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Condensed Statements of Cash Flows (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Operating activities      
Net loss $ (36,375)us-gaap_NetIncomeLoss $ (26,088)us-gaap_NetIncomeLoss $ (11,742)us-gaap_NetIncomeLoss
Adjustments to reconcile net loss to net cash used in operating activities:      
Depreciation expense 160us-gaap_Depreciation 71us-gaap_Depreciation 139us-gaap_Depreciation
Amortization of debt discount and beneficial conversion 15espr_AmortizationOfDebtDiscountPremiumAndBeneficialConversion 459espr_AmortizationOfDebtDiscountPremiumAndBeneficialConversion 117espr_AmortizationOfDebtDiscountPremiumAndBeneficialConversion
Amortization of debt issuance costs 16us-gaap_AmortizationOfFinancingCosts 19us-gaap_AmortizationOfFinancingCosts 15us-gaap_AmortizationOfFinancingCosts
Amortization of premiums and discounts on investments 202us-gaap_AccretionAmortizationOfDiscountsAndPremiumsInvestments 47us-gaap_AccretionAmortizationOfDiscountsAndPremiumsInvestments  
Revaluation of warrants   2,587us-gaap_FairValueAdjustmentOfWarrants (32)us-gaap_FairValueAdjustmentOfWarrants
Noncash interest expense on convertible notes   459us-gaap_PaidInKindInterest 1,370us-gaap_PaidInKindInterest
Stock-based compensation expense 3,679us-gaap_ShareBasedCompensation 1,227us-gaap_ShareBasedCompensation 80us-gaap_ShareBasedCompensation
Loss related to assets held for sale 29us-gaap_GainLossOnSaleOfPropertyPlantEquipment 27us-gaap_GainLossOnSaleOfPropertyPlantEquipment 87us-gaap_GainLossOnSaleOfPropertyPlantEquipment
Loss/(Gain) on sale of assets 2us-gaap_GainLossOnSaleOfOtherAssets (148)us-gaap_GainLossOnSaleOfOtherAssets (3)us-gaap_GainLossOnSaleOfOtherAssets
Changes in assets and liabilities:      
Prepaids and other assets (332)us-gaap_IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets 27us-gaap_IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets (429)us-gaap_IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets
Accounts payable (299)us-gaap_IncreaseDecreaseInAccountsPayable 1,756us-gaap_IncreaseDecreaseInAccountsPayable (215)us-gaap_IncreaseDecreaseInAccountsPayable
Other accrued liabilities 882us-gaap_IncreaseDecreaseInOtherAccruedLiabilities 1,443us-gaap_IncreaseDecreaseInOtherAccruedLiabilities (195)us-gaap_IncreaseDecreaseInOtherAccruedLiabilities
Net cash used in operating activities (32,021)us-gaap_NetCashProvidedByUsedInOperatingActivitiesContinuingOperations (18,114)us-gaap_NetCashProvidedByUsedInOperatingActivitiesContinuingOperations (10,808)us-gaap_NetCashProvidedByUsedInOperatingActivitiesContinuingOperations
Investing activities      
Purchases of investments (48,088)us-gaap_PaymentsToAcquireInvestments (24,677)us-gaap_PaymentsToAcquireInvestments  
Proceeds from sales/maturities of investments 12,351us-gaap_ProceedsFromMaturitiesPrepaymentsAndCallsOfShorttermInvestments 3,505us-gaap_ProceedsFromMaturitiesPrepaymentsAndCallsOfShorttermInvestments  
Proceeds from sale of assets 12us-gaap_ProceedsFromSaleOfProductiveAssets 201us-gaap_ProceedsFromSaleOfProductiveAssets 5us-gaap_ProceedsFromSaleOfProductiveAssets
Purchase of property and equipment (873)us-gaap_PaymentsToAcquirePropertyPlantAndEquipment (31)us-gaap_PaymentsToAcquirePropertyPlantAndEquipment (7)us-gaap_PaymentsToAcquirePropertyPlantAndEquipment
Net cash used in investing activities (36,598)us-gaap_NetCashProvidedByUsedInInvestingActivitiesContinuingOperations (21,002)us-gaap_NetCashProvidedByUsedInInvestingActivitiesContinuingOperations (2)us-gaap_NetCashProvidedByUsedInInvestingActivitiesContinuingOperations
Financing activities      
Proceeds from issuance of common stock, net of issuance costs 91,731us-gaap_ProceedsFromIssuanceOfCommonStock 72,194us-gaap_ProceedsFromIssuanceOfCommonStock  
Proceeds from issuance of preferred stock, net of issuance costs   16,824espr_ProceedsFromIssuanceOfPreferredStockAndPreferenceStockNetOfIssuanceCosts  
Proceeds from exercise of common stock options 473us-gaap_ProceedsFromStockOptionsExercised 123us-gaap_ProceedsFromStockOptionsExercised 41us-gaap_ProceedsFromStockOptionsExercised
Proceeds from warrant issuance 78espr_ProceedsFromIssuanceOfWarrantsFairValueEstimate   298espr_ProceedsFromIssuanceOfWarrantsFairValueEstimate
Proceeds from debt issuance, net of issuance costs 4,838us-gaap_ProceedsFromDebtNetOfIssuanceCosts   15,412us-gaap_ProceedsFromDebtNetOfIssuanceCosts
Net cash provided by financing activities 97,120us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperations 89,141us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperations 15,751us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperations
Net increase in cash and cash equivalents 28,501us-gaap_NetCashProvidedByUsedInContinuingOperations 50,025us-gaap_NetCashProvidedByUsedInContinuingOperations 4,941us-gaap_NetCashProvidedByUsedInContinuingOperations
Cash and cash equivalents at beginning of period 56,537us-gaap_CashAndCashEquivalentsAtCarryingValue 6,512us-gaap_CashAndCashEquivalentsAtCarryingValue 1,571us-gaap_CashAndCashEquivalentsAtCarryingValue
Cash and cash equivalents at end of period 85,038us-gaap_CashAndCashEquivalentsAtCarryingValue 56,537us-gaap_CashAndCashEquivalentsAtCarryingValue 6,512us-gaap_CashAndCashEquivalentsAtCarryingValue
Supplemental disclosure of cash flow information:      
Deferred offering costs not yet paid 107espr_DeferredOfferingCostsIncurred    
10% Convertible promissory notes      
Supplemental disclosure of cash flow information:      
Conversion of promissory notes, including accrued interest into Series A - 1 preferred stock   16,623us-gaap_DebtConversionConvertedInstrumentAmount1
/ us-gaap_DebtConversionByUniqueDescriptionAxis
= us-gaap_ConvertibleNotesPayableMember		  
Pfizer note      
Supplemental disclosure of cash flow information:      
Conversion of promissory notes, including accrued interest into Series A - 1 preferred stock   $ 7,803us-gaap_DebtConversionConvertedInstrumentAmount1
/ us-gaap_DebtConversionByUniqueDescriptionAxis
= espr_PfizerConvertibleNoteMember