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Shareholders’ equity (Details) (USD $) In Thousands, except Share data, unless otherwise specified | 1 Months Ended | 3 Months Ended | 14 Months Ended |
--- | --- | --- | --- | ---
Aug. 31, 2011 | Mar. 31, 2013 | Sep. 30, 2012 | Dec. 31, 2012
Increase (Decrease) in Stockholders' Equity [Roll Forward] |  |  |  |
Repurchase of common stock (in shares) |  | 400,000 |  |
Common stock, shares authorized (in shares) |  | 195,000,000 |  | 195,000,000
Stock repurchase program, Number of shares authorized to be repurchased (in shares) | $ 60,000 |  |  |
Number of shares Treasury stock acquired (in shares) |  |  | 4,786,521 |
Treasury stock acquired, Aggregate purchase price |  |  | 49,924 |
Stock repurchase program, Remaining authorized repurchase amount (in shares) |  | $ 10,076 |  |
Series A Convertible Preferred Stock [Member] |  |  |  |
Increase (Decrease) in Stockholders' Equity [Roll Forward] |  |  |  |
Shares, Beginning balance |  | 80,283 |  |
Repurchase of common stock (in shares) |  | 0 |  |
Conversion of convertible preferred stock (in shares) |  | (199) |  |
Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued in Period |  | 0 |  |
Shares, Ending balance |  | 80,084 |  |
Treasury Stock [Member] |  |  |  |
Increase (Decrease) in Stockholders' Equity [Roll Forward] |  |  |  |
Shares, Beginning balance |  | 4,786,521 |  |
Repurchase of common stock (in shares) |  | 0 |  |
Conversion of convertible preferred stock (in shares) |  | 0 |  |
Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued in Period |  | 0 |  |
Shares, Ending balance |  | 4,786,521 |  |
Common Stock [Member] |  |  |  |
Increase (Decrease) in Stockholders' Equity [Roll Forward] |  |  |  |
Shares, Beginning balance |  | 88,548,637 |  |
Repurchase of common stock (in shares) |  | 0 |  |
Conversion of convertible preferred stock (in shares) |  | 19,864 |  |
Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued in Period |  | 27,708 |  |
Shares, Ending balance |  | 88,596,209 |  |

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- Definition The number of shares converted in a noncash (or part noncash) transaction. Noncash is defined as transactions during a period that do not result in cash receipts or cash payments in the period. "Part noncash" refers to that portion of the transaction not resulting in cash receipts or cash payments in the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4332-108586 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4313-108586 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4304-108586 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 95 -Paragraph 32 -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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