Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2018	Nov. 02, 2018
Document And Entity Information [Abstract]
Entity Registrant Name	CSG SYSTEMS INTERNATIONAL INC
Entity Central Index Key	0001005757
Document Type	10-Q
Document Period End Date	Sep. 30, 2018
Amendment Flag	false
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Ex Transition Period	false
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	Q3
Trading Symbol	CSGS
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		33,328,416