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Stock-based compensation - Plan activity (Details) (CAD) | 12 Months Ended
--- | ---
Mar. 31, 2013 | Mar. 31, 2012 | Mar. 31, 2011
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward] |  |  |  |  |
Exercised (options) |  | (8,480) |  | (193,250) |
Share option plan |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward] |  |  |  |  |
Beginning balance (options) | 1,834,794 | 1,647,474 |  | 2,250,804 |
Granted (options) | 1,517,400 | 287,700 |  | 260,000 |
Exercised (options) |  | (8,480) | [1] | (193,250) | [1]
Forfeited (options) | (366,472) | (91,900) |  | (120,080) |
Modified (options) |  |  |  | (550,000) | [2]
Ending balance (options) | 2,985,722 | 1,834,794 |  | 1,647,474 |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Roll Forward] |  |  |  |  |
Beginning balance (CAD per share) | 8.79 | 9.25 |  | 7.84 |
Granted (CAD per share) | 2.83 | 6.56 |  | 9.77 |
Exercised (CAD per share) |  | 4.15 | [1] | 4.98 | [1]
Forfeited (CAD per share) | 9.10 | 10.42 |  | 10.30 |
Modified (CAD per share) |  |  |  | 5.00 | [2]
Ending balance (CAD per share) | 5.72 | 8.79 |  | 9.25 |
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract] |  |  |  |  |
Weighted average fair value per option granted (CAD per share) | 1.88 | 4.38 |  | 6.79 |
Share option plan | Stock options |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract] |  |  |  |  |
Dividend yield | 0.00% | 0.00% |  | 0.00% |
Expected volatility | 74.52% | 75.22% |  | 78.59% |
Risk-free interest rate | 1.02% | 1.32% |  | 2.65% |
Expected life (years) | 6 years 4 months 24 days | 6 years 3 months 18 days |  | 6 years 1 month 6 days |
Senior executive stock option plan |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward] |  |  |  |  |
Granted (options) | 550,000 | 550,000 |  | 550,000 |
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract] |  |  |  |  |
Weighted average fair value per option granted (CAD per share) | 1.68 | 2.40 |  | 9.30 |
Senior executive stock option plan | Stock options |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract] |  |  |  |  |
Dividend yield | 0.00% | 0.00% |  | 0.00% |
Expected volatility | 71.74% | 74.99% |  | 76.74% |
Risk-free interest rate | 0.26% | 0.54% |  | 1.77% |
Expected life (years) | 2 years 1 month 6 days | 3 years 1 month 10 days |  | 6 years 1 month 6 days |
Deferred performance share unit plan |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward] |  |  |  |  |
Ending balance (options) | 0 |  |  |  |
Deferred performance share unit plan | Deferred performance share units (DPSUs) |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward] |  |  |  |  |
Beginning balance (units) |  | 432,519 |  | 507,295 |
Forfeited (units) |  |  |  | (74,776) |
Expired (units) |  | (41,117) |  |  |
Converted to (units) |  | (391,402) |  |  |
Ending balance (units) |  | 0 |  | 432,519 |
Restricted share unit plan | Restricted share units (RSUs) |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward] |  |  |  |  |
Beginning balance (units) | 1,175,565 | 382,476 |  | 468,815 |
Granted (units) | 625,405 | 695,086 |  |  |
Settled (units) | (345,679) | (27,850) |  |  |
Forfeited (units) | (364,446) | (102,022) |  | (86,339) |
Converted from (units) |  | 227,875 |  |  |
Ending balance (units) | 1,090,845 | 1,175,565 |  | 382,476 |
Directors' deferred share unit plan | Deferred stock units (DSUs) |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward] |  |  |  |  |
Beginning balance (units) | 465,266 | 337,018 |  | 263,266 |
Granted (units) | 266,091 | 128,248 |  | 73,752 |
Settled (units) | (63,413) | 0 |  |  |
Ending balance (units) | 667,944 | 465,266 |  | 337,018 |
[1] | All stock options exercised resulted in new common shares being issued (note 19(a))
[2] | 550,000 stock options were modified as senior executive stock options on September 22, 2010 (note 24(c))

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- Definition The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period. + 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)(iv)(3) -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(2)(e) -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 number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan). + 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)(2)(iii)(1) -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(2)(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. + Details
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- Definition The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date. + 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)(2)(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(2)(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. Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 123R -Paragraph A240 -Subparagraph b(2)(a) -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 estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term. + 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 (f)(2)(iii) -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 e(2)(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. + Details
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- Definition Expected term of share-based compensation awards, 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 S99 -Paragraph 1 -Subparagraph (SAB TOPIC 14.D.2) -URI http://asc.fasb.org/extlink&oid=6793087&loc=d3e301413-122809 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2)(i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 14 -Section D -Subsection 2 + Details
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- Definition The risk-free interest rate assumption that is used in valuing an option on its own shares. + 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 (f)(2)(iv) -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 e(2)(d) -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 grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 123R -Paragraph A240 -Subparagraph c(1) -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 (d)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The addition or reduction in the number of reserved shares that could potentially be issued under the option plan attributable to reasons other than grants, exercises, forfeitures, and expirations during the reporting period. + 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) -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) -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 number of shares reserved for issuance under stock option agreements awarded under the plan that validly exist and are outstanding as of the balance sheet date, including vested options. + 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)(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. Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 Reference 3: 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 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 123R -Paragraph A240 -Subparagraph b(1)(a) -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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