Shares Used in Computing Net Income Per Share Attributable to Fiserv, Inc. - Schedule of Weighted-Average Number of Shares (Details) - shares shares in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Earnings Per Share [Abstract]
Weighted-average common shares outstanding used for the calculation of net income attributable to Fiserv, Inc. per share – basic (in shares)	670.0	392.5	674.1	392.1
Common stock equivalents (in shares)	10.8	7.1	11.9	7.3
Weighted-average common shares outstanding used for the calculation of net income attributable to Fiserv, Inc. per share – diluted (in shares)	680.8	399.6	686.0	399.4