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Income tax expense (Details) (USD $)
 3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Dec. 31, 2014
Income Tax Disclosure [Abstract]      
Income tax expense $ 3,272,000us-gaap_IncomeTaxExpenseBenefit $ 3,882,000us-gaap_IncomeTaxExpenseBenefit  
Effective tax rate (percent) 16.30%us-gaap_EffectiveIncomeTaxRateContinuingOperations 26.00%us-gaap_EffectiveIncomeTaxRateContinuingOperations  
Gross unrecognized tax benefits 16,900,000us-gaap_UnrecognizedTaxBenefits   16,600,000us-gaap_UnrecognizedTaxBenefits
Unrecognized tax benefits that would be recorded as reduction in income tax provision, if recognized $ 16,900,000us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate