normal operating cycle, whichever is longer), for goods or services (including trade receivables) that have been delivered or sold in the normal course of business, reduced to the estimated net realizable fair value by an allowance established by the entity of the amount it deems uncertain of collection.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

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-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 3

-Subparagraph a(1)

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-Name Regulation S-X (SX)

-Number 210

-Section 02

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Amount of direct write-downs of receivables charged against the allowance for doubtful accounts.

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- Definition

A valuation allowance for trade and other receivables due to an Entity within one year (or the normal operating cycle, whichever is longer) that are expected to be uncollectible.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 4

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Amount of recoveries of receivables doubtful of collection that were previously charged off.

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Amount of the current period expense charged against operations, the offset which is generally to the allowance for doubtful accounts for the purpose of reducing receivables, including notes receivable, to an amount that approximates their net realizable value (the amount expected to be collected).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 5

-Article 5

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-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 95

-Paragraph 28

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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