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
	Mar. 31, 2021	Mar. 31, 2020
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)	668.6	678.1
Common stock equivalents (in shares)	11.3	13.1
Weighted-average common shares outstanding used for the calculation of net income attributable to Fiserv, Inc. per share – diluted (in shares)	679.9	691.2