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Activity of stock options exercised (Details) (USD $)
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Total intrinsic value of stock options exercised $ 20,684us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue $ 4,140us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue
Cash received from stock option exercises $ 44,771us-gaap_EmployeeServiceShareBasedCompensationCashReceivedFromExerciseOfStockOptions $ 6,480us-gaap_EmployeeServiceShareBasedCompensationCashReceivedFromExerciseOfStockOptions