Stockholders' Equity - Reserved Shares of Common Stock for Future Issuance (Details) - shares						12 Months Ended
	Jan. 01, 2020	Jun. 06, 2019	Jan. 01, 2019	Dec. 12, 2018	Jan. 01, 2018	Dec. 31, 2018	Dec. 31, 2017	Sep. 30, 2020	Dec. 31, 2019	Jan. 30, 2019	Feb. 28, 2016	Feb. 28, 2014
Class of Stock [Line Items]
Common stock, shares reserved for future issuance (in shares)								24,800,299	21,403,890
Warrant to purchase common stock
Class of Stock [Line Items]
Common stock, shares reserved for future issuance (in shares)								509,611	509,611
Common Stock Options and RSUs Outstanding
Class of Stock [Line Items]
Common stock, shares reserved for future issuance (in shares)								15,653,443	12,195,031
Shares available for issuance under Akebia equity plans
Class of Stock [Line Items]
Common stock, shares reserved for future issuance (in shares)								3,156,911	2,983,256
2014 Plans
Class of Stock [Line Items]
Common stock, shares reserved for future issuance (in shares)												1,785,000
Increase in common stock, shares reserved for future issuance (in shares)	4,031,376		3,801,198	2,323,213	1,575,329
Inducement Award Program
Class of Stock [Line Items]
Common stock, shares reserved for future issuance (in shares)										3,150,000
Shares available for issuance under the ESPP
Class of Stock [Line Items]
Common stock, shares reserved for future issuance (in shares)								5,480,334	5,715,992
Increase in common stock, shares reserved for future issuance (in shares)		5,200,000				0	0
Maximum aggregate number of shares available for purchase (in shares)											739,611