Document And Entity Information - USD ($)	12 Months Ended
	Sep. 30, 2019	Dec. 03, 2019	Mar. 29, 2019
Document and Entity Information [Abstract]
Entity Registrant Name	CSP INC /MA/
Document Type	10-K
Current Fiscal Year End Date	--09-30
Entity Common Stock, Shares Outstanding		4,153,742
Entity Public Float			$ 40,671,108
Amendment Flag	false
Entity Central Index Key	0000356037
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Non-accelerated Filer
Entity Well-known Seasoned Issuer	No
Document Period End Date	Sep. 30, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	FY
Entity Small Business	true
Entity Emerging Growth Company	false
Entity Shell Company	false