Financial Instruments − Schedule of Company's Financial Instruments Measured at Fair value on Recurring Basis (Details) - USD ($) $ in Millions	Sep. 30, 2026	Dec. 31, 2025
Investment securities, current:
Debt securities, current	$ 4,673.6	$ 4,332.9
Investment securities, non-current:
Total	102.7	94.2
Executive deferred compensation plan trust	18.0	18.4
Location, Statement of Financial Position, Balance [Axis]: us-gaap:OtherAssetsNoncurrent
Investment securities, non-current:
Executive deferred compensation plan trust	16.0	16.3
Location, Statement of Financial Position, Balance [Axis]: us-gaap:OtherLiabilitiesNoncurrent
Liabilities:
Contingent consideration		17.9
Location, Statement of Financial Position, Balance [Axis]: us-gaap:PrepaidExpenseAndOtherAssetsCurrent
Investment securities, non-current:
Executive deferred compensation plan trust	2.0	2.2
Location, Statement of Financial Position, Balance [Axis]: wex:AccruedExpensesAndOtherCurrentLiabilities
Liabilities:
Contingent consideration	18.6	51.0
Corporate and sovereign debt securities
Investment securities, current:
Debt securities, current	1,667.0	1,349.6
Municipal bonds
Investment securities, current:
Debt securities, current	62.8	66.6
Asset-backed securities
Investment securities, current:
Debt securities, current	1,028.4	868.2
Mortgage-backed securities
Investment securities, current:
Debt securities, current	1,590.4	1,651.0
Mutual fund
Investment securities, non-current:
Fixed-income mutual fund and Pooled investment fund measured at NAV	26.9	27.6
Pooled investment fund measured at NAV
Investment securities, non-current:
Fixed-income mutual fund and Pooled investment fund measured at NAV	12.9	12.9
Level 1
Investment securities, non-current:
Executive deferred compensation plan trust	18.0	18.4
Level 1 \| Mutual fund
Investment securities, non-current:
Fixed-income mutual fund and Pooled investment fund measured at NAV	26.9	27.6
Level 1 \| Money market mutual funds
Financial Assets:
Cash and cash equivalents	85.3	64.3
Level 2
Investment securities, non-current:
Available-for-sale debt securities	62.9	53.7
Liabilities:
Contingent consideration	18.6	68.9
Level 2 \| U.S. treasury notes
Investment securities, current:
Debt securities, current	325.0	397.5
Level 2 \| Corporate and sovereign debt securities
Investment securities, current:
Debt securities, current	1,667.0	1,349.6
Level 2 \| Municipal bonds
Investment securities, current:
Debt securities, current	62.8	66.6
Level 2 \| Asset-backed securities
Investment securities, current:
Debt securities, current	1,028.4	868.2
Level 2 \| Mortgage-backed securities
Investment securities, current:
Debt securities, current	1,590.4	1,651.0
Level 2 \| U.S. Treasury bills
Financial Assets:
Cash and cash equivalents	0.0	99.9
Net Asset Value \| Pooled investment fund measured at NAV
Investment securities, non-current:
Fixed-income mutual fund and Pooled investment fund measured at NAV	$ 12.9	$ 12.9

Amount of liability recognized from contingent consideration in asset acquisition.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 805
-SubTopic 50
-Name Accounting Standards Codification
-Section 15
-Paragraph 3
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480123/805-50-15-3

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Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 820
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-Name Accounting Standards Codification
-Section 50
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Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
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Reference 3: http://www.xbrl.org/2003/role/disclosureRef
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-Section 50
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-Publisher FASB
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Reference 4: http://www.xbrl.org/2003/role/disclosureRef
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Reference 5: http://www.xbrl.org/2003/role/disclosureRef
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+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 326
-SubTopic 30
-Name Accounting Standards Codification
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-Publisher FASB
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Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 210
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-Name Accounting Standards Codification
-Section S99
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-URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 210
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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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