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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 9 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Earnings Per Share [Abstract]        
Stock options excluded from the calculation of diluted weighted-average outstanding shares because their impact was anti-dilutive (in shares) 1.2 1.2 1.1 1.0