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Shareholders' equity Additional Information (Details) - USD ($) | Dec. 31, 2014 | Jul. 14, 2014 | Dec. 19, 2013 | Mar. 20, 2013 | Mar. 19, 2013
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Equity [Abstract] |  |  |  |  |
Common stock reserved for future issuance | 18,372,187 |  |  |  |
Common stock reserved for future issuance in equity forward transaction | 4,700,000 |  |  |  |
Public offering related to equity forward transaction (in shares) |  |  |  |  | 6,100,000
Sale of common stock related to equity forward transaction (in dollars per share) |  |  |  |  | $ 26.75
Closing price of common stock (in dollar per share) |  |  |  |  | $ 27.01
Offering related to underwriters exercising their over-allotment option under equity forward transaction (in shares) |  |  |  | 900,000 |
Shares borrowed by forward counterparty from third party |  |  |  |  | 7,000,000
Underwriting discount (in dollars per share) |  |  |  |  | $ 1.00312
Forward sale price (in dollars per share) | $ 22.63 |  |  |  | $ 25.74688
Initial fair value (in dollars per share) | $ 0 |  |  |  |
Delivery of net shares on settlement | 1,500,000 | 1,000,000 | 1,300,000 |  |
Delivery of cash on settlement | $ 51,000,000 | $ 23,900,000 | $ 32,100,000 |  |
Underwriting discount included in delivery of cash | 5,000,000 | $ 1,000,000 | $ 1,300,000 |  |
Cash in exchange of physical delivery of shares on settlement | $ 106,000,000 |  |  |  |

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- Definition Represents the amount of delivery of cash to forward counterparty on settlement of equity forward transaction. + References No definition available. + Details
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- Definition Aggregate number of common shares reserved for future issuance. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.29) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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