Derivatives - Undesignated Hedges (Parenthetical) (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2026	Sep. 30, 2025	Sep. 30, 2026	Sep. 30, 2025
Cash Flow and Fair Value Hedges
Foreign exchange gains/(losses) on settlement assets and obligations, cash balances, and other assets and liabilities	$ (11.3)	$ (12.4)	$ (35.6)	$ 25.5

Represents Foreign Exchange Gain Loss on Settlement Assets and Obligations, Other Assets and Other Liabilities and Cash Balances

No definition available.

Name:	wu_ForeignExchangeGainLossOnSettlementAssetsAndObligationsOtherAssetsAndOtherLiabilitiesAndCashBalances
Namespace Prefix:	wu_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration