Derivatives - Undesignated Hedges (Parenthetical) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Cash Flow and Fair Value Hedges
Foreign exchange gains/(losses) on settlement assets and obligations, cash balances, and other assets and liabilities	$ (56.1)	$ (37.0)	$ (33.1)

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