Shares Used in Computing Net Income (Loss) Per Share Attributable to Fiserv, Inc. - Schedule of Weighted-Average Number of Shares (Details) - shares shares in Millions |
3 Months Ended | 9 Months Ended | ||
|---|---|---|---|---|
Sep. 30, 2026 |
Sep. 30, 2025 |
Sep. 30, 2026 |
Sep. 30, 2025 |
|
| Earnings Per Share [Abstract] | ||||
| Weighted-average common shares outstanding used for the calculation of net income (loss) attributable to Fiserv, Inc. per share – basic (in shares) | 531.8 | 540.2 | 532.9 | 550.8 |
| Common stock equivalents (in shares) | 0.0 | 1.6 | 0.0 | 2.2 |
| Weighted-average common shares outstanding used for the calculation of net income (loss) attributable to Fiserv, Inc. per share – diluted (in shares) | 531.8 | 541.8 | 532.9 | 553.0 |