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Shares Used in Computing Net Income Per Share Attributable to Fiserv, Inc. - Narrative (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]        
Stock options excluded from the calculation of diluted weighted-average outstanding shares because their impact was anti-dilutive (in shares) 3.1 1.2 1.8 1.0