Common Stock, Common Stock Warrants and Stock Option Plan - 2015 Employee Stock Purchase Plan (Details) - USD ($)							1 Months Ended	3 Months Ended	12 Months Ended
	Jan. 31, 2020	Jul. 31, 2019	Jan. 31, 2019	Jul. 31, 2018	Jan. 31, 2018	Jul. 29, 2016	Sep. 30, 2015	Mar. 31, 2020	Dec. 31, 2017	Dec. 31, 2019
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, capital shares reserved for future issuance (in shares)								15,035,095		14,093,625
Employee Stock [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, capital shares reserved for future issuance (in shares)							150,000	547,513		373,428
Employee stock purchase plan description								The number of shares reserved for issuance under our 2015 Employee Stock Purchase Plan will increase automatically on the first day of January following the first offering date by the number of shares equal to 1% of the total outstanding shares of our common stock as of the immediately preceding December 31 (rounded to the nearest whole share).
Percentage threshold of outstanding shares increased annually under the plan (as a percent)							1.00%
Common stock shares authorized for issuance (in shares)							2,250,000
Payroll deduction (as a percent)								15.00%
Percentage of fair market value of common stock (as a percent)								85.00%
Shares issued (in shares)	128,601	154,116	129,815	69,761	55,806	68,392			110,711
Maximum fair market value, that would permit employee to purchase common stock under plan								$ 25,000
Maximum number of shares issued each participants (in shares)								2,500