Document and Entity Information - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Feb. 28, 2022	Jun. 30, 2021
Cover [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2021
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	FY
Entity Registrant Name	REDWOOD MORTGAGE INVESTORS IX
Entity Central Index Key	0001448038
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Interactive Data Current	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	false
Entity Shell Company	false
ICFR Auditor Attestation Flag	false
Entity Common Stock, Shares Outstanding		78,350,147
Document Annual Report	true
Document Transition Report	false
Entity File Number	000-55601
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	26-3541068
Entity Address, Address Line One	177 Bovet Road
Entity Address, Address Line Two	Suite 520
Entity Address, City or Town	San Mateo
Entity Address, State or Province	CA
Entity Address, Postal Zip Code	94402
City Area Code	650
Local Phone Number	365-5341
Entity Public Float			$ 0
Auditor Firm ID	243
Auditor Name	BDO USA, LLP
Auditor Location	San Francisco, California