Nature of Business - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended		6 Months Ended
	May 31, 2017	Mar. 31, 2016	Jun. 30, 2017	Jun. 30, 2016	Mar. 31, 2017	Dec. 31, 2016
Nature of Business [Line Items]
Proceeds from public stock offering, net of issuance costs				$ 52,318
Common stock, shares authorized			100,000,000			100,000,000
Preferred stock, shares authorized			10,000,000			10,000,000
Aggregate net proceeds received from the offering, net of underwriting discounts and commissions and estimated offering expenses payable			$ 48,836
Minimum [Member] | Entinostat or SNDX-6352 [Member]
Nature of Business [Line Items]
Anticipated period of approval			2 years
Initial Public Offering [Member]
Nature of Business [Line Items]
Common stock, shares issued		4,809,475
Sale of stock, price per share		$ 12.00
Proceeds from public stock offering, net of issuance costs		$ 50,500
Underwriting discounts and commissions		4,000
Offering expenses		$ 3,100
Convertible preferred stock converted in to common stock		12,872,551
Shares of Common Stock Issuable Under the Warrant		357,840
Reclassification of common stock warrant liability to additional paid-in capital		$ 4,600
Common stock, shares authorized					100,000,000
Preferred stock, shares authorized					10,000,000
Over-Allotment Option [Member]
Nature of Business [Line Items]
Common stock, new shares issued	200,190	409,475
Follow-on Public Offering [Member]
Nature of Business [Line Items]
Common stock, new shares issued	3,950,190
Sale of stock, price per share	$ 13.25
Aggregate net proceeds received from the offering, net of underwriting discounts and commissions and estimated offering expenses payable	$ 48,700