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Shares Used in Computing Net Income Per Share Attributable to Fiserv, Inc. - Narrative (Details) - shares
shares in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Sep. 30, 2025
Sep. 30, 2024
Earnings Per Share [Abstract]        
Stock options excluded from the calculation of diluted weighted-average outstanding shares because their impact was anti-dilutive (in shares) 1,300 0 956 0