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Share-based compensation The 2011 Director Plan (Details) (USD $) In Millions, except Share data, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Income tax benefit (in dollars) | $ 3.4 us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense | $ 2.8 us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense | $ 2.4 us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense
Common stock |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Shares granted | 33,170 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod / us-gaap_AwardTypeAxis = us-gaap_CommonStockMember | 33,184 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod / us-gaap_AwardTypeAxis = us-gaap_CommonStockMember | 29,448 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod / us-gaap_AwardTypeAxis = us-gaap_CommonStockMember
Fair value of vested stock (in dollars) | 0.8 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue / us-gaap_AwardTypeAxis = us-gaap_CommonStockMember | 0.8 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue / us-gaap_AwardTypeAxis = us-gaap_CommonStockMember | 0.8 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue / us-gaap_AwardTypeAxis = us-gaap_CommonStockMember
Income tax benefit (in dollars) | $ 0.3 us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense / us-gaap_AwardTypeAxis = us-gaap_CommonStockMember | $ 0.3 us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense / us-gaap_AwardTypeAxis = us-gaap_CommonStockMember | $ 0.3 us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense / us-gaap_AwardTypeAxis = us-gaap_CommonStockMember

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- Definition The total recognized tax benefit related to compensation cost for equity-based payment arrangements recognized in income during the 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 (h)(1)(i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Fair value of share-based awards for which the grantee gained the right by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash. + 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 (d)(2) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Number of share instruments newly issued under a share-based compensation 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 -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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