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Stock-based compensation - Options by exercise price range (Details) (Share option plan, CAD) | 12 Months Ended
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Mar. 31, 2013 | Mar. 31, 2012
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items] |  |
Number of options outstanding | 2,985,722 |
Options outstanding, weighted average remaining life | 7 years 1 month 6 days | 6 years 3 months 18 days
Options outstanding, weighted average exercise price | 5.72 |
Number of options exercisable | 1,122,362 | 1,031,774
Options exercisable, weighted average remaining life | 4 years 2 months 12 days |
Options exercisable, weighted average exercise price | 8.82 | 8.82
$2.75 |  |
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items] |  |
Exercise price (CAD per share) | 2.75 |
Number of options outstanding | 705,600 |
Options outstanding, weighted average remaining life | 9 years 6 months |
Options outstanding, weighted average exercise price | 2.75 |
Number of options exercisable | 0 |
Options exercisable, weighted average remaining life | 0 years |
Options exercisable, weighted average exercise price | 0.00 |
$2.79 |  |
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items] |  |
Exercise price (CAD per share) | 2.79 |
Number of options outstanding | 750,000 |
Options outstanding, weighted average remaining life | 9 years 2 months 12 days |
Options outstanding, weighted average exercise price | 2.79 |
Number of options exercisable | 0 |
Options exercisable, weighted average remaining life | 0 years |
Options exercisable, weighted average exercise price | 0.00 |
$3.69 |  |
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items] |  |
Exercise price (CAD per share) | 3.69 |
Number of options outstanding | 115,840 |
Options outstanding, weighted average remaining life | 6 years 4 months 24 days |
Options outstanding, weighted average exercise price | 3.69 |
Number of options exercisable | 91,700 |
Options exercisable, weighted average remaining life | 6 years 7 months 6 days |
Options exercisable, weighted average exercise price | 3.69 |
$4.90 |  |
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items] |  |
Exercise price (CAD per share) | 4.90 |
Number of options outstanding | 40,000 |
Options outstanding, weighted average remaining life | 9 years |
Options outstanding, weighted average exercise price | 4.90 |
Number of options exercisable | 0 |
Options exercisable, weighted average remaining life | 0 years |
Options exercisable, weighted average exercise price | 0.00 |
$5.00 |  |
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items] |  |
Exercise price (CAD per share) | 5.00 |
Number of options outstanding | 403,982 |
Options outstanding, weighted average remaining life | 1 year 9 months 18 days |
Options outstanding, weighted average exercise price | 5.00 |
Number of options exercisable | 403,982 |
Options exercisable, weighted average remaining life | 1 year 9 months 18 days |
Options exercisable, weighted average exercise price | 5.00 |
$6.56 |  |
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items] |  |
Exercise price (CAD per share) | 6.56 |
Number of options outstanding | 188,260 |
Options outstanding, weighted average remaining life | 8 years 1 month 6 days |
Options outstanding, weighted average exercise price | 6.56 |
Number of options exercisable | 42,140 |
Options exercisable, weighted average remaining life | 7 years 4 months 24 days |
Options exercisable, weighted average exercise price | 6.56 |
$8.28 |  |
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items] |  |
Exercise price (CAD per share) | 8.28 |
Number of options outstanding | 80,000 |
Options outstanding, weighted average remaining life | 6 years 2 months 12 days |
Options outstanding, weighted average exercise price | 8.28 |
Number of options exercisable | 48,000 |
Options exercisable, weighted average remaining life | 6 years 2 months 12 days |
Options exercisable, weighted average exercise price | 8.28 |
$8.58 |  |
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items] |  |
Exercise price (CAD per share) | 8.58 |
Number of options outstanding | 60,000 |
Options outstanding, weighted average remaining life | 7 years 6 months |
Options outstanding, weighted average exercise price | 8.58 |
Number of options exercisable | 24,000 |
Options exercisable, weighted average remaining life | 7 years 6 months |
Options exercisable, weighted average exercise price | 8.58 |
$9.33 |  |
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items] |  |
Exercise price (CAD per share) | 9.33 |
Number of options outstanding | 125,560 |
Options outstanding, weighted average remaining life | 6 years 6 months |
Options outstanding, weighted average exercise price | 9.33 |
Number of options exercisable | 79,200 |
Options exercisable, weighted average remaining life | 6 years 7 months 6 days |
Options exercisable, weighted average exercise price | 9.33 |
$10.21 |  |
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items] |  |
Exercise price (CAD per share) | 10.13 |
Number of options outstanding | 128,780 |
Options outstanding, weighted average remaining life | 7 years 4 months 24 days |
Options outstanding, weighted average exercise price | 10.13 |
Number of options exercisable | 55,640 |
Options exercisable, weighted average remaining life | 7 years 3 months 18 days |
Options exercisable, weighted average exercise price | 10.13 |
$13.21 |  |
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items] |  |
Exercise price (CAD per share) | 13.21 |
Number of options outstanding | 75,000 |
Options outstanding, weighted average remaining life | 4 years 9 months 18 days |
Options outstanding, weighted average exercise price | 13.21 |
Number of options exercisable | 75,000 |
Options exercisable, weighted average remaining life | 4 years 9 months 18 days |
Options exercisable, weighted average exercise price | 13.21 |
$13.50 |  |
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items] |  |
Exercise price (CAD per share) | 13.50 |
Number of options outstanding | 194,940 |
Options outstanding, weighted average remaining life | 4 years 7 months 6 days |
Options outstanding, weighted average exercise price | 13.50 |
Number of options exercisable | 194,940 |
Options exercisable, weighted average remaining life | 4 years 7 months 6 days |
Options exercisable, weighted average exercise price | 13.50 |
$15.37 |  |
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items] |  |
Exercise price (CAD per share) | 15.37 |
Number of options outstanding | 40,000 |
Options outstanding, weighted average remaining life | 5 years |
Options outstanding, weighted average exercise price | 15.37 |
Number of options exercisable | 40,000 |
Options exercisable, weighted average remaining life | 5 years |
Options exercisable, weighted average exercise price | 15.37 |
$16.46 |  |
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items] |  |
Exercise price (CAD per share) | 16.46 |
Number of options outstanding | 50,000 |
Options outstanding, weighted average remaining life | 5 years |
Options outstanding, weighted average exercise price | 16.46 |
Number of options exercisable | 40,000 |
Options exercisable, weighted average remaining life | 5 years |
Options exercisable, weighted average exercise price | 16.46 |
$16.75 |  |
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items] |  |
Exercise price (CAD per share) | 16.75 |
Number of options outstanding | 27,760 |
Options outstanding, weighted average remaining life | 3 years 6 months |
Options outstanding, weighted average exercise price | 16.75 |
Number of options exercisable | 27,760 |
Options exercisable, weighted average remaining life | 3 years 6 months |
Options exercisable, weighted average exercise price | 16.75 |

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- Definition Weighted average exercise price as of the balance sheet date for those equity-based payment arrangements exercisable and outstanding. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(iii) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Weighted average remaining contractual term of exercisable stock options, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (e)(2) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The number of shares reserved for issuance pertaining to the outstanding exercisable stock options as of the balance sheet date in the customized range of exercise prices for which the market and performance vesting condition has been satisfied. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 123R -Paragraph A240 -Subparagraph b(1)(c) -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. Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(iii) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The number of shares reserved for issuance pertaining to the outstanding stock options as of the balance sheet date for all option plans in the customized range of exercise prices. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(i)-(ii) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 123R -Paragraph A240 -Subparagraph b(1)(a, b) -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. + Details
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- Definition The weighted average price as of the balance sheet date at which grantees could acquire the underlying shares with respect to all outstanding stock options which are in the customized range of exercise prices. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(i)-(ii) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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