Fair Value Measurements - Assets and Liabilities Measured at Fair Value On a Recurring Basis (Details) - USD ($) $ in Millions	Sep. 30, 2025	Dec. 31, 2024
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Obligation to purchase redeemable noncontrolling interest	$ 0	$ 95
Fair Value, Recurring \| Level 2 \| Prepaid expenses and other current assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cross-currency rate swap contract designated as net investment hedge	0	2
Fair Value, Recurring \| Level 2 \| Other long-term assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cross-currency rate swap contract designated as net investment hedge	0	6
Fair Value, Recurring \| Level 2 \| Accounts payable and other current liabilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Forward exchange contracts designated as cash flow hedges	9	6
Fair Value, Recurring \| Level 2 \| Accounts payable and other current liabilities \| Fair Value Hedging
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cross-currency rate swap contract	0	12
Fair Value, Recurring \| Level 2 \| Accounts payable and other current liabilities \| Net Investment Hedging
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cross-currency rate swap contract	29	9
Fair Value, Recurring \| Level 2 \| Other long-term liabilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Forward exchange contracts designated as cash flow hedges	2	2
Fair Value, Recurring \| Level 2 \| Other long-term liabilities \| Net Investment Hedging
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cross-currency rate swap contract	76	17
Fair Value, Recurring \| Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Contingent consideration	6	0
Contingent consideration	21	0
Obligation to purchase redeemable noncontrolling interest	0	95
Contingent debt guarantee	$ 8	$ 15

FISERV INC 10-Q