v3.19.3

Stockholders' Equity - ATM Offering Program - Additional Information (Detail) - ATM Offering Program [Member] - USD ($)	3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2019	Dec. 31, 2018	Aug. 31, 2018
Class of Stock [Line Items]
Issuance of common stock	329,588	329,588	340,307
Proceeds from issuance of common stock, net	$ 400,000	$ 0.4	$ 5,000,000
Issuance of common stock, commissions paid	18,000	18,000	200,000
Issuance of common stock, other offering expenses	$ 100,000	$ 0.1	$ 400,000
Cowen [Member]
Class of Stock [Line Items]
Amount which can be raised under ATM offering program				$ 75,000,000
Stock issuance costs under sales agreement as a percent of gross proceeds				3.00%

Common stock aggregate offering price authorized under sales agreement.

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Common stock issuance costs under sales agreement as percent of gross proceeds.

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Issuance of common stock under ATM agreement net of issuance costs value.

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The cash outflow for cost incurred directly with the issuance of an equity security.

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Number of new stock issued during the period.

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Sienna Biopharmaceuticals, Inc. 10-Q