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Convertible debentures (Details)
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12 Months Ended | 12 Months Ended | 38 Months Ended | 0 Months Ended | 12 Months Ended | 38 Months Ended | 0 Months Ended | 12 Months Ended | 0 Months Ended | 12 Months Ended | 0 Months Ended | 12 Months Ended | ||||||||||||||
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Dec. 31, 2012
USD ($)
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Dec. 31, 2011
USD ($)
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Dec. 31, 2010
USD ($)
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Dec. 31, 2006
6.25% 2006 Debentures due 2011
USD ($)
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Dec. 31, 2006
6.25% 2006 Debentures due 2011
CAD
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Dec. 31, 2012
6.25% 2006 Debentures due 2011
CAD
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Dec. 31, 2012
6.5% Debentures due October 2014
USD ($)
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Dec. 31, 2011
6.5% Debentures due October 2014
USD ($)
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Feb. 27, 2013
6.5% Debentures due October 2014
CAD
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Dec. 31, 2009
6.5% Debentures due October 2014
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Dec. 17, 2009
6.25% Debentures due March 2017
USD ($)
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Dec. 17, 2009
6.25% Debentures due March 2017
CAD
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Dec. 31, 2012
6.25% Debentures due March 2017
USD ($)
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Dec. 31, 2012
6.25% Debentures due March 2017
CAD
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Dec. 31, 2011
6.25% Debentures due March 2017
USD ($)
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Feb. 27, 2013
6.25% Debentures due March 2017
CAD
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Oct. 20, 2010
5.6% Debentures due June 2017
USD ($)
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Oct. 20, 2010
5.6% Debentures due June 2017
CAD
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Dec. 31, 2012
5.6% Debentures due June 2017
USD ($)
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Dec. 31, 2012
5.6% Debentures due June 2017
CAD
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Dec. 31, 2011
5.6% Debentures due June 2017
USD ($)
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Dec. 31, 2012
5.75% convertible unsecured subordinated debentures
USD ($)
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Jul. 05, 2012
5.75% Debentures due June 2019
USD ($)
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Dec. 31, 2012
5.75% Debentures due June 2019
USD ($)
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Dec. 11, 2012
6.00% Debenture due December 2019
CAD
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Dec. 31, 2012
6.00% Debenture due December 2019
USD ($)
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| Convertible debentures | ||||||||||||||||||||||||||
| Aggregate principal amount of convertible debentures issued | $ 230,640,000 | 60,000,000 | 86,300,000 | 80,500,000 | $ 130,000,000 | $ 130,000,000 | $ 130,000,000 | 100,000,000 | $ 100,640,000 | |||||||||||||||||
| Convertible debentures stated interest rate percentage | 6.25% | 6.25% | 6.50% | 6.25% | 6.25% | 5.60% | 5.60% | 5.75% | 6.00% | |||||||||||||||||
| Proceeds from issuance of convertible debenture, net of offering costs | 230,640,000 | 74,575,000 | 52,800,000 | 82,100,000 | 78,900,000 | |||||||||||||||||||||
| Conversion rate per Cdn$1,000 principal amount (in shares) | 80.6452 | 80.6452 | 76.9231 | 76.9231 | 55.2486 | 55.2486 | 57.9710 | 68.9655 | ||||||||||||||||||
| Principal amount used for convertible debentures conversion ratio | 1,000 | 1,000 | 1,000 | 1,000 | 1,000 | |||||||||||||||||||||
| Conversion price of shares | 12.40 | 13.00 | 18.10 | $ 17.25 | 14.50 | |||||||||||||||||||||
| Net proceeds from issuance convertible debentures issued, after underwriting discounts and expenses | 124,000,000 | 95,500,000 | ||||||||||||||||||||||||
| Movement in convertible debentures | ||||||||||||||||||||||||||
| Balance at the beginning of the period | 189,563,000 | 220,616,000 | 44,103,000 | 56,104,000 | 66,306,000 | 83,575,000 | 79,154,000 | 80,937,000 | ||||||||||||||||||
| Issuance of convertible debentures | 230,640,000 | 60,000,000 | 86,300,000 | 80,500,000 | 130,000,000 | 130,000,000 | 130,000,000 | 100,000,000 | 100,640,000 | |||||||||||||||||
| Principal amount converted to equity | 32,000 | 26,357,000 | 7,147,000 | (32,000) | (10,862,000) | 15,200,000 | (15,567,000) | 18,800,000 | ||||||||||||||||||
| Foreign exchange loss | (547,000) | (13,838,000) | 1,014,000 | 978,000 | (1,139,000) | 1,470,000 | (1,702,000) | 1,757,000 | (1,783,000) | (130,000) | ||||||||||||||||
| Balance at the end of the period | 424,246,000 | 189,563,000 | 220,616,000 | 45,049,000 | 44,103,000 | 67,776,000 | 66,306,000 | 80,911,000 | 79,154,000 | 130,000,000 | 130,000,000 | 100,510,000 | ||||||||||||||
| Common shares issued on conversion | 1,200,000 | 1,400,000 | ||||||||||||||||||||||||
| Aggregate interest expenses | $ 15,800,000 | $ 12,100,000 | $ 9,900,000 | |||||||||||||||||||||||