ACQUISITIONS - Narrative (Details) $ in Thousands	3 Months Ended	6 Months Ended
	Mar. 31, 2020 USD ($)	Mar. 31, 2020 USD ($)
Business Acquisition [Line Items]
Goodwill acquired	$ 34,911
TSYS
Business Acquisition [Line Items]
Adjustment to goodwill	(54,742)
Goodwill acquired	17,300,000
Revenue of acquiree	1,055,000
Operating income (loss) of acquiree	115,500
Employee termination benefits obligation	48,300	$ 48,300
Employee termination benefits charges	17,600	74,700
Employee termination benefits charges related to stock-based compensation	2,600	$ 19,900
Merchant Solutions
Business Acquisition [Line Items]
Goodwill acquired	34,911
Merchant Solutions | TSYS
Business Acquisition [Line Items]
Goodwill acquired	7,100,000
Issuer Solutions
Business Acquisition [Line Items]
Goodwill acquired	0
Issuer Solutions | TSYS
Business Acquisition [Line Items]
Goodwill acquired	7,900,000
Business and Consumer Solutions
Business Acquisition [Line Items]
Goodwill acquired	0
Business and Consumer Solutions | TSYS
Business Acquisition [Line Items]
Goodwill acquired	$ 2,300,000