REVENUE - Performance Obligations (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Sep. 30, 2020
Disaggregation of Revenue [Line Items]
Revenue remaining performance obligation	$ 1,034,026	$ 837,990
Revenue remaining performance obligation, percentage	100.00%	100.00%
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-01-01
Disaggregation of Revenue [Line Items]
Expected timing of satisfaction		2 years
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-10-01
Disaggregation of Revenue [Line Items]
Expected timing of satisfaction		1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-01-01
Disaggregation of Revenue [Line Items]
Expected timing of satisfaction	2 years	3 years
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-10-01
Disaggregation of Revenue [Line Items]
Expected timing of satisfaction	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2022-01-01
Disaggregation of Revenue [Line Items]
Expected timing of satisfaction	3 years
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2023-01-01
Disaggregation of Revenue [Line Items]
Expected timing of satisfaction
1 year or less
Disaggregation of Revenue [Line Items]
Revenue remaining performance obligation	$ 577,650	$ 456,777
Revenue remaining performance obligation, percentage	56.00%	54.00%
1-2 years
Disaggregation of Revenue [Line Items]
Revenue remaining performance obligation	$ 261,071	$ 207,214
Revenue remaining performance obligation, percentage	25.00%	25.00%
2-3 years
Disaggregation of Revenue [Line Items]
Revenue remaining performance obligation	$ 127,188	$ 119,382
Revenue remaining performance obligation, percentage	12.00%	14.00%
Greater than 3 years
Disaggregation of Revenue [Line Items]
Revenue remaining performance obligation	$ 68,117	$ 54,617
Revenue remaining performance obligation, percentage	7.00%	7.00%
Pega Cloud
Disaggregation of Revenue [Line Items]
Revenue remaining performance obligation	$ 571,461	$ 506,807
Pega Cloud | 1 year or less
Disaggregation of Revenue [Line Items]
Revenue remaining performance obligation	284,359	211,661
Pega Cloud | 1-2 years
Disaggregation of Revenue [Line Items]
Revenue remaining performance obligation	177,214	157,500
Pega Cloud | 2-3 years
Disaggregation of Revenue [Line Items]
Revenue remaining performance obligation	79,775	93,283
Pega Cloud | Greater than 3 years
Disaggregation of Revenue [Line Items]
Revenue remaining performance obligation	30,113	44,363
Maintenance
Disaggregation of Revenue [Line Items]
Revenue remaining performance obligation	327,696	239,148
Maintenance | 1 year or less
Disaggregation of Revenue [Line Items]
Revenue remaining performance obligation	196,667	170,643
Maintenance | 1-2 years
Disaggregation of Revenue [Line Items]
Revenue remaining performance obligation	59,360	40,631
Maintenance | 2-3 years
Disaggregation of Revenue [Line Items]
Revenue remaining performance obligation	37,734	18,277
Maintenance | Greater than 3 years
Disaggregation of Revenue [Line Items]
Revenue remaining performance obligation	33,935	9,597
Term license
Disaggregation of Revenue [Line Items]
Revenue remaining performance obligation	66,802	63,185
Term license | 1 year or less
Disaggregation of Revenue [Line Items]
Revenue remaining performance obligation	49,265	50,788
Term license | 1-2 years
Disaggregation of Revenue [Line Items]
Revenue remaining performance obligation	16,872	5,341
Term license | 2-3 years
Disaggregation of Revenue [Line Items]
Revenue remaining performance obligation	420	7,052
Term license | Greater than 3 years
Disaggregation of Revenue [Line Items]
Revenue remaining performance obligation	245	4
Perpetual license
Disaggregation of Revenue [Line Items]
Revenue remaining performance obligation	22,971	10,408
Perpetual license | 1 year or less
Disaggregation of Revenue [Line Items]
Revenue remaining performance obligation	15,686	8,708
Perpetual license | 1-2 years
Disaggregation of Revenue [Line Items]
Revenue remaining performance obligation	1,064	1,700
Perpetual license | 2-3 years
Disaggregation of Revenue [Line Items]
Revenue remaining performance obligation	4,094	0
Perpetual license | Greater than 3 years
Disaggregation of Revenue [Line Items]
Revenue remaining performance obligation	2,127	0
Consulting
Disaggregation of Revenue [Line Items]
Revenue remaining performance obligation	45,096	18,442
Consulting | 1 year or less
Disaggregation of Revenue [Line Items]
Revenue remaining performance obligation	31,673	14,977
Consulting | 1-2 years
Disaggregation of Revenue [Line Items]
Revenue remaining performance obligation	6,561	2,042
Consulting | 2-3 years
Disaggregation of Revenue [Line Items]
Revenue remaining performance obligation	5,165	770
Consulting | Greater than 3 years
Disaggregation of Revenue [Line Items]
Revenue remaining performance obligation	$ 1,697	$ 653