Fair Value Measurements - Summary of Warrants Liability and Remeasured Liability to Estimated Fair Value (Detail)	12 Months Ended
	Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Alternative Investment, Measurement Input [Extensible List]	evok:BlackScholesOptionPricingModelMember
Risk-free Interest Rate [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Estimated fair value assumptions	2.09
Expected Volatility [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Estimated fair value assumptions	100.39
Expected Term [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Expected term	4 years 29 days
Expected Dividend Yield [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Estimated fair value assumptions	0.0