COVER PAGE - USD ($) $ in Billions	12 Months Ended
	Dec. 31, 2019	Feb. 03, 2020	Jun. 28, 2019
Cover page.
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2019
Document Transition Report	false
Entity File Number	1-11859
Entity Registrant Name	PEGASYSTEMS INC.
Entity Incorporation, State or Country Code	MA
Entity Tax Identification Number	04-2787865
Entity Address, Address Line One	One Rogers Street
Entity Address, City or Town	Cambridge
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	02142-1209
City Area Code	617
Local Phone Number	374-9600
Title of 12(b) Security	Common Stock, $.01 par value per share
Trading Symbol	PEGA
Security Exchange Name	NASDAQ
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Public Float			$ 2.7
Entity Common Stock, Shares Outstanding		79,657,420
Amendment Flag	false
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	FY
Entity Central Index Key	0001013857
Current Fiscal Year End Date	--12-31