Entity Information (USD $)	3 Months Ended
	Mar. 31, 2012	May 03, 2012	Jun. 30, 2011
Entity Registrant Name	ON ASSIGNMENT INC
Entity Central Index Key	0000890564
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Accelerated Filer
Entity Common Stock, Shares Outstanding		37,621,871
Document Fiscal Year Focus	2012
Document Fiscal Period Focus	Q1
Entity Public Float			$ 308,131,042