Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2019	Oct. 29, 2019
Cover [Abstract]
Entity Registrant Name	CSG SYSTEMS INTERNATIONAL INC
Entity Central Index Key	0001005757
Document Type	10-Q
Document Period End Date	Sep. 30, 2019
Amendment Flag	false
Entity Small Business	false
Entity Emerging Growth Company	false
Document Fiscal Year Focus	2019
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		32,946,549
Entity Current Reporting Status	Yes
Entity Shell Company	false
Entity File Number	0-27512
Entity Tax Identification Number	47-0783182
Entity Address, Address Line One	6175 S. Willow Drive
Entity Address, Address Line Two	10th Floor
Entity Address, City or Town	Greenwood Village
Entity Address, State or Province	CO
Entity Address, Postal Zip Code	80111
City Area Code	(303)
Local Phone Number	200-2000
Entity Incorporation, State or Country Code	DE
Document Quarterly Report	true
Document Transition Report	false
Title of 12(b) Security	Common Stock, Par Value $0.01 Per Share
Security Exchange Name	NASDAQ
Entity Interactive Data Current	Yes