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Summary of Significant Accounting Policies - Net Income Per Share (Details) - shares
shares in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Accounting Policies [Abstract]      
Weighted-average shares excluded from calculations of common stock equivalents for anti-dilutive stock options 1.3 1.1 1.1
Weighted Average Number of Shares Outstanding, Diluted [Abstract]      
Weighted-average common shares outstanding used for the calculation of net income attributable to Fiserv, Inc. per share – basic 672.1 512.3 405.5
Common stock equivalents (in shares) 11.3 10.3 8.2
Weighted-average common shares outstanding used for the calculation of net income attributable to Fiserv, Inc. per share – diluted 683.4 522.6 413.7