Acquisitions and Dispositions - Pro Forma Combined Financial Information (Details) - First Data - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Business Acquisition, Pro Forma Information, Nonrecurring Adjustment [Line Items]
Total revenue	$ 15,775	$ 15,284
Net income	1,520	1,125
Net income attributable to Fiserv, Inc.	$ 1,457	$ 1,040
Net income per share attributable to Fiserv, Inc.:
Net income per share attributable to Fiserv, Inc., Basic (in dollars per share)	$ 2.14	$ 1.50
Net income per share attributable to Fiserv, Inc., Diluted (in dollars per share)	$ 2.10	$ 1.47