REVENUE - Performance Obligations (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Sep. 30, 2019
Disaggregation of Revenue [Line Items]
Revenue remaining performance obligation	$ 837,990	$ 608,583
Revenue remaining performance obligation, percentage	100.00%	100.00%
1 year or less
Disaggregation of Revenue [Line Items]
Revenue remaining performance obligation	$ 456,777	$ 338,787
Revenue remaining performance obligation, percentage	54.00%	56.00%
1-2 years
Disaggregation of Revenue [Line Items]
Revenue remaining performance obligation	$ 207,214	$ 129,185
Revenue remaining performance obligation, percentage	25.00%	21.00%
2-3 years
Disaggregation of Revenue [Line Items]
Revenue remaining performance obligation	$ 119,382	$ 83,976
Revenue remaining performance obligation, percentage	14.00%	14.00%
Greater than 3 years
Disaggregation of Revenue [Line Items]
Revenue remaining performance obligation	$ 54,617	$ 56,635
Revenue remaining performance obligation, percentage	7.00%	9.00%
Perpetual license
Disaggregation of Revenue [Line Items]
Revenue remaining performance obligation	$ 10,408	$ 9,848
Perpetual license | 1 year or less
Disaggregation of Revenue [Line Items]
Revenue remaining performance obligation	8,708	7,689
Perpetual license | 1-2 years
Disaggregation of Revenue [Line Items]
Revenue remaining performance obligation	1,700	853
Perpetual license | 2-3 years
Disaggregation of Revenue [Line Items]
Revenue remaining performance obligation	0	1,306
Perpetual license | Greater than 3 years
Disaggregation of Revenue [Line Items]
Revenue remaining performance obligation	0	0
Term license
Disaggregation of Revenue [Line Items]
Revenue remaining performance obligation	63,185	30,422
Term license | 1 year or less
Disaggregation of Revenue [Line Items]
Revenue remaining performance obligation	50,788	25,948
Term license | 1-2 years
Disaggregation of Revenue [Line Items]
Revenue remaining performance obligation	5,341	3,798
Term license | 2-3 years
Disaggregation of Revenue [Line Items]
Revenue remaining performance obligation	7,052	591
Term license | Greater than 3 years
Disaggregation of Revenue [Line Items]
Revenue remaining performance obligation	4	85
Maintenance
Disaggregation of Revenue [Line Items]
Revenue remaining performance obligation	239,148	190,092
Maintenance | 1 year or less
Disaggregation of Revenue [Line Items]
Revenue remaining performance obligation	170,643	158,220
Maintenance | 1-2 years
Disaggregation of Revenue [Line Items]
Revenue remaining performance obligation	40,631	18,590
Maintenance | 2-3 years
Disaggregation of Revenue [Line Items]
Revenue remaining performance obligation	18,277	8,323
Maintenance | Greater than 3 years
Disaggregation of Revenue [Line Items]
Revenue remaining performance obligation	9,597	4,959
Pega Cloud
Disaggregation of Revenue [Line Items]
Revenue remaining performance obligation	506,807	363,372
Pega Cloud | 1 year or less
Disaggregation of Revenue [Line Items]
Revenue remaining performance obligation	211,661	133,785
Pega Cloud | 1-2 years
Disaggregation of Revenue [Line Items]
Revenue remaining performance obligation	157,500	105,081
Pega Cloud | 2-3 years
Disaggregation of Revenue [Line Items]
Revenue remaining performance obligation	93,283	72,915
Pega Cloud | Greater than 3 years
Disaggregation of Revenue [Line Items]
Revenue remaining performance obligation	44,363	51,591
Consulting
Disaggregation of Revenue [Line Items]
Revenue remaining performance obligation	18,442	14,849
Consulting | 1 year or less
Disaggregation of Revenue [Line Items]
Revenue remaining performance obligation	14,977	13,145
Consulting | 1-2 years
Disaggregation of Revenue [Line Items]
Revenue remaining performance obligation	2,042	863
Consulting | 2-3 years
Disaggregation of Revenue [Line Items]
Revenue remaining performance obligation	770	841
Consulting | Greater than 3 years
Disaggregation of Revenue [Line Items]
Revenue remaining performance obligation	$ 653	$ 0