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Stock-Based Compensation (Details 4) (Employee Stock Purchase Plan, USD $)
 3 Months Ended
Mar. 31, 2015
May 31, 2011
Employee Stock Purchase Plan    
Maximum employee salary to purchase shares (in dollars) $ 25,000exh_ShareBasedCompensationArrangementByShareBasedPaymentAwardMaximumEmployeeSubscriptionAmount  
Maximum percentage of employee eligible pay to purchase shares 10.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardMaximumEmployeeSubscriptionRate  
Maximum number of shares available under the Plan   1,000,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized
Remaining shares available for purchase 174,466us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant  
Employee Stock Purchase plan discount from market price (as a percent) 5.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardDiscountFromMarketPriceOfferingDate  
Minimum
    
Employee Stock Purchase Plan    
Purchase price as a percent of the fair market value 85.00%us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardPurchasePriceOfCommonStockPercent
/ us-gaap_PlanNameAxis
= us-gaap_EmployeeStockMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember		  
Maximum
    
Employee Stock Purchase Plan    
Purchase price as a percent of the fair market value 100.00%us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardPurchasePriceOfCommonStockPercent
/ us-gaap_PlanNameAxis
= us-gaap_EmployeeStockMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember