Fair Value Measurements - Additional Information (Detail)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value transfer amount			$ 0		$ 0
Measurement Input, Option Volatility [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of warrant	0.7802		0.7802
Measurement Input, Risk Free Interest Rate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of warrant	0.275		0.275
Measurement Input, Expected Dividend Rate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of warrant	0		0
Measurement Input, Expected Term [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of warrant,term	10 years		10 years
Creabilis [Member] \| Discount Rate [Member] \| Minimum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Contingent consideration measurement input	0.053		0.053
Creabilis [Member] \| Discount Rate [Member] \| Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Contingent consideration measurement input	0.159		0.159
Creabilis [Member] \| Probability of Multiple Scenarios [Member] \| Minimum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Contingent consideration measurement input	0.036		0.036
Creabilis [Member] \| Probability of Multiple Scenarios [Member] \| Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Contingent consideration measurement input	0.526		0.526
In-process Research and Development Intangible Assets [Member] \| Creabilis [Member] \| Discount Rate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Intangible asset measurement input	0.205		0.205
In-process Research and Development Intangible Assets [Member] \| Creabilis [Member] \| Probability of Multiple Scenarios [Member] \| Minimum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Intangible asset measurement input	0.001		0.001
In-process Research and Development Intangible Assets [Member] \| Creabilis [Member] \| Probability of Multiple Scenarios [Member] \| Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Intangible asset measurement input	0.225		0.225
Contingent Consideration [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Change in fair value	$ (300,000)	$ 800,000	$ 4,300,000	$ 2,300,000

Sienna Biopharmaceuticals, Inc. 10-Q