Stockholders' Equity (Deficit) - Sales of Convertible Preferred Stock (Details) - USD ($)	1 Months Ended	4 Months Ended	12 Months Ended
	Nov. 30, 2018	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2019	Jun. 07, 2019	Feb. 28, 2018
Class Of Stock [Line Items]
Preferred stock, shares issued			0	0
Preferred stock, par value			$ 0.001	$ 0.001	$ 0.001
Net cash proceeds from issuance of preferred stock			$ 17,873,000
Series B-2 Convertible Preferred Stock | Private Oncternal
Class Of Stock [Line Items]
Preferred stock, shares issued		1,662,494
Preferred stock, par value		$ 6.13
Net cash proceeds from issuance of preferred stock		$ 8,900,000
Preferred stock, value collected						$ 1,100,000
Series C Convertible Preferred Stock | Private Oncternal
Class Of Stock [Line Items]
Preferred stock, shares issued	2,495,114
Preferred stock, par value	$ 6.81
Net cash proceeds from issuance of preferred stock	$ 16,800,000
Preferred stock, value collected	$ 0